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diff --git a/43363-0.txt b/43363-0.txt new file mode 100644 index 0000000..1b41490 --- /dev/null +++ b/43363-0.txt @@ -0,0 +1,3967 @@ +*** START OF THE PROJECT GUTENBERG EBOOK 43363 *** + +Transcriber's note: + Spelling and punctuation inconsistencies have been harmonized. + Obvious printer errors have been repaired. Italic text has been + marked with _underscores_. The symbol for male has been + rendered as [M] and for female as [F]. + + + + + The Cambridge Manuals of Science and + Literature + + EARTHWORMS AND THEIR ALLIES + + + + + CAMBRIDGE UNIVERSITY PRESS + London: FETTER LANE, E.C. + C. F. CLAY, MANAGER + + [Illustration] + + Edinburgh: 100, PRINCES STREET + London: WILLIAM WESLEY & SON, 28, ESSEX STREET, STRAND + Berlin: A. ASHER AND CO. + Leipzig: F. A. BROCKHAUS + New York: G. P. PUTNAM'S SONS + Bombay and Calcutta: MACMILLAN AND CO., LTD. + + _All rights reserved_ + + + + + EARTHWORMS AND + THEIR ALLIES + + BY + + FRANK E. BEDDARD + + M.A. (Oxon.), F.R.S., F.R.S.E. + + Cambridge: + at the University Press + + New York: + G. P. Putnam's Sons + + 1912 + + + Cambridge: + PRINTED BY JOHN CLAY, M.A. + AT THE UNIVERSITY PRESS + +_With the exception of the coat of arms at the foot, the design on +the title page is a reproduction of one used by the earliest known +Cambridge printer, John Siberch, 1521_. + + + + +PREFACE + + +The importance of earthworms in questions relating to geographical +distribution is so universally admitted that it seemed to me +convenient to embody in a short volume the principal facts. + +It became necessary in order to accomplish this task in an adequate +fashion to preface the distributional facts with some anatomical and +zoological data. I have reduced this section of the book to a minimum +and I trust that the illustrations will enable the reader, who is +not specially acquainted with the structure of these animals, to +obtain an idea of their general features and variability in external +character and internal anatomy. While the use of technical terms is +inevitable in presenting such details, it will be found, I think, +that a reference to the figures will render them intelligible. + +Since this volume mainly deals with the phenomena of distribution, I +have included in my survey nearly all of the usually admitted genera +of worms, particularly of the terrestrial forms, which are in the +light of our present knowledge the more important in considering this +subject. + + F. E. B. + + ZOOLOGICAL SOCIETY OF LONDON. + _December_, 1911. + + + + +CONTENTS + + + CHAP. PAGE + + I. Structural and Systematic 1 + + II. Mode of Life 43 + + III. The external features of Earthworms and their + relation to habit and environment 59 + + IV. Sense Organs and Senses of Earthworms 64 + + V. Relative frequency of Earthworms in different + regions of the World 70 + + VI. Peregrine forms 96 + + VII . The Earthworms of Oceanic Islands 109 + + VIII. Movement and Migration among Earthworms 113 + + IX. The Geographical Distribution of Earthworms 129 + + List of Literature referring to Earthworms 144 + + Index 146 + + + + +CHAPTER I + +STRUCTURAL AND SYSTEMATIC + + +The group of segmented, bristle-bearing, worms, termed Oligochaeta by +zoologists, comprises what are popularly known as earthworms together +with certain forms, inhabitants of ponds, lakes and rivers, which are +not so well known as to have received a more distinctive name than +merely 'worms.' Their next allies are apparently the leeches and--a +little more remote--marine bristle-bearing worms termed Polychaeta; +the three groups, together with perhaps a certain number of other +forms belonging to smaller groups, constitute the Annelida which are +a distinct and separate assemblage of invertebrate animals. + +The most interesting features about these Oligochaetous worms +are their very great anatomical variation and the facts of their +distribution over the globe. Their importance as geological agents +in levelling the ground was made known a long time ago by Darwin, +and that aspect of earthworms has remained in much the same position +as Darwin left it. We shall concern ourselves here only with the +structure, habits, and range of the earthworms and their immediate +allies, the aquatic Oligochaeta. These three aspects of the animals +dovetail into each other more thoroughly than is the case with some +other groups. This is due to the fact that they have of late years +been very thoroughly studied from the anatomical and distributional +side. So lately as 1889, M. Vaillant in a very comprehensive treatise +was only able to enumerate 369 species, of which a large number were +but incompletely differentiated, and some are no longer admitted. +There are at the moment of writing perhaps 1500 species, the vast +majority of which are well known owing to careful investigation. +Furthermore there are but few parts of the world, and these are +not of large area, from which earthworms at any rate have not been +gathered. Though there can be no doubt that a very considerable +number of species await discovery, it would seem that we are in +possession of information which is not likely to be seriously +affected by future researches. + + +THE ANATOMY OF EARTHWORMS. + +Although it is not contemplated to make the present volume a guide +to the structure of this group of worms, it is necessary to give +some little anatomical sketch of the group in order first of all to +illustrate their diversity of structure, secondly to give reasons +for the classification of them, and thirdly to enable the reader to +realise certain structural details which it is absolutely necessary +to give some account of in order to explain other matters. + +It is for example impossible to attempt any account of the fitness of +some of these animals for their terrestrial life and of others for an +aquatic life without treating of anatomy to some extent. + +I shall take one particular species as a type and indicate later the +principal divergencies shown by other forms. According to the general +opinion among those who have studied the Oligochaeta I take as a +representative form a Megascolecid (this and the other families are +dealt with _seriatim_ on p. 14 et seq.), as this group is presumed to +be the oldest, and within that group a representative of the genus +_Notiodrilus_ which is with some reason held to be the most primitive +genus in the group. Finally I have no particular reason for selecting +the species _Notiodrilus tamajusi_ except that there happens to be a +longer and fuller description of it than of many. + + [Illustration: Fig. 1. _Notiodrilus tamajusi._ The worm shown + from the ventral surface. About natural size. (After Eisen.)] + +This earthworm is a native of Guatemala and is some six inches in +length with a diameter of perhaps a quarter of an inch. The front +part of the body is thicker than posteriorly. The body will be seen +to be divided into some 218 rings by circular furrows which run right +round the body. These divisions are termed segments or somites. At +the head the mouth is surrounded by the first of these, and on the +dorsal surface of that segment is a projection like an incomplete +segment which is known as the prostomium. From the XIIIth segment to +the middle of the XXth the body has a different appearance, and this +region is known as the clitellum. Each of the 218 segments of the +body except the first, and possibly one or two at the hinder end, is +furnished with eight minute projecting bristles, the setae; these are +disposed in pairs and all lie upon the ventral aspect of the worm. +The movement of these by special muscles aids in locomotion. + + [Illustration: Fig. 2. The same species; first two segments and + prostomium shown from ventral surface, I prostomium, II first + segment without any setae, III second segment with its four pairs + of setae (the dorsalmost seta of each outer pair is not visible + in this view).] + + [Illustration: Fig. 3. A section through the body of the same + species showing the ventral position of the pairs of setae. + (After Eisen.)] + +An examination with even a hand lens shows a number of external pores +which are important. Anteriorly there is the mouth which is overhung +by the prostomium referred to above. At the extreme hind end--and +surrounded by the last segment of the body--is the vent. Along the +middle line of the back are a series of pores, one just at the very +anterior edge of each segment, through which, when the worm is dried +and then slightly pressed, liquid is seen to be ejected. These are +called the dorsal pores and they belong one to each segment with the +exception of the first seven, or--in some cases--more, segments. +In front of one or other of the pair of setae which is situated +most laterally, _i.e._ furthest from the ventral median line, is an +orifice on each side in all but the first one or two segments of the +body. These paired pores are the external outlets of the excretory +organs frequently termed on account of their regular repetition +with the segments 'segmental organs,' but more conveniently to be +named nephridia. In the clitellar region and in fact on each of the +segments XVII, XVIII, XIX are a pair of pores of which those on the +XVIIIth segment are the least conspicuous. The large pair of pores on +each of segments XVII and XIX occupy the position of the ventralmost +pair of setae, which are here absent, or rather replaced by a very +long curved and ornamented seta, which projects out of the orifice. +These two pairs of pores are the outlets of the prostatic glands as +they have been termed. The minute pair of pores on segment XVIII do +not take up the position of the ventral setae; for these are present +and to the inside of each pore. A groove, shaped something like a +reversed 3 or the Greek letter [Greek: S], connects the orifices of +each side of the body, the middle part of the groove, where the two +semicircular halves of which it is composed meet, coinciding with the +minute pores on segment XVIII which are the orifices of the sperm +ducts. + + [Illustration: Fig. 4. The same species. Ventral view of segments + XVI-XX (numbered in the figure) which form the clitellum, the + posterior boundary of which is shown by a curved line on segment + XX. The figure will be understood from the annexed description.] + +On segment XIV are a pair of very minute pores a little in front of +the ventralmost setae and thus very near together. These are the +openings of the oviducts. Finally, near to the anterior border-line +of segments VIII and IX and on a line with the ventral pair of setae +is a pore on each side through which the cavity of the spermathecae +reaches the exterior. + + [Illustration: Fig. 5. A longitudinal section through the middle + of the first nineteen segments of the body of _Notiodrilus + vasliti_ (a species very closely allied to that described in the + text), the segments are numbered I, II, &c. _D.v._ dorsal blood + vessel, _G_ gizzard, _H_ hearts, _oe_ oesophagus lying in front + of gizzard behind which another tract of still narrow oesophagus + is seen opening into _Int._ intestine. The whole alimentary canal + is supported by the intersegmental septa (_Pr.c._) between which + is the system of spaces forming the coelom, _ov.d._ pore of + oviduct, _Splh._ orifices of spermathecae, [M] orifice of sperm + duct, _o_ ovary, _t_ spermaries. (After Eisen.)] + +So much then for the external characters of our worm. We next turn +to the internal anatomy. When the worm is opened by a longitudinal +section from end to end, and the two flaps of skin are turned +outwards and pinned down, the internal structure is almost completely +revealed. Running from end to end is seen the alimentary canal; +the general cavity of the body (coelom) in which it lies, as do of +course the other organs to be enumerated, is seen to be divided by +cross divisions, the intersegmental septa, into a series of chambers +which correspond with the external division into segments. The +septa are in fact inserted on to the body-wall along the furrows +which mark the divisions between adjacent segments. Anteriorly +the large pharynx is responsible for confusing the arrangement of +the septa, which become subdivided and fused or are prolonged a +greater way backwards and thus present a less obviously segmental +disposition. Certain of the more anteriorly placed of these septa +are much thicker than the rest. This is the case with the septa +which separate segments V to XII. The alimentary canal is perfectly +straight and runs in the middle line, being supported by the septa +which it perforates. The mouth leads into a buccal cavity which later +becomes the pharynx, a portion of the tube which is much thickened +by muscular walls dorsally. Then follows a very short section of +the oesophagus and in the fifth segment this becomes the gizzard, +a very characteristic organ with thick muscular walls quite smooth +and with a very thick lining of structureless membrane. After this +is a narrower tube, the rest of the oesophagus. Into this open in +each of segments VII, VIII, IX a pair of calciferous glands; these +are diverticula of the gut with much folded walls, the cells of +which secrete carbonate of lime. In the XIIth segment or so, the +oesophagus suddenly widens out to form the intestine which runs as +such to the end of the body. This wider tube has a ridge running +along its dorsal side, the typhlosole. Along the dorsal surface of +the intestine and the oesophagus is seen a red tube, contractile +during the life of the worm, which is the dorsal blood vessel and +whose contained blood is coloured red, as is the blood of vertebrated +animals, by haemoglobin. But in the earthworm the colouring matter +is not situated in corpuscles as in the vertebrate. The dorsal +vessel is connected by a few pairs of equally contractile transverse +trunks with a ventral vessel which is not contractile. There are +other branches of these main longitudinal trunks and some minor +longitudinal trunks which we shall not stop to describe further. +The nervous system of the worm consists of a pair of ganglia which +lie above the gut in the third segment; they are connected by a +commissure running round the gut with a chain of closely fused pairs +of ganglia, one for each segment to the very end of the body. In each +of the segments, except the first two or three, there are a pair of +excretory organs known as nephridia; these are essentially coiled +glandular tubes opening on to the exterior by the regularly placed +pores already referred to in considering the external characters. The +tube ends in a funnel-shaped, and therefore dilated, mouth, which +opens into the segment in front of that which contains the rest of +the organ; a nephridium therefore lies in two segments. The only +other important organs which are left for consideration are those +devoted to the reproduction of the species. The essential organs are +the spermaries and the ovaries. Of the former there are two pairs +of minute whitish bodies which lie in segments X and XI on either +side of the nerve cord attached to the anterior septal wall of their +segments. The ovaries are not in the following, but in the XIIIth, +segment, and occupy an identical position in that segment. A short +tube with a funnel or trumpet-shaped and wide orifice opens into the +cavity of the XIIIth segment opposite to each, and, perforating +the septum, opens on to the exterior on the XIVth segment. A similar +but larger and more folded pair of trumpet-shaped funnels opens in +the same way opposite to each spermary. But in this case the two +tubes of the sperm ducts run backwards for some way and those of +each side after joining open on to the XVIIIth segment by the pores +already mentioned. On the XVIIth and XIXth segments open two glands +which are long and tubular in form and much coiled. These are the +spermiducal glands and each opens in common with a muscular sac +containing the long and ornamented seta referred to in describing +the various external orifices. It will be noticed that the sperm +duct has no direct connection with these glands but only indirectly +through the external gutter which connects the three male orifices +of each side of the body. Segments IX-XII inclusive contain certain +sacs which depend from, and are formed as outgrowths of, the septa +of those segments. These are the sperm sacs in which the male germ +cells undergo their development. A corresponding body (but very much +smaller) is sometimes found in relation to the ovary but has not +been actually described in the particular species dealt with here. +Finally, in segments VIII and IX are a pair (that is four altogether) +of roundish sacs, with two or three minute diverticula, known as the +spermathecae. In the diverticula of these sacs are stored the sperm +derived from another individual. + + [Illustration: Fig. 6. Genital region of _Maoridrilus dissimilis_ + (in which species the different organs are essentially as in + _Notiodrilus tamajusi_). The oesophagus is cut away between the + Xth and XVIIIth segments to display the various organs. _n_ + external orifices of nephridia which alternate in different + segments; the one lettered opens in front of dorsalmost pair of + setae, that of segment XVI opens in front of ventralmost pair, + _p_ coiled glands opening on to XVIth and XIXth segments, _sc._ + sacs containing long seta, associated with these glands, _t_ + spermary of segment; another pair in an exactly similar position + in segment XI. Behind spermaries are funnel-shaped openings + of sperm ducts which are seen running along the body to their + external orifice on XVIIIth segment. _v.s._ sperm sacs behind the + posterior of these and in segment XIII is seen the large ovary + corresponding in position to the spermary and opposite to each + ovary the oviduct. (x 3.)] + + [Illustration: Fig. 7. A spermatheca of _Notiodrilus tamajusi_. + The external orifice is shown at _o_; above this are the + diverticula, _s_ is a portion of an intersegmental septum. (After + Eisen.)] + +This completes the general sketch of the structure of _Notiodrilus +tamajusi_ which we have selected as a type. In this same genus are a +large number of species which differ from that selected in various +small structural points. Thus in _N. annectens_ (Beddard), a species +from New Zealand, the spermaries and ovaries are attached to the +posterior, instead of to the anterior, wall of their segments, and +there are neither calciferous glands nor modified setae upon segments +XVII and XIX. In all essentials however the two types agree and are +thus to be looked upon as referable to the same genus. Starting from +the structure of these types we may now sketch in quite a brief way +the main divergencies of structure shown in the group of Oligochaeta. + +We shall naturally begin with the family Megascolecidae of which a +type has just been described. + +Within the limits of the same sub-family as that which contains +_Notiodrilus_, _i.e._ the Acanthodrilinae, the changes of structure +affect all the principal organs of the body except the nervous +system, but are not very large and vary from genus to genus. They +are mainly perhaps in the direction of reduction and simplification. +Thus in _Chilota_, _Maheina_ and _Yagansia_ the spermaries are +reduced to one pair in either the Xth or XIth segment, while in +_Yagansia_ one pair of spermathecae and of spermiducal glands have +also disappeared. In _Microscolex_ the spermaries remain normal, but +one pair of spermathecae and of spermiducal glands have disappeared, +the remaining organs of these series being in the IXth and XVIIth +segments respectively. In _Microscolex_, _Chilota_ and _Yagansia_, +moreover, there is a further degeneration in the disappearance of the +calciferous glands. These glands are often absent and sometimes less +developed in the New Zealand _Maoridrilus_, which is otherwise not a +degenerate form and differs characteristically from _Notiodrilus_ +by the fact that the paired nephridia alternate in position in +successive segments, being now in front of the dorsal, and in other +segments in front of the ventral, pairs of setae. While these genera +are somewhat degenerate, the New Zealand _Plagiochaeta_ has undergone +specialisation in an upward direction. For the setae of each segment +are increased to a large number much exceeding eight. + +It is not a long step to the sub-families Diplocardiinae and +Trigastrinae. In the first of these, an American race confined to the +northern and central parts of that continent, the male pore shows +a tendency to move backwards, being situated on any of segments +XVIII-XXI. The two spermiducal glands follow it, but are always +placed one pair in front and one behind, as in _Notiodrilus_. In this +group we get a new feature of specialisation in the duplication or +triplication of the gizzard. + +So too with the Trigastrinae where there are either two or three +gizzards; but in this sub-family another modification has become +apparent. The paired nephridia have disappeared and their place +is taken by several, often quite numerous, pairs of much smaller +nephridia called on that account 'micronephridia' instead of +'meganephridia.' To this sub-family belong the especially African but +also American and Malayan _Dichogaster_, whose name is derived from +the important fact that it possesses two gizzards. + +Not far off is to be placed another sub-family, that of the +Octochaetinae, which is New Zealand and Indian in range, the +intermediate countries being, strange to say, not populated by this +race of Oligochaeta. The group contains several genera of which +_Octochaetus_, _Eutyphoeus_, and _Dinodrilus_ are the best known. +All these worms agree in the main features of their anatomy with +_Notiodrilus_; but they have diverged in different directions. Thus +in _Octochaetus_ the typical two pairs of gonads and glands belonging +to the generative system have been retained, while the nephridial +system consists of micronephridia; in _Eutyphoeus_, one pair of +spermiducal glands has disappeared, and as a general rule the species +of this genus have only one pair of spermaries and the corresponding +pair of sperm ducts. They are close to _Octochaetus_. The third +genus mentioned, _Dinodrilus_, is a New Zealand form specialised in +possessing 12 setae in each segment. Otherwise it is not far removed +from _Octochaetus_. + + [Illustration: Fig. 8. Ventral view of _Eutyphoeus masoni_. _p_ + papillae, [M] male pores, [F] oviduct pores. (x 3.)] + +A fifth sub-family is also easily referable to the type whose +structure has been dealt with as a preliminary to the present survey. +That sub-family is the Ocnerodrilinae which is American and African +in range. These worms are somewhat degenerate in comparison with +their allies. Thus the calciferous glands are reduced to a single +pair or to a single gland in the IXth segment, the nephridia though +regular and paired have no covering plexus of blood vessels, and +the worms themselves are slender and delicate, being indeed often +aquatic in habit. The spermiducal glands, which are as in the former +sub-families independent of the sperm ducts though sometimes opening +in common with them into a short pocket-like ingrowth of the skin, +are reduced in their minute structure and much simpler than in the +other types. + +The genus _Kerria_ is the least reduced perhaps. It has the male +pores on segment XVIII with a pair of spermiducal glands on the +segments preceding and following this in the typical Acanthodriline +fashion. There are two pairs of spermathecae in VIII and IX, +but the spermaries are reduced to one pair in X. The gizzard is +present. OCNERODRILUS is a little further reduced from this last. +The gizzard has gone; there is but one pair of spermiducal glands +(as a rule) opening in common with the extremity of the male duct +on to segment XVII; the spermathecae also are reduced to one pair, +but there are two pairs of spermaries. The African _Nannodrilus_ is +more robustly built. There are two gizzards, the male duct opens +into a muscular pouch, into which also open one of the two or three +pairs of spermiducal glands. There are two or three other genera +and sub-genera not showing any great divergencies from the range of +structure indicated in briefly defining those enumerated above. + +Finally, we have the largest of all the sub-families of this family, +viz. that of the Megascolecinae. These worms are mainly tropical in +range and also mainly found in the Indo-Australian portion of the +world. In them the sperm ducts open in common with the usually single +pair of spermiducal glands and prevalently upon the XVIIIth segment. +The glands moreover have not always, though they often have, the +tubular form shown in all the sub-families hitherto considered. In +many forms they are branched and lobate glands, and if there are two +pairs one may be of one type and the other of the second and derived +type, as for instance in _Megascolex ceylonicus_. Furthermore, it is +much commoner among the genera of this sub-family for the setae to +become numerous and to spread right round the segment; this condition +is seen in the genera _Pheretima_, _Megascolex_, _Diporochaeta_, +_Perionyx_, _Plionogaster_. The spermathecae also are commonly more +than the typical two pairs of the forms already considered, and in +certain species (for example _Pheretima hexatheca_) there are as many +as six pairs of those organs which are moreover--and in this they +resemble the majority of species of the last sub-families--nearly +always furnished with a diverticulum or diverticula. The nephridia +are either paired or numerous and these various characters allow of +the sub-family being split up into sixteen genera or thereabouts. + +As an example of another type of organisation and as contrasting with +_Notiodrilus_ we may now briefly describe the structure of the genus +_Pontoscolex_ (better known as _Urochaeta_), a member of the American +and African family Geoscolecidae. + +The worm is some four inches long and composed of as many as 212 +segments. Each of these except the first has eight setae which for +the first few segments of their occurrence are disposed in four pairs +in the usual fashion. Behind this point however the setae cease to +present this symmetrical arrangement and are irregularly disposed +so that a given seta is not in the same line with the corresponding +seta of the segments in front or behind. There is thus no region of +the body which has not a seta implanted upon it; and the effect is +therefore comparable to the condition obtaining in those worms, such +as _Pheretima_, where circles of numerous setae are met with. There +are however only eight in a given segment. The clitellum extends +from segment XV to XXII or XXIII and is developed only dorsally and +laterally. It has setae like the rest of the body; but those upon +the clitellum are longer and more markedly ornamented than are those +of the body generally. The latter are not only sculptured with +fine ridges but are bifid at their free extremity. The prostomium +is often apparently completely absent. It is however really present +but is retractile. As to the pores which are visible externally the +dorsal pores are completely absent. The pores of the nephridia lie in +front of the dorsal pair of setae or in a line corresponding to the +position of those setae where the arrangement has become irregular. +The spermathecal pores are three pairs and are in the very front +of segments VII, VIII, IX. The male pores, very inconspicuous, lie +on the ventral side of segment XXI just within the clitellum. The +oviducal pores are on segment XIV. + +As to internal anatomy the general plan of the segmentation shows +no great differences. Certain septa only show a difference, _i.e._ +those separating segments VI-XI which are specially thickened. In the +alimentary canal a gizzard in segment VI is to be noted and three +pairs of calciferous glands in segments VII-IX. The nephridia are +paired structures and commence early. The first two or three segments +are occupied by a pair of large glands opening into the buccal +cavity which are apparently a slightly modified pair of nephridia +and are generally termed 'peptonephridia' since they are associated, +as it would appear, with the function of alimentation and are not +purely excretory organs. There is but a single pair of spermaries in +segment XI, and of ovaries in segment XIII. The sperm ducts open on +to the exterior in the position already mentioned and they are not +associated at their pore with any glands comparable to spermiducal +glands. A pair of sperm sacs depend from segment XI and traverse a +considerable number of segments, being thus long and tongue-shaped +instead of short and limited to one segment. The spermathecae are +three pairs of elongated sacs in segments VII-IX, without any +diverticula at all. + +It will appear therefore that many and considerable differences +divide _Pontoscolex_ from _Notiodrilus_ and indeed from all of the +Megascolecidae whose structure has been touched upon in the foregoing +pages. The most important of these are the ornamented setae and their +arrangement and the modification of the setae upon the clitellum: +the absence of diverticula to the spermathecae: the absence of +terminal glands associated with the male ducts. Although taken in +their entirety these characters are distinctive of the American +Geoscolecidae (sub-family Geoscolecinae), there is no one of them +which is not to be found in some Megascolecid. Thus the subgenus +_Ilyogenia_ (of _Ocnerodrilus_) has sometimes no spermiducal glands: +the genus _Perionyx_ has spermathecae without diverticula in some +species, and other genera of Megascolecinae are in a like condition. +The setae of _Dichogaster_ are sometimes ornamented, while in +_Pheretima houlleti_ the clitellar setae are different from those +upon the other segments. + +We can in fact only define the family Geoscolecidae by an assemblage +of characters which are mainly these: dorsal pores absent, only a +few in the neck region being occasionally present; setae generally +ornamented, those of the clitellum being larger and more marked than +the others; spermathecae without diverticula; often instead of a +pair of those pouches in the segment a large number of very small +sacs, as in _Microchaeta_, _Kynotus_. Sperm ducts without terminal +glandular or muscular sac, except in a few cases; setae always eight +in a segment except in the genus _Periscolex_ which has acquired +the 'perichaetous' condition. The range of variation shown in the +anatomy of the Geoscolecidae will be best taken in connection with +the several sub-families into which it has been subdivided. In the +first of these, the Geoscolecinae, no great differences divide the +genera from that selected as the type, viz. _Pontoscolex_. The long +sperm sacs attain to an extraordinary length in _Trichochaeta_ (or +_Hesperoscolex_) where the single pair extends through no less than +109 segments. Though as a general rule the sperm ducts open directly +on to the exterior they do so through the intermediary of a large +pouch in _Glossoscolex_ (= _Titanus_). In _Onychochaeta_ the setae on +the last segments of the body are very much enlarged and thus form a +more effective means of holding on to the soil than is possessed by +other species. + +The sub-family Hormogastrinae which contains but a single genus +_Hormogaster_ is remarkable for being limited in range to the +Mediterranean coasts. The genus is mainly distinguished by +possessing three gizzards; otherwise it is not very different +from the sub-family just described. The African and Madagascar +forms are associated (together with a few Asiatic forms) into a +third sub-family Microchaetinae. These worms frequently possess a +considerable number of very small spermathecae in segments XII, XIII +or thereabouts instead of the usual paired arrangement. They have too +very often glands connected with the enlarged setae already mentioned +which are however (in the genus _Kynotus_ at any rate) usually in +front of the clitellum. The latter organ, contrary to what we find +among the Geoscolecinae, is often behind the point of orifice of the +male pores. This is so with _Kynotus_. + + [Illustration: Fig. 9. Ventral view of _Polytoreutus + Kilindinensis_, head end (x 2). _p_ papillae, [M] male pore, [F] + female pore.] + +The last sub-family, Criodrilinae, has but three genera _Criodrilus_, +_Sparganophilus_ and _Alma_. These worms do not show any very marked +differences from other Geoscolecids. _Alma_ is noteworthy for the +facts that the male pores are borne upon long processes of the body +which bear specially modified setae and that one species at any rate +has gills. + + [Illustration: Fig. 10. Ventral view of _Polytoreutus finni_, + head end (x 2), lettering as in fig. 9.] + +Another type of structure is offered by the Eudrilid earthworms +which form rather a restricted family. These worms are as a rule +quite easy to distinguish by their external characters only. For the +apertures of the spermathecae and sperm ducts are apt to be very +large and conspicuous. They are also generally unpaired, a character +which is however not confined to the Eudrilidae; for there are +Megascolecids, such as _Fletcherodrilus_, and Geoscolecids in which +the same unpaired character occurs. The principal feature of the +family is that the ovaries are commonly enclosed in sacs--comparable +to the sperm sacs which frequently envelop the spermaries in +other earthworms--and that these sacs not only contain the mouths +of the oviducts but are directly continuous with the single or +double spermatheca. This is usually a large sac, always single or +consisting of one pair only, which opens on to the exterior close +to the oviducal pores; these spermathecae in the Eudrilidae are not +comparable to the spermathecae of other earthworms; for they are in +a way comparable to the sperm sacs, being formed as outgrowths of +the septa. There is some variation of structure within the family. +In a number which are associated into a sub-family Eudrilacea there +are two paired calciferous glands and a single unpaired one, while +the paired nephridia open by a large pore on to the exterior. In a +parallel sub-family, the Pareudrilacea, the calciferous glands are +apt to be more numerous and have a totally different structure: they +have been apparently converted into non-digestive glands bearing some +relation to the vascular system. The nephridia moreover do not open +on to the exterior by single pores, but form a network within the +thickness of the body wall and then open by numerous pores. There is +however no resemblance here to the micronephridia of _Dichogaster_ +and other Megascolecids. In _Libyodrilus_ (as an example of the +Pareudrilacea) each nephridium forms a network out of the duct +leading to the exterior. In the interior of the body a series of +paired meganephridia are visible. + + [Illustration: Fig. 11. Organs of reproduction of _Eudriloides + durbanensis_.] + +The earthworms of Europe belonging to the family Lumbricidae +offer again a rather different type of structure, which is more +reminiscent of the Geoscolecidae than of the Megascolecidae or +Eudrilidae. In this family there are no glands appended to, or in +the neighbourhood of, the orifices of the sperm ducts, such as are +found in the other forms. As in the Geoscolecidae the clitellum is +furnished with setae somewhat different in form from those which +deck the body generally. These setae are never more than eight in a +segment. Dorsal pores (absent in Geoscolecidae and in Eudrilidae) +are invariably present. The spermathecae are without appendices and +nearly always simply paired, though rarely we get numerous much +smaller spermathecae in a single segment, as in _Kynotus_ among the +Microchaetine Geoscolecids. Internally the most striking feature of +this family is to be seen in the position of the gizzard at the end +of the oesophagus and at the beginning of intestine. The apertures +of the male pores are--save for two or three exceptions where they +are further forward--invariably upon the fifteenth segment, and +the clitellum, often very long, usually begins behind this point, +features which are also seen in _Kynotus_. + +Finally we have the Moniligastridae which differ from all the +types hitherto considered in a few rather important particulars. +These worms are named on account of the fact that they possess +several gizzards upon the oesophagus, a character which is however +met with in the Megascolecid genus _Plionogaster_ and in certain +Eudrilids, _e.g._ _Hyperiodrilus_. The main peculiarity of the +family is that the sperm ducts are very short and open on to the +next segment to that which contains the spermaries, as in the +water-living Oligochaeta generally. The terminal sac into which the +male ducts open is moreover rather like that of such a family as the +Lumbriculidae. + + +THE AQUATIC FAMILIES OF OLIGOCHAETA. + +It would seem to be quite possible that when the fresh waters of the +world have been as well examined for Oligochaeta as have so many +parts of the land areas, the number of purely aquatic Oligochaeta +will be found to equal those inhabiting the land. In any case we +are quite justified at the present moment in stating that there are +rather more families of these smaller Oligochaeta than there are of +the bulkier terrestrial forms. But while there are certainly seven or +eight distinct families, these do not between them contain at present +so many genera as do the fewer families of earthworms; and the number +of species of the latter that are known to science enormously exceeds +that of the 'Limicolae' as the fresh-water worms were at one time +called in common. The fact that there are purely marine forms of +these water worms was hardly appreciated at the time that the term +Limicolae was used; now however we are acquainted with a few such +forms, and even with some which live at will in either fresh, salt, +or brackish water. Of these something will be said later. + +These forms have also been collectively treated of as Microdrili, a +term which expresses the undoubted fact that they are all of small +size and sometimes even minute; others however reach the dimensions +of the smaller species of earthworms. There are a certain number of +characters shared by the various families which may be considered +first of all, before dividing them into their several subdivisions. +These aquatic Oligochaetes are usually tender and transparent, the +muscular layers of the body wall being much reduced as compared with +the tougher terrestrial forms. The clitellum is also thinner and +consists of a single layer of cells only, thus contrasting with the +double layered clitellum of earthworms. As a rule the alimentary +tract is simplified, there being no gizzard or glandular appendices +of the oesophagus comparable to the calciferous glands of most +earthworms. But this rule is not without exceptions; for we find in +_Haplotaxis_ a gizzard occasionally developed, and in the remarkable +genus _Agriodrilus_ from the Baikal lake a continuous gizzard +formation along the oesophagus, while the Enchytraeidae may show +something very like calciferous glands: and even a Tubificid, called +by Pierantoni _Limnodriloides_, has a pair of diverticula of the gut. + +Other internal organs show certain points of likeness in all or in +the great majority of the aquatic families. Thus the nephridia are +without a plexus of blood capillaries surrounding them, a state of +affairs which also occurs in some of the slender Ocnerodrilinae +among the earthworms. These paired organs also are very frequently +not found in the anterior segments of the body and these include +also as a general rule the segments in which the reproductive +elements are formed. Save for an exceptional case among the genera +of Enchytraeidae the dorsal pores are not found among the Limicolae, +but in some cases at least a single pore, the head pore, is found. +The sperm ducts, which among earthworms usually (and indeed always +save in the anomalous Moniligastridae) traverse a considerable +number of segments on their way from the internal opening into the +body-cavity to the external pore, do not show the same phenomenon +among the Limicolous Oligochaeta. They are sometimes indeed limited +to a single segment, that is to say the funnels and the external +pore lie in one segment. In other cases they open on to the exterior +in the segment next to that which bears the funnel, and it is only +rarely that they traverse more than one segment. There are also +points of difference of general applicability to be noted in the +sperm sacs and egg sacs. The latter are large and extensive, which +is not the case among earthworms, and the former are as a rule more +extensive in the number of segments that they occupy than among the +terrestrial forms. Another difference which they show is that their +cavity is quite simple and not divided up by trabeculae into numerous +intercommunicating chambers as in the earthworms. Finally the eggs of +the aquatic Oligochaeta are large and full of yolk and thus contrast +with the very small ova of the earthworms which are moreover much +more abundant. These features are either of general or universal +occurrence and together form an assemblage of characters which mark +out the aquatic families of Oligochaeta from their, at least mainly, +terrestrial allies. + +We may also refer to certain structures which although not universal +among these aquatic families are nevertheless found only in +them--that is, are not found in any family of the terrestrial worms +of this order. The most salient of such characters are the long and +hair-shaped setae tapering to a fine point and often provided with +a series of delicate branches like a feather; such setae are often +of very great length and they occur in their various modifications +among the Aeolosomatidae, Naididae, and Tubificidae. It is clear +that these delicate setae, though they may not be due in any way to +the aquatic life, are rendered possible by it. To drag such tender +processes through stiff clay would surely break and tear them out. +It may be also mentioned that among the aquatic families as a rule +the intersegmental septa do not show that thickening in some of the +anterior segments of the body which is so general a feature of the +land-dwelling species. Finally it is only among the aquatic forms, +and among them only in the families Aeolosomatidae and Naididae, that +asexual reproduction by budding takes place. Indeed so common and +usual is it in the genera of these families that even yet there are +considerable lacunae in our knowledge of the organs of reproduction +in the said families. + +Together with these general similarities are many points of +structural difference among the worms inhabiting ponds, lakes, and +rivers, which allow of their being divided into a number of quite +distinct families. + +One of the most distinct of these families and lying in any case +quite at the base of the series is the family Aeolosomatidae which +includes a number of distinct species of delicate and transparent +worms, and in whose integument are embedded round cells bearing a +large brightly coloured oil drop; this is reddish or green in colour, +or--and this more rarely--colourless, but still recognisable as an +oil drop. The green sometimes even verges upon blue on the one side +and yellow on the other, while the red may approach brown or purple. +These worms are assigned for the most part to the genus _Aeolosoma_ +which is found in all of the great continents and of which seven +or eight species are known. To a more doubtful genus _Pleurophleps_ +are assigned a few small worms which have the general appearance of +_Aeolosoma_, but are without the coloured or colourless oil drops +in the skin. These worms have a very large prostomium which is +ciliated on the lower surface, and the body is not markedly segmented +externally by constrictions or internally by septa. The bristles are +slender and hair-like, but among them are in some species the shorter +stouter bristles bifid at the free tip, which are so general among +the aquatic families of the Oligochaeta. These worms are not uncommon +objects in pools containing weed; and they are to be found usually +crawling among the weed. They consist as a rule of but few segments +to most of which a pair of nephridia belong. The ovaries and the +spermaries are only known in a few forms and appear to be unpaired +and lie respectively in the fifth and sixth segments. There are 1-3 +pairs of spermathecae, and the sperm ducts if distinct from, are at +least very like, nephridia. The ova appear to make their way to the +exterior by a large aperture in the ventral middle line of a middle +segment of the body. The vascular system contains uncoloured blood +and is greatly simplified. + +The next family to the Aeolosomatidae in zoological position is +clearly the Naididae. These are also small worms, but show in some +respects a higher grade of organisation than their allies. While +asexual generation is general, the reproductive organs are more +commonly found than in _Aeolosoma_, though there are still many +hiatus in our knowledge of the same in certain genera. Where they are +known it has been found that the spermaries and ovaries are very far +forward in the body, in the fifth and sixth segments respectively. +The spermathecae are in segment six and the male ducts open into a +terminal chamber, called 'atrium,' which is on the whole not unlike +that of the Tubificidae. The blood in these worms is red as in the +higher types, and thus differs from that of the genus _Aeolosoma_. +The setae are rather varied, being in some cases long and slender, +sometimes greatly exaggerated in length as in _Ripistes_; other setae +are forked at the free end, and in _Paranais_ this is the only kind +of setae met with. A marked feature of this family is that the dorsal +bundles of setae do not always begin like the ventral setae upon the +second segment of the body. Indeed in _Schmardaella_ there are no +bundles of dorsal setae at all. The Indian genus _Branchiodrilus_ +is remarkable for the fact that it has paired processes of the body +which may be termed gills and which in some segments involve the +dorsal setae. Another kind of gill is found in the genus _Dero_ +(which has many species) and in the allied _Aulophorus_. These +are placed round the vent, and are ciliated. Other genera are +_Nais_, _Chaetogaster_, _Vejdovskyella_, _Amphichaeta_, _Stylaria_, +_Macrochaetina_, _Pristina_, _Naidium_. + +Several genera, _Pristina_, _Nais_, _Dero_, are found in many parts +of the world; but it is not possible at present to consider very +seriously the facts of their geographical distribution. + +Next to the Naids a group of aquatic worms present themselves for +consideration which are usually placed in three distinct families, +which families are however rather hard to define. These three +families are the _Tubificidae_, _Phreodrilidae_, and _Lumbriculidae_. +The Phreodrilidae were at one time placed with the Tubificidae by +Michaelsen. It is now perhaps the general opinion that they form a +family of their own, at any rate since the discovery of two other +genera _Phreodriloides_ and _Astacopsidrilus_, besides the original +genus founded by myself, and named _Phreodrilus_ from the fact that +the species was found in a deep well in New Zealand. + +The distribution of this family especially of the genus _Phreodrilus_ +is extremely interesting. The genus _Phreodrilus_ is the only one +genus of the aquatic Oligochaeta which has, like _Notiodrilus_, a +circumpolar range, the pole being the south pole. It occurs in New +Zealand, in Kerguelen, and, if we are to accept the opinions of Drs +Michaelsen and Benham that my genus _Hesperodrilus_ is to be merged +in _Phreodrilus_, in Patagonia also. + +In this genus the male pores are upon the XIIth segment while the +spermaries lie in segment XI. Moreover the spermathecae lie behind +the male pores. In the Tubificidae on the other hand it is at least +the rule for the spermaries and male pores to be pushed a segment +further forwards. And the spermathecae are before the male pores. +_Phreodriloides_ is like _Phreodrilus_ but has no spermathecae. +It is also New Zealand in range. _Astacopsidrilus_ is Australian +and is semi-parasitic upon the Crayfish _Astacopsis_, whence its +generic name. _Phreodrilus branchiatus_ is one of the few forms of +Oligochaeta that possesses gills. Of these there are a series of +pairs on about the last eleven segments of the body. They are lateral +in position, and thus contrast with the also gilled _Branchiura +sowerbii_, where the gills, also on the posterior segments of the +body, are more numerous and lie dorsally and ventrally, a pair to +each segment. + +The Tubificidae differ from the Phreodrilidae mainly in the points +already noted. There are a considerable number of genera of which +the following are the best known, viz., _Tubifex_, _Limnodrilus_, +_Limnodriloides_, _Branchiura_, _Lophochaeta_, _Ilyodrilus_, +_Psammoryctes_, _Clitellio_, _Telmatodrilus_, _Bothrioneuron_, +_Lycodrilus_. + +The Tubificidae are mainly northern temperate forms, and a few of +them such as _Clitellio arenarius_ and '_Peloryctes inquilina_[1]' +are found on the sea coast. There are also a few of this family in +the southern hemisphere. These forms include _Clitellio abjornseni_ +from Australia, and a few species of _Branchiura_ from New Zealand +and the islands of the Antarctic ocean. There is also to be mentioned +_Rhizodrilus_ (or _Vermiculus_) _aucklandicus_ from the island of +that name in the New Zealand area. The only tropical species appears +to be the Indian and Malayan _Bothrioneuron iris_, though this genus +also occurs in Europe and in southern South America. It is quite +likely however that _Branchiura sowerbii_, a species known at present +from tanks in hot houses, may be a tropical American species. + + [1] With many synonyms, including _Tubifex ater_ (see p. 53). + +The family Lumbriculidae is yet more restricted in its range. It has +not yet been met with away from the temperate northern hemisphere, +and the great variety of species recently described from Lake Baikal +by Dr Michaelsen is a very remarkable fact. The Lumbriculidae are +entirely fresh water in habit and not even partially terrestrial. +The following are the principal known genera: _Lumbriculus_, +_Trichodrilus_, _Rhynchelmis_, _Phreatothrix_, _Claparedilla_, +_Stylodrilus_, _Mesoporodrilus_, _Sutroa_, _Eclipidrilus_, +_Aurantina_, _Athecospermia_, _Lamprodrilus_, _Teleutoscolex_. + +In the worms of this family the male pores are usually upon the tenth +segment but sometimes upon the eighth or eleventh. And in addition to +a pair of funnels in the antecedent segment there is also commonly a +second pair in the same segment as that which contains the external +pore. The two sperm ducts however open into the same distended atrial +cavity before opening on to the exterior. In _Lamprodrilus_ however +each sperm duct opens by its own separate atrium on to the exterior +and in two consecutive segments. In _Teleutoscolex_ there is but one +pair of funnels opening into the same segment with the atrial pore. + +Near perhaps to the Lumbriculidae comes a very small family indeed, +that of the Alluroididae. So small is it that it consists of but +a single genus _Alluroides_ of which there are two species _A. +pordagei_ and _A. tanganyikae_. Both of these species were described +by myself, one of them from but a single specimen, the other from +only two. Both species--and the name of one denotes the region--are +from eastern tropical Africa. The Tanganyika worm is purely aquatic; +the other species was found in a swamp of the mainland opposite to +Mombasa. This remarkable genus has a pair of spermaries in segment +X; but the ovaries are as in earthworms in the XIIIth segment. The +male pores are upon that segment also, _i.e._ further back than +in the aquatic worms. The spermathecae open close to the median +dorsal line of the body in one species; in _A. tanganyikae_ there is +but one spermatheca which opens actually in the dorsal median line +between segments VIII and IX. This family seems to come nearest to +the Lumbriculidae but has also obvious points of likeness to the +terrestrial Moniligastridae. It fully deserves a separate family, +which was founded for it by Dr Michaelsen. + +Not obviously related to any of the other families of Oligochaeta is +the family Enchytraeidae. This consists of a very large number of +species, which are placed in about a dozen genera, and whose habitat +is nearly confined to the cold and temperate regions of the world. A +large number of species for example have been described by Dr Eisen +from Alaska, while others inhabit the verge of Patagonia. It is only +a few which are found in warmer countries. There is for instance a +solitary Enchytraeid in India and the neighbouring parts of Asia +described by myself as _Henlea lefroyi_ but doubtfully of that genus +according to Dr Michaelsen. I have also myself obtained another +Enchytraeid from British Guiana. Apart from such rare exceptions the +family is arctic or temperate in its range and is even met with upon +the ice of glaciers. These little worms--they are very rarely of more +than minute size--are both aquatic and terrestrial and inhabitants of +the sea shore. They have four bundles of short often straight and +somewhat stumpy setae; _Anachaeta_ is entirely without setae. That +they bear some relation to the lowest group of Oligochaeta, that of +the Aeolosomas and Naids, appears to be shown by the very anterior +position of the spermathecae, _i.e._ in the fourth or fifth segment. +The spermaries and ovaries on the other hand are in segments XI and +XII. They are peculiar among the aquatic families in having complex +glands appended to the oesophagus which recall the calciferous glands +of the earthworms. The funnel of the sperm duct is extraordinarily +deep and lined with glandular cells except in an apparently primitive +genus from Lake Baikal. + +The remaining family of the Limicolae is that of the _Haplotaxidae_ +which contains two genera, viz., Haplotaxis, better known as +_Phreoryctes_, and _Pelodrilus_. These two genera overlap somewhat +in their characters and it is in the present state of our knowledge +rather difficult, if indeed possible, to differentiate them +thoroughly. They are slender worms which frequent either damp +earth or water and are thus somewhat intermediate in habit between +the Limicolae and the Terricolae. The chief peculiarity of their +structure lies in the fact that the sperm ducts are unprovided with +any kind of terminal apparatus whatever, but open directly upon +the exterior. Moreover there are generally two pairs of testes in +segments X and XI, and in some species two pairs of ovaries in the +two following segments. The small family is very widely distributed +in more temperate regions, principally of the antarctic hemisphere. +It occurs for instance in New Zealand, South Australia, the Cape, and +in the northern hemisphere in Europe, Western Asia, and North America. + + + + +CHAPTER II + +MODE OF LIFE + + +We have now completed a brief survey of the general characters of +the group of the Oligochaeta which will at least serve to impress +upon the reader the fact that these animals are somewhat diverse in +structure, and that even as regards outward appearance it is not +difficult to distinguish a large number of different types. These +facts become all the more remarkable when we reflect upon the very +similar conditions which surround all the species of earthworms. +It is not clear how far the influence of the soil differs in a +tropical forest in South America and in Central Africa. With +divergent external conditions anatomical differentiation becomes more +accountable. But in this case we have a lavish anatomical variation +which would appear to have no connection with any kind of need that +we can as yet appreciate. Comparing the Terricolous Oligochaeta with +other large groups of the animal kingdom, all or nearly all the +members of which lead a closely similar life, such as the frogs and +toads, or the land mollusca, or snakes, we get a much wider range of +structural change in the Oligochaeta than in any of these. + +We shall now consider their mode of life and their relations to the +environment. + +The mode of life of earthworms seems at first sight to need no +special chapter or section. They simply live in and beneath the soil, +leaving it at times to wander over the surface especially at night +and during wet weather. But there are a number of species which +habitually lead an aquatic life and whose characteristics therefore +demand consideration. + + +AQUATIC EARTHWORMS. + +Although it is perhaps somewhat of a contradiction to speak of +aquatic earthworms the collocation of words will serve to emphasise +the fact that there are species of Oligochaeta belonging to the tribe +Megadrili or terrestrial group, which are as purely aquatic in their +habits as is a _Tubifex_ or _Limnodrilus_. In such cases we may +fairly assume rather a return to an aquatic life than the persistence +of such a habit. For we do not find among these genera and species +much evidence of particular resemblance with the purely aquatic +familes of Oligochaeta. It is therefore particularly interesting to +examine into the characteristics of these water-living genera; for +we may expect to be able to deduce from them some hints as to what +characters are really to be associated with the purely aquatic life. +We can in fact hope to differentiate between adaptive and fundamental +characters in these animals. + +These secondarily aquatic species can be referred to two categories. +There are examples of particular species which differ from their +congeners in being aquatic; and there are whole genera, even +sub-families, which are exclusively, or very nearly so, aquatic +in habit. The former division need not detain us; for the actual +occurrence of the worms in fresh water instead of upon dry land +may be a temporary affair and not a mark of habitual sojourn. Thus +I myself found the British and European earthworm _Eiseniella_ +(_Allurus_) _tetraedrus_ in the River Plym in Devonshire, while it +has been generally met with upon land. The Patagonian and Falkland +Island species _Notiodrilus aquarum dulcium_ was so called on +account of its having been collected in fresh water. But its near +ally _N. georgianus_ (which is perhaps even identical with it) was +found on the sea shore in the same region of the world. While the +differences which the small species of _Notiodrilus_ shows from other +purely terrestrial members of the same genus are trifling, further +information may prove that this case is on all fours with that of +_Eiseniella_ referred to above. There are plenty of similar instances +which I shall not enumerate. + +We may now therefore pass on to the second category. These examples +are obviously much more important because they are of worms which +appear to be wholly aquatic, or very nearly so, and which belong to +definite genera easily distinguishable as such from their allies. The +examples are not however very numerous. And they belong practically +exclusively to the family Geoscolecidae, a family which, it will be +seen later, is confined to South America, South Africa, Madagascar, +certain parts of India and Burmah and of Europe. It is not a family +which has reached the greater part of the East or which has been +carried to the Antarctic parts of the globe. It is furthermore very +important to bear in mind that there are reasons for regarding this +family Geoscolecidae as one of the more modern branches of the +Oligochaeta; this latter statement tends to prove that the aquatic +life is, as already suggested, a secondary matter in these worms, and +is not due to their belonging to an ancient race which has never left +the waters of the land. + +A very interesting fact offers itself first of all in considering +this family of earthworms. The Geoscolecidae are one of the only +division[2] of the Oligochaeta terricolae which are generally found +to be without those characteristic series of pores in the middle line +of the back known as the dorsal pores. They are thus eminently suited +for an aquatic life; for it is to be supposed from the fact that the +purely aquatic 'Limicolae' are also without these pores that their +existence is prejudicial to a water-living worm. Indeed the entrance +of water into the body-cavity would presumably be dangerous to the +worm. The Geoscolecidae are thus already marked out, as it were, for +an aquatic life. No modification is here necessary for them. It is +also to be noted in this connection that various species of the genus +_Notiodrilus_ to which reference has been made as a partly aquatic +genus have no dorsal pores. They too are thus fitted for at least an +amphibious life. + + [2] In the Eudrilidae also these pores are very frequently absent. + +The rule however regarding the absence of dorsal pores in the +Geoscolecidae is not absolute. A few species and among them two +species at any rate of the aquatic genus _Sparganophilus_ have a few +pores between some of the anterior segments which have been spoken of +as 'neck pores.' They are not, it is to be believed, of a different +nature from the generally distributed dorsal pores of other worms but +are in fact limited to the 'neck' region. + +There are no other obvious characters of the family Geoscolecidae as +a whole which might be regarded as fitting them for a purely aquatic +life. + +Of this family one entire sub-family, the Criodrilinae, is very +nearly purely aquatic in habit. Two genera, viz. _Callidrilus_ and +_Glyphidrilus_, out of another sub-family, Microchaetinae, which +contains perhaps five other genera, are also aquatic in their mode +of life. In examining into the characters of the various aquatic +species with a view to searching for common characters which might +be put down to modifications induced by the aquatic life, there +are two or three which arrest attention. In the first place the +Criodrilinae never possess a well-developed gizzard, having at most +a rudimentary gizzard, or even two. However this character is not of +universal applicability, for both _Callidrilus_ and _Glyphidrilus_ +have got a gizzard and a strong one. These later genera however have +no calciferous glands or oesophageal pouches of any description, +structures which are also absent among the Criodrilinae. It will +be remembered that the purely aquatic families, Tubificidae, +Lumbriculidae, etc., rarely show signs of a gizzard and rarely +possess oesophageal pouches of any kind. In view of the fact that in +the case of a life in fresh water no calcareous salts are necessary +to resist the acids of the soil, and that the mud passed through the +alimentary canal is already finely divided, it is not surprising to +find gizzard and calciferous glands absent or rudimentary. + +Another not unusual feature among these aquatic Geoscolecidae is the +quadrangular form of the posterior end of the body. This is shown--as +its specific name denotes--by _Glyphidrilus quadrangulus_, by species +of _Alma_ and in all the species of the genus _Criodrilus_. It is +to be noted in this connection that a species of the partly aquatic +_Eiseniella_ has been named _tetraedrus_ on account of precisely +the same phenomenon. In these cases it is the posterior part of +the body which is thus quadrangular; the anterior segments down to +the ninth in _Criodrilus_ being rounded in the usual Oligochaetous +fashion. As the paired setae are apt to lie in the four projections +of the quadrangular body, one is tempted to see in this arrangement +of structures a faint approach to the dorsal and ventral parapodia +of the marine worms, and in any case it seems possible that by this +means the worms can cling more effectively and continuously to the +stems and leaves of aquatic plants among which they so largely live. + +It is a very remarkable fact that in the genera _Criodrilus_ and +_Alma_ the vent is dorsal in position instead of being surrounded as +in earthworms generally by the last segment of the body. This fact +might be put down to the near affinity between these two genera, +were it not for the fact that _Glyphidrilus_ shows precisely the +same state of affairs. These facts gain additional significance +in my opinion from the fact that among the leeches which are now +universally admitted to be allies of the earthworms the same position +of the vent is met with. This abnormal position of the end of the +alimentary canal may thus be fairly quoted in connection with +structures modified by the aquatic life. + +Finally, and this seems to be very important, the only genus among +the Megadrili which possesses gills is the Nile worm _Alma nilotica_. + + +MARINE SPECIES. + +There are a few, but relatively speaking very few, worms of the order +Oligochaeta which lead a marine life. And of these the majority +are shore forms not extending into the waters of the sea. The most +salient example, at any rate the best known perhaps, is the genus +_Pontodrilus_, the name of which fixes its habitat, and was naturally +given to it on that account. It was originally found on the sea +shore of the South of France, and I have myself examined examples +from Nice. The worm lives among bunches of sea-weed cast up by the +sea, and which are thoroughly salt. Besides the two forms that have +been met with in this Mediterranean region but which are united by +Dr Michaelsen into but one species, other _Pontodrilus_ have been +described from so many and such diverse parts of the world as the +following. The West Indies (Bermudas, Jamaica etc.), the coasts +of South America, of both West and Eastern Africa, the Red Sea, +Christmas Island near Java, Sharks Bay in West Australia, the Hawaii +Archipelago, Celebes, South West Australia etc. In fact there is no +great tract of the ocean excepting the antarctic region where this +genus is not to be found. It is possible however that this latter +statement is not correct and that New Zealand ought to be added. +But the species described from those islands, viz. _Pontodrilus +lacustris_, is not a marine form at all as its specific name denotes; +nor is it quite certainly to be included in the genus. On the other +hand a form from the Chatham Islands in the same quarter of the +globe, described originally as _Pontodrilus chathamensis_, is to +be referred to the antarctic region. Altogether some dozen species +of _Pontodrilus_ have been described by different naturalists; but +quite recently Dr Michaelsen has reduced these to three only, which +are _P. bermudensis_ (F. E. B.), _P. litoralis_ (Grube) and _P. +matsushimensis_ (Izuka), with the doubtful addition of _P. lacustris_ +already referred to. Whatever may be the ultimate verdict upon this +question of species it is clear that the genus is widely spread upon +the sea shores of the world and that forms from different regions +show some fixed variations, which others may eventually agree with +their original describers in regarding as definite species. + +It cannot be said that any salient characters in the organisation +of these worms mark them out from either terrestrial or fresh-water +Oligochaeta. There are no such important variations of structure as +can be seized upon to characterise them as inured to salt water. +The genus agrees with many aquatic forms in the fact that the +nephridia are not present in the earlier segments of the body, not +indeed putting in an appearance until about the thirteenth segment +or even later. They are thin delicate worms; but there is nothing +distinguishing about this, while the feeble or absent gizzard is +a character which is really difficult to correlate with habitat. +Still we have here a whole genus which is marine in its habit. Among +the Megadrili or earthworms there are not many other examples of +these 'euryhaline' forms as they have been named. On the shores of +Patagonia however and Kerguelen shore-living species of the mainly +antarctic genus of earthworms _Notiodrilus_ have been met with. +And there are a few allied cases among the antarctic genera of +Acanthodrilinae. + +In addition to these terrestrial forms there are a few limicoline +genera which are partly marine in their habit. Thus several species +of the prevalently arctic and antarctic family Enchytraeidae are +shore living. There are also marine Tubificids such as _Clitellio +arenarius_ and _Tubifex ater_ (not uncommon on British shores), +marine Lumbriculids and a marine Naid from the Italian coast. These +forms show no great differences from their fresh-water allies. + + +EARTHWORMS ORIGINALLY PURELY AQUATIC ANIMALS. + +The very name Earthworm, so distinctive as it is of the habitat of +these animals, seems to have been expressly invented in order to +crystallise into one word the remarkable distributions of these +creatures. They are with very few exceptions the most purely +terrestrial animals that we know. There are a few Mammals like the +mole and several underground Amphibians and Snakes in the tropics +which share this habitat with the worms, probably because they +chiefly prey upon them. But there is no group of animals that is +characterised by a subterranean existence in the way that earthworms +are. For we cannot put burrowing animals, such as the prairie dog +and many rodents, into the same category. These make and seek their +burrows for protective purposes and in order to bring forth their +young in security. They do not feed beneath the surface of the ground +or pass their entire lives in that situation. We have already in a +previous chapter dealt with such exceptional forms of earthworms as +do not lead an entirely subterranean existence; but as was pointed +out in chapter I these exceptions are but few and the immense bulk of +earthworms fully justify their name. + +Nevertheless there are many arguments which tend to show that these +purely land-dwellers have grown out of exclusively water-dwellers and +even that the change from the one mode of life to the other has been +accomplished comparatively recently. For there are here and there +vestiges of structures which seem only fitted for an aquatic life; +and in other cases structural changes have commenced which would +appear to be in definite relation to the underground mode of life +prevalent to-day. Let us consider for a moment the differences which +obtain between the conditions of life in water or in soft mud at the +bottom of pool or river, and those which are undergone by a dweller +in stiff soil or vegetable débris. In the first case the medium is +fluid or at most very soft, while the soil is at least stiffer and +harder to traverse. + +Secondly the transition between the very bottom of a pool and the top +layers of the water is more or less gradual, while the stiff soil +ends abruptly in the tenuity of the atmosphere. + +A third point of difference is doubtless the smaller supply of +readily available oxygen in the still pools and even rapid rivers, +which in certain stagnant pools and in the bottom waters of deep +lakes must produce a very vast difference in physiological conditions. + +We have already dealt with the characteristics of the aquatic genera +of earthworms, not only in detailing the general characters of the +families which are found in this situation but also in studying +the features which earthworms show in those cases where they have +reverted to an aquatic mode of life. It remains in the present +section to attempt to descry in the purely terrestrial forms the +remnants of adaptations to an aquatic life which are no longer of +service to them. + +It is a noteworthy fact, that the continuous circle of setae which +is met with in certain earthworms is by no means a character of such +classificatory importance as it was at one time, perhaps, thought +to be. It is true that we meet with this character in the genera +_Megascolex_ and _Pheretima_ which are not very far from each other +in the system and are at any rate members of the same sub-family, +the Megascolecinae. But we also find the continuous circle of setae +well developed in _Plagiochaeta_ which is not so near to _Pheretima_, +and an approach towards it in _Dinodrilus_ and _Dinodriloides_ which +are equally remote perhaps from both _Pheretima_ and _Megascolex_ +on the one hand and _Plagiochaeta_ on the other. Still it may be +urged that all of these genera are at least members of the family +Megascolecidae and that the question of a character which thus +merely shows affinity is not yet eliminated. It is therefore of +particularly great importance that Dr Cognetti de Martiis should have +met with the South American genus _Periscolex_ which, undoubtedly a +member of a totally distinct family, the Geoscolecidae, yet shows +the same complete circle of setae. The reason for dwelling upon this +particular anatomical character in the present connection is because +it would seem to be a character specially suited to an underground +life where there is an equal pressure all round the body and where +progression would seem therefore to be best attained by a continual +leverage round the circular body. And this view is strengthened by +the sporadic occurrence of this modification in different families. +We thus come to the conclusion that the opposite state of affairs is +a remnant of an aquatic life, a conclusion which it is the object of +the present section to discuss. More than this, it would seem that +an equal disposition of the two bundles of setae on each side of +the body was a less modified state of affairs than the restriction +of the two bundles or pairs of setae to the ventral surface, such +as occurs for example in the genus _Dichogaster_ and which is very +obvious in some of the larger-sized members of this extensive genus. +For the restriction of the setae to the ventral surface obviously +favours progression upon a surface and not through a medium. And +it is only among the terrestrial Oligochaeta that this mode of +progression occurs. It might also be urged, and with some reason, +that the retention of rather longer setae upon the clitellum in the +Lumbricidae and Geoscolecidae, and the possession of equally long +or in many cases much longer setae corresponding to one of the two +pairs of setae of the generative segment in certain Megascolecidae, +is a feature in which an aquatic condition--so to speak--is retained. +The setae would represent a vestige of the general presence of long +setae over the body generally such as is convenient or at least +not inconvenient to an Annelid living in water or soft mud. But +probably it will be thought the modified genital setae are a recent +development and not a retention. + +There is no more thoroughly terrestrial family of earthworms than +that of the Moniligastridae and yet this family in its general +anatomical characters shows many points of likeness to aquatic forms +as has been now pointed out by many observers. It is true that these +characters are not those which might be associated at first sight +with an aquatic life. But none the less they are characteristic of +most of the families which live in the waters of the earth. Thus +_Moniligaster_ and its allies (_Eupolygaster_, _Drawida_, etc.) have +quite short sperm ducts which open on to the exterior at furthest in +the segment next to that in which their internal funnel lies. Again +the simple structure of the terminal gland into which they open and +which in its turn opens on to the exterior is very like that of such +a family as that of the Lumbriculidae. Another fact is the simple +undivided cavity of the sperm sacs which is unlike that of typical +earthworms but again like that of all of the Limicolous families. +We may fairly see in these worms evidence of origin from aquatic +ancestors. Evidence of the same nature, _i.e._ not as showing the +retention more or less of anatomical characters commonly associated +with a life in water, but as affording indirect evidence of an origin +from actually aquatic forms, is to be seen in certain members of +the families Geoscolecidae and Eudrilidae. In both of these it not +infrequently happens that the sperm sacs are but a single pair and +that that pair consists of sacs of extraordinary length. Thus in +_Trichochaeta_ (or _Hesperoscolex_) _barbadensis_ Miss Fedarb and I +have shown that the long thin sperm sacs extend through no less than +109 segments, which is vastly in excess of the length of those of +the majority of earthworms in which they are most commonly limited +to a single segment. In the same way the Eudrilid worm _Polytoreutus +magilensis_ has a pair of long and thin sperm sacs which extend +through some fifty segments. This elongation of the sperm sacs in the +ripe worms is a very common feature of the Limicolous genera. + + + + +CHAPTER III + +THE EXTERNAL FEATURES OF EARTHWORMS AND THEIR RELATION TO HABIT AND +ENVIRONMENT + + +To the very inexperienced eye all earthworms might appear to be +quite similar in detail as they undoubtedly are in general form. +But it needs not a great deal of examination to detect even salient +characteristics whereby one kind may be distinguished from another; +to the expert it is possible in very many cases to go no further +than the outside before assigning its correct place in the system +to a given example. The general external features of this group +of worms have been already dealt with in another chapter. To some +of these we again direct attention in a more elaborate fashion in +order to emphasise the possible meanings of the variations met with +apart from their use in systematic arrangement. It is difficult to +say in comparing one worm with another what is the most salient +external difference. There are however a few which may be regarded +as equally conspicuous on a nearer examination of the specimens. +The varying position and greater or less extent of the clitellum, +the longer or shorter retractile or nonretractile prostomium, the +position of the usually conspicuous male pores, and the existence +of in the first place and--when present--the numbers and situation +of the so-called genital papillae are among the most obvious. The +setae and their position we treat of under the heading of the +modification of the worms to lead a terrestrial life; and though +these chitinous organs differ greatly they do not concern us in +the present section. The girdle or clitellum ('eminentia quasi +ulcerata') has been long observed as a character of these animals +and it is one which distinguishes them from all other worms except +the leeches and a very few marine Polychaeta. This modified region +of the body is often of a different colour to the rest and has a +glandular look which readily enables one to recognise its position +and limits, though its obviousness is less in some cases. It either +forms a complete ring round the body or is developed upon the dorsal +surface and only to a slight extent upon the ventral surface. Its +use, as is well known, is to secrete the cocoon in which the eggs are +deposited; and the epidermis which forms it is thickened and more +glandular than that in other regions of the body. Among earthworms +it is doubtful whether the clitellum ever occupies less than three +segments; it consists of three only in the great majority of species +of the marked genus _Pheretima_. From this lowest level it extends +in other forms, and in the partially aquatic African genus _Alma_ +it may occupy as many as forty segments. The position also varies +from genus to genus and from species to species. It is sometimes +further forward and sometimes further back. In the remarkable family +Moniligastridae this organ is developed earlier in the body than in +any other group of true earthworms, consisting of four segments or +so commencing with the tenth. As a rule the clitellum begins further +back than this--the thirteenth or fourteenth being a common place for +the first commencement of the organ among the Megascolecidae, while +among the Geoscolecidae and Lumbricidae it is generally much further +back, commencing in _Alma_ at the forty-fifth. These details might +be increased to many pages; but enough has been said to emphasise +the variability of the organ. What reason can be assigned to this +variability, which might be supposed unnecessary in view of its +functions? There are perhaps two suggestions that may be made, though +many facts are lacking which might offer confirmation or refutation +of either of these. It is to be noticed that on the whole the older +types such as the Moniligastridae and the Megascolecidae (including +for this purpose the Eudrilidae) have clitella which are short. There +are a few but not many exceptions. These older types do not seem +capable of extending their range with any rapidity. It is true that +here again there are exceptions, notably many species of _Pheretima_ +which are considered under the section which deals with the +migration of these animals. On the other hand the Lumbricidae have on +the whole a more extensive clitellum and so have many Geoscolecidae. +It is obvious that of all earthworms the Lumbricidae is the family +which has the greatest capacity of migration and adaptation to new +circumstances. The reason for this may be that in the latter case +the more extensive clitellum produces a larger cocoon which in +its turn can hold and cherish while they reach maturity a larger +number of embryos. Much remains to be learnt under this heading. But +the comparatively small clitellum of the large Ceylon _Megascolex +coeruleus_ only contains two embryos, while the also comparatively +small cocoon of the large and restricted _Octochaetus multiporus_ +(limited to the South Island of New Zealand) only contains a single +embryo. This latter fact may be regarded as fairly well established +since I myself examined quite fifty cocoons. + +On the other hand larger numbers seem to arrive at maturity in the +cocoons of _Allolobophora_. The more extensive clitellum must produce +a relatively larger cocoon, and it is interesting to note that the +cocoon of the widely distributed genus _Criodrilus_ (Europe and South +America) is very long although not of great diameter. However the +facts are not sufficiently great to dogmatise much upon this subject. +Another conceivable reason for differences in the clitellum is--as +I also think is the case with the genital papillae--to prevent +hybridisation. That the sense of touch is delicate in these animals +seems clear from the abundant development of epidermal sense-organs. +It may be that the feel of the clitellum during union enables two +individuals of a given species to come together and prevents those +of different species from mating. In any case there is no positive +evidence that hybridisation does occur in this group of animals. +The astounding variability and yet constancy in a given species +of the genital papillae is in favour of regarding these organs as +tactile recognition marks; and it will be noted that they are not by +any means characteristic of some of the older types of earthworms. +Furthermore they are particularly conspicuous in such genera as +_Pheretima_, _Megascolex_ etc., which possess a large number of +species. In these of course mutual recognition would otherwise be +more difficult. + + [Illustration: Fig. 12. Ventral view of _Pheretima solomonis_ to + show papillae which are to be compared with those of fig. 13. (x + 2.)] + + [Illustration: Fig. 13. Ventral view of _Pheretima sedgwickii_. + (x 2.)] + + + + +CHAPTER IV + +SENSE ORGANS AND SENSES OF EARTHWORMS + + +As this is not an anatomical treatise we shall not attempt any +detailed anatomical and histological account of the sense organs in +this group of worms. But a few facts must be given in illustration +and explanation of the senses of touch and sight that the Oligochaeta +undoubtedly possess. These Annelids, unlike their allies the marine +Polychaeta, and even their allies on two other sides, the leeches and +flatworms, have no complexly organised eyes or other sense organs. +They have in fact no organs to which a definite sense can be attached +on histological grounds. There is nothing comparable to an eye or to +the auditory sacs of other low worms. There are only particular cells +of the epidermis, often associated into small groups, and those again +into larger associations of rows of such groups of cells. It is to be +presumed that these modified groups of cells have a sense function; +but no more can be said than that they are doubtless tactile and +also to some extent receptive of the influence of light. True visual +cells have been asserted to exist in earthworms, consisting of cells +of which a part is clear and transparent and has been supposed to +serve as a lens for the rest of the cell which represents a retina. +But belief in the function of these cells is by no means unanimous. +On the other hand many investigations have proved the existence +of groups of epidermic cells of an oval form which are at present +arranged in definite rows upon the segments of both terrestrial and +aquatic forms, which are moreover connected with nerve terminations, +or are at least--according to more modern views--in close contact +with the terminations of nerve fibres. These are furnished often at +their free tips with minute sensitive processes. There is nothing in +the structure of these to associate them definitely with any sense in +particular. They suggest of course tactile organs more than organs +of any other sense. In addition to these are certain problematical +organs which are found in the Eudrilidae and are present in +the members of one section of that group, the section which is +represented by the universally found _Eudrilus_. + +These bodies have been compared to a Pacinian body (a sense-organ +found in Vertebrates) and they bear no little resemblance to it. +For each consists of a darkly staining core surrounded by a layer +of cells arranged like the coats of an onion. In any case it would +appear that these bodies must be looked upon as of a sensory nature, +though they do not reach the surface of the body but underlie the +epidermis. Their function must remain purely a matter of guesswork +at present, for nothing to the point is known of the habits of the +Eudrilidae. It has been suggested by Dr Gustav Eisen that these cells +are auditory and serve to warn the worm of the footsteps of birds and +other enemies. That too is his view of certain peculiar but different +cells found in the epidermis of _Pontoscolex_. In the latter +something like an otosome has been found which is certainly lacking +in the Eudrilidae, whatever may be the function of the cellular +epidermic bodies here briefly referred to. + +While there is thus nothing very positive to be gleaned from an +examination of the structure of the Oligochaeta as to the senses +which they may possess, experiment has done something towards an +elucidation of their behaviour under stimuli and their reaction +to light and to other forces which come into play during their +lives. There is some evidence that earthworms can see, using that +expression of course in a very broad way. At any rate they react to +changes in the intensity of light. The gross experiment of flashing +a lantern upon earthworms which are reaching out from their burrows +with the tail end inserted in those burrows shows that they have an +appreciation of light. More refined experiments have been conducted +upon the sense of light. Dr Graber used a box with two compartments, +the one of which was dark and the other illuminated with diffused +daylight, _i.e._ not direct sunlight. The worms were allowed equally +free access to both and were examined at the close of every hour, and +their positions noted. The investigator found that on the average the +dark half of the box contained 5·2 times as many worms as the light +chamber, thus indicating a very marked preference for absence of +light. + +Not only is this the case, but the same observer proved that +earthworms can distinguish between degrees of intensity of light. +This obviously indicates a more complete 'visual' sense. He +illuminated the light-box of the former experiment with light +admitted through a ground glass screen, thus diminishing its +intensity. The other chamber was left as before but the screen was +removed, thus admitting full daylight. In this experiment the number +of worms in each compartment proved to be different. The results were +not so striking as before, since only rather more than one-half were +found in the more dimly illuminated chamber. It is a well-known fact +that if earthworms are abroad at all from their burrows, it is during +the night that this movement takes place, the numbers decreasing +towards morning though worms are often seen crawling about well after +sunrise. Some experiments have been made in attempt of explanation of +this apparent anomaly. It would appear from these experiments that +while worms are negatively phototropic to strong and moderate light +as has already been pointed out, they are positively phototropic to +very dim light; hence the advent of evening calls them forth from +their burrows. It will be noted that this perception is of very great +advantage to the worm since its more active enemies above ground are +diurnal. It was held originally that the head end of the worm only +was thus sensitive to light; but more recent experiments have shown +that this is not the case, and that all of the body is sensitive. +This disposes of course of the existence of special light-receiving +organs in the anterior part of the worm's body. Not only this, but an +interesting extension of the view has been promulgated. It has been +shown by Prof. G. Parker and a colleague that in the common Brandling +worm, _Allolobophora foetida_, the response to light stimuli was +related to the amount of the body exposed to its influence. This +is very important as showing that the light perception in these +creatures is probably not due to special organs having a limited +position on the body, but is due to collective sense impressions of +many cells scattered over the whole body, the impression being the +greater when the whole body is exposed and less when only parts of +it are exposed. Furthermore, and this has quite another importance, +these observers noted that the reaction effects differed when only a +part of the body was exposed; that they were greater in the front of +the body, less in the middle, and less still at the tail end. Indeed +they found that the reactions in the case of the front end of the +body alone being exposed were rather more than one-third as compared +with those which were shown when the whole body was subjected to the +light stimulus. The fact that the least sensitive region of the body +is the posterior end has, it is true, only been definitely proved in +the case of the worm whose specific name has been mentioned. But +it is possible that others are similarly affected. And it is highly +important to note the prevalent habit among the Tubificidae of lying +with the head end imbedded in the mud of the pool which they inhabit, +while the tail end emerges and waves freely in the flood. The +additional fact that this tail end occasionally bears gills (as in +_Branchiura sowerbii_ and _Phreodrilus branchiatus_) has a collateral +importance not to be mistaken. + + + + +CHAPTER V + +RELATIVE FREQUENCY OF EARTHWORMS IN DIFFERENT REGIONS OF THE WORLD + + +It will be of use for various purposes to be considered later to +arrive at a comprehensive view of the relative numbers of species +and genera of earthworms in the four quarters of the globe. And in +making this general census we shall not take into consideration the +purely aquatic forms, but shall limit ourselves to the earthworms, +_sensu stricto_, or Megadrili, of which, however, it is true that +some members are actually lake and river dwellers. This latter fact +will not, however, interfere with the usefulness of the comparative +survey. + +Two preliminary remarks are necessary. The opinions of naturalists +vary as to the limits of genera; and a species may be a species to +one and a mere variety to another. Thus it will be impossible to give +a summary of the facts to be enumerated presently, which will be +either absolutely accurate or which will satisfy everyone in every +detail. But it is asserted that the following survey is substantially +correct. + +In the second place it is often possible to eliminate from the +fauna of a given region those species and even genera which have +been accidentally imported, a matter which will receive careful +consideration on a later page. Such forms are therefore, in those +cases at any rate where the evidence seems to be overwhelming, +withdrawn from the list. In other cases, particularly in the Eastern +region of the world, it has been found less easy to rectify the +catalogues by removing what Dr Michaelsen has termed 'peregrine' +forms. + +We shall commence with a census of South America; the entire +Continent will be divided for the present purpose into three +divisions, viz. South America, Central and North America, and in the +third place the West Indian Islands. + +In South America we find that the bulk of the indigenous earthworms +belong to the family Geoscolecidae and to a definite sub-family, +viz. Geoscolecinae. These genera are _Onychochaeta_ with one +species, _Hesperoscolex_ of which one species is known from the +area, _Periscolex_ with one species, _Anteoides_ with two species, +_Pontoscolex_ one species. _Opisthodrilus_ with two species, +_Rhinodrilus_ (including either as synonyms or as sub-genera, +_Thamnodrilus_, _Anteus_, _Tykonus_, _Urobenus_ and _Aptodrilus_) +with no less than 49 species: _Andiodrilus_ with five species, +_Holoscolex_ with one species, _Glossoscolex_ ten species, +_Fimoscolex_, _Andiorrhinus_ and _Enantiodrilus_ with one species +apiece. + +Thus of this sub-family of Geoscolecidae we have in South America +a large number of genera and a much larger number of species. Of +a second sub-family of Geoscolecidae there are three species of +_Criodrilus_ found in the South American Continent. + +We now turn to the Megascolecidae of which a large number of species +occur within the area now under consideration. The bulk of these +belong to the sub-family Acanthodrilinae and they are as follows: + +Of the genus _Notiodrilus_ there are ten species, of _Microscolex_ +two species, of _Chilota_ 19, of _Yagansia_ 13. + +A second sub-family Trigastrinae also occurs in this Continent and +the following genera are permanent inhabitants, viz.:-- + +_Dichogaster_ (= _Benhamia_) with only three species, of which +two at least are found elsewhere, and of which therefore the +autochthonism is doubtful. + +Finally, there is the sub-family Ocnerodrilinae comprising the +following genera: _Kerria_ with ten species, _Ocnerodrilus_ (with +sub-genera _Liodrilus_, _Ilyogenia_ and _Haplodrilus_) four species, +which again are rather doubtful indigenes of the South American +Continent. + +Leaving aside certain species (of the genera _Lumbricus_, +_Pheretima_, etc.) which are clearly not indigenous, the South +American Continent harbours 150 kinds of earthworms which are +distributed in some 19 genera. But of these a few species (_e.g._ +_Onychochaeta windlei_, _Kerria macdonaldi_) stray into neighbouring +regions, _i.e._ the West Indies and California. It is doubtful +therefore whether they are to be referred to as limited to one of +these regions and accidentally imported into the others, or whether +they are genuine inhabitants of both. + +The South American Continent shares with the West Indies the +following genera, but the species (except in the case of +_Onychochaeta windlei_, _Glossoscolex peregrinus_) are distinct; +these genera are _Hesperoscolex_, _Pontoscolex_, _Dichogaster_, and +_Ocnerodrilus_. _Diachaeta_ is limited to the West Indies. + +The following South American genera are also found in Central and +warmer North America (Mexico, California), viz. _Hesperoscolex_, +_Periscolex_, _Rhinodrilus_, _Pontoscolex_, _Andiodrilus_, +_Glossoscolex_, _Notiodrilus_, _Microscolex_, _Dichogaster_, _Kerria_ +and _Ocnerodrilus_. But with the exception of _Hesperoscolex_, which +seems to belong rather to Central America and the West Indies, +_Microscolex_ and _Pontoscolex_ which are world-wide and whose +original home is therefore difficult to fix, and _Dichogaster_ +and _Ocnerodrilus_ which would seem rather to be rare immigrants +(perhaps not truly indigenous) into South America, these genera are +practically distinctively South American. + +Thus we may fairly say that the genera _Anteoides_, _Opisthodrilus_, +_Andiodrilus_, _Holoscolex_, _Glossoscolex_, _Rhinodrilus_, +_Andiorrhinus_, _Fimoscolex_, _Enantiodrilus_, _Notiodrilus_, +_Chilota_, _Yagansia_ and _Kerria_ are at least very distinctive of +S. America and that they are represented by the large majority of +species found in that continent, the total being 120 or slightly more. + +Leaving the American Continent and adjacent islands and archipelagos, +the next great land-mass to receive attention will be the Continent +of Africa. In giving a census of the species and genera of earthworms +which inhabit this quarter of the globe it must be premised that +the facts relate only to Africa south of the Sahara. But little is +known of the genera which occur in Algeria and Morocco, but from what +little is known it is clear that they should come into consideration +in connection with the fauna of Europe and not with that of tropical +and south temperate Africa. + +We have in the first place to consider an entirely peculiar family, +of fair extent in genera and species, which is limited to this region +of the world; that is to say with one apparent exception which is +clearly only apparent. The genus _Eudrilus_ is one of the few kinds +of worms that turns up in collections from every tropical part of +the world and even at times from more temperate countries. It is one +of those 'peregrine' forms, as Dr Michaelsen has termed them, which +possess unusual facilities for extending their range. Presumably its +real home is Africa. This family is known as the Eudrilidae though +by some it is only regarded as a sub-family of the Megascolecidae. +In this family we have the following genera: _Eudriloides_ +with 11 species, _Platydrilus_ with 11 species, _Megachaetina_ +with two species, _Reithrodrilus_ with one, _Stuhlmannia_ with +five species, _Notykus_ and _Bogertia_ with one species each, +_Metadrilus_ also with but one species, _Pareudrilus_ with perhaps +five, _Libyodrilus_ with one and _Nemertodrilus_ with two species, +_Metschaina_ with two species, _Eudrilus_ with two (or possibly +more?) species, _Parascolex_ with four species, _Preussiella_ with +two, _Buttneriodrilus_ with two, _Eminoscolex_ with 16 species, +_Hyperiodrilus_, _Heliodrilus_, _Alvania_, _Rosadrilus_, _Kaffania_, +_Euscolex_, _Metascolex_, _Beddardiella_, _Gardullaria_ with only one +species to each genus, _Bettonia_ with three species, _Teleudrilus_ +with 15 species, _Polytoreutus_ with 22 species, _Iridodrilus_ +with two, _Malodrilus_ also with two, _Neumanniella_ with eight, +_Eupolytoreutus_ with two and _Teleutoreutus_ with only one species. + +This completes the list of the Eudrilidae. We will take the huge +family Megascolecidae next; and we find in tropical Africa that the +genus _Dichogaster_ alone contains at least 93 species confined to +Africa, as well as a few which it shares in common with America, +and the common and widely spread _D. bolavi_ which has even made +its way to Europe. Of the sub-family Ocnerodrilinae we have the +genus _Gordiodrilus_ with seven species, a genus which also occurs +in Madagascar and the West Indies: _Nannodrilus_ with three +species, _Diaphorodrilus_ with one species and a few examples of +_Ocnerodrilus_ and its sub-genera, some of which are also forms that +occur elsewhere in the world, for example _Nematogenia panamensis_. +Of _Pygmæodrilus_ there are eight or nine species. + +Of the sub-family Acanthodrilinae the Cape region of South Africa +harbours some seven species of the genus _Notiodrilus_, of which one +however is a West African form. The allied _Chilota_ has 13 species, +and there is a peculiar genus _Holoscolex_ near to _Yagansia_ with +one species. We next have to deal with the Geoscolecidae, of which +a sub-family, Microchaetinae, is mainly found in Africa, the rest +being found in the neighbouring Madagascar and some few in the East. +_Microchaetus_ contains about 14 species, _Tritogenia_ perhaps +three, _Callidrilus_ two, and the genus _Glyphidrilus_, mainly found +in Asia, has one species in the region now under consideration. In +addition to these Geoscolecids there is the peculiar and largely +aquatic _Alma_ with six or seven species in East, West, and Central, +Africa, and in the Nile region. + +Summing up the genera which are found in tropical and South Africa we +find that there are 44 which are abundant in, or entirely confined +to, the region. In addition to these four or so occur in Africa but +are either more abundant elsewhere or (as in the case of _Chilota_ +and _Notiodrilus_) are equally distinctive of other parts of the +world. The number of species may be estimated at 270, possibly rather +more. Clearly therefore this part of the world is much richer than +South America, both in numbers of genera actually found, and peculiar +to the country, and numbers of species. + +Passing from Africa the next definite quarter of the globe which +will detain us here is the island of Madagascar, so remarkable for +the Mammalian fauna which characterises it, for its lemurs, peculiar +Insectivora and Carnivora, and above all by reason of the absence of +the prevalent African types such as antelopes, zebras, rhinoceros +etc. It is probable that a good deal still remains for discovery +among the earthworms of this island; but a considerable number are +already known and they are as follows: + +The Eudrilidae are completely absent, a state of affairs which is +paralleled by the absence of antelopes among mammals. + +The sub-family Acanthodrilinae of the family Megascolecidae are +represented by four species of the genus _Notiodrilus_, with which +perhaps _Maheina braueri_ from the Seychelles should be included as +it presents small differences from _Notiodrilus_. + +Among the Megascolecinae a good many species of _Pheretima_ have been +collected both on Madagascar and on certain adjacent islands; but +these, with one possible exception, are forms which occur elsewhere +and are often indeed very widely distributed 'peregrines,' so that +it is hardly permissible to place them among the real inhabitants of +Madagascar. The same arguments hold in the case of _Lampito mauritii_ +and the ubiquitous _Eudrilus_ and _Pontoscolex_. But there is the +peculiar _Howascolex_. + +Among the Ocnerodrilinae a distinct species of _Gordiodrilus_ occurs, +and an obviously introduced _Ocnerodrilus_. + +It is among the Geoscolecidae that the most characteristic forms are +met with. These belong exclusively to a genus of the Microchaetinae, +_Kynotus_, which is found nowhere else but in Madagascar, where it +is represented by at least twelve different species. + +We have therefore in Madagascar and the surrounding islands only two +peculiar genera, only four genera which can be regarded as endemic, +and only about seventeen peculiar species. + +Passing eastward and crossing the Indian Ocean we come to the +Continent of Asia, and we shall first direct attention to the +peninsula of India and adjacent parts of Burmah and the island of +Ceylon, of which there has been accumulated a great deal of knowledge +concerning the Oligochaetous fauna. + +This quarter of the globe differs quite as much from any that have +been hitherto considered as they do among themselves. We have left +behind us the Geoscolecidae with the exception of the ubiquitous and +clearly peregrine _Pontoscolex_, and the genus _Glyphidrilus_, which, +being aquatic at times, is perhaps hardly to be considered in the +present survey. + +We have also in this Indian region the equally ubiquitous _Eudrilus +eugeniae_ which need not of course detain us further. The Lumbricidae +are for the most part of European forms with the exception of +_Helodrilus indicus_ conceivably an actual inhabitant of India. +It is among the Megascolecidae that the vast majority of the +forms endemic in India are to be found. We shall take this family +according to its sub-families. In the first place we note that the +sub-family Acanthodrilinae is totally unrepresented. The large +sub-family Megascolecinae has very numerous representatives. Of +the genus _Megascolex_ itself there are some 30 species, of which, +however, two or three are looked upon as varieties. Of the allied +genus _Notoscolex_ there are ten species and of _Perionyx_ about +13. The genera _Megascolides_, _Diporochaeta_, _Spenceriella_ and +_Woodwardia_ have only six species between them of which three belong +to _Megascolides_. _Plutellus_ has five species in this region. +_Lampito_, which is a widely spread form with but one species _L. +mauritii_, may or may not find here its original home. It also occurs +in Africa and Madagascar. _Pheretima_ is represented by no less +than 12 species, of which at any rate the very great majority are +peregrine forms, and not to be safely regarded as forming an integral +part of the fauna of the Indian peninsula and adjoining countries. + +Of the sub-family Octochaetinae the type genus _Octochaetus_ has +ten species in this region and _Eutyphoeus_, which is restricted to +the region, has 15 species or perhaps rather more. _Hoplochaetella_ +has but one. Of the Trigastrinae there are four or five examples +of the genus _Dichogaster_ which occur within the region now +under consideration; but it is doubtful whether they are truly +indigenous. On the other hand _Eudichogaster_, closely allied both to +_Dichogaster_ and _Trigaster_, seems to be confined to this part of +the world where it is represented by five species. Finally we come to +the last sub-family--that of the Ocnerodrilinae, which is represented +by a species apiece of the genera _Ocnerodrilus_, _Nematogenia_ and +_Gordiodrilus_. The latter species _G. travancorensis_ is alone +to be regarded as endemic and it is very near to the African _G. +zanzibaricus_, described some years since by the present author. + +The remaining family of terricolous Oligochaeta found in India is +the family Moniligastridae which is practically limited to this part +of the world and consists of at least twenty species distributed +among the genera _Moniligaster_, _Eupolygaster_, _Desmogaster_ and +_Drawida_, the majority belonging to the last-named genus. + +This quarter of the globe is therefore inhabited by 18 genera which +are certainly truly endemic, and which comprise between them about +120 species. But only four or five genera are peculiar. + +The remainder of the Asiatic Continent is not very well explored +with regard to its earthworm inhabitants. It seems clear however +that the southern and coastal region of China and Japan with the +Malay peninsula are really continuous with the mass of islands which +lie between India and Australia and form together a tract of land +which is characterised by an Oligochaetous fauna differing from both +that of India on the one hand and Australia on the other. We shall +therefore consider this huge portion of the globe as one region +comparable to the other divisions that have been hitherto considered. +With reference to the Lumbricidae and Geoscolecidae the same remarks +may be made as in the case of India. The indigenous forms of the +latter family are to be looked upon as outside of the present survey +since they are largely or entirely aquatic forms. _Pontoscolex +corethrurus_, and _Eudrilus eugeniae_, need not detain us for reasons +already amply stated. We now come to the great family Megascolecidae. +Of this family the genus _Pheretima_ stands at the head; and of the +200 or so species that have been or can be assigned to this genus +all, with merely two or three exceptions, are natives of the Eastern +Archipelago and adjoining mainlands of Asia. Of other Megascolecinae +the region has yielded the following genera. In Java one species +of _Woodwardia_ (_W. javanica_) has lately been described. It is +regarded by Michaelsen however as doubtfully indigenous. And the same +remark may be made of _Perionyx_. _Plionogaster_, however, with four +or five species, is limited, as far as our present knowledge goes, to +the Philippines and to neighbouring islands. The Acanthodrilinae and +Octochaetinae are totally absent from this part of the world, there +being no record even of peregrine species of these sub-families. +The sub-family Trigastrinae is not however unrepresented; for of +_Dichogaster_ several species occur, such as _D. malayana_ and _D. +saliens_. But these are by no means certainly to be looked upon as +real natives of the situations within this area where they occur. +This completes the scanty list of genera found in the region under +consideration; for one Moniligastrid (_M. barwelli_) is hardly to +be looked upon as indigenous. We have therefore to record here but +five genera, of which only two are certainly indigenous and probably +also confined to the region; they contain between them certainly two +hundred species. + +We next come to the Continent of Australia. The earthworm fauna is +again quite without Geoscolecidae and of course Lumbricidae. As to +the former there are not even doubtful cases like _Glyphidrilus_ of +the East; for we find only recorded _Pontoscolex corethrurus_, a +species concerning whose extraordinary powers of migration there is +no possible doubt. The same may be said of _Eudrilus_ also recorded +from Australia. Here the Megascolecidae are as conspicuous as in the +old world generally. We find, however, a great many members of the +sub-family Acanthodrilinae. Of the antarctic genus _Notiodrilus_ +there are some six species. _Microscolex_ occurs; but the real +habitat of this genus is very doubtful. _Diplotrema_, with one +species, _D. fragilis_, is not only indigenous to, but confined to, +Australia. + +The most prevalent sub-family is that of the Megascolecinae. Of +the genus _Pheretima_ there are two species which may or may not be +truly indigenous. Of _Plutellus_ there are forty species; there are +over thirty of _Diporochaeta_, while _Notoscolex_ (with which Dr +Michaelsen associates _Digaster_, _Didymogaster_, _Perissogaster_) +includes more than forty species, _Fletcherodrilus_ has but one +species, _Megascolex_ has seventy species, _Woodwardia_ sixteen, +_Spenceriella_ five, and _Megascolides_ seventeen. There are four +species of _Perionyx_ which were at one time regarded by Michaelsen +as necessitating a new but allied genus _Perionychella_; the two are +now merged. Thus there are not far short of 150 species and eleven +genera represented, of which only two are limited to Australia. + +Having completed the survey of the central and southern land masses +of the globe we next direct attention to the northern land masses, +viz. North and Central America on the one hand, and Europe and +northern Asia on the other. With regard to America we find besides +many species of Lumbricidae the genera _Notiodrilus_, _Microscolex_ +among the Acanthodrilinae, _Megascolides_ and _Plutellus_ of the +Megascolecinae, no member at all of the Octochaetinae, a considerable +number of species of _Ocnerodrilus_ and sub-genera belonging to the +Ocnerodrilinae, a good many species of _Dichogaster_ and at least +one of _Trigaster_ among the Trigastrinae, while one sub-family, +that of the Diplocardiinae, is only found here and contains two +genera _Diplocardia_ and _Zapotecia_ with quite ten species between +them of which only one belongs to the last-named genus. There are +no Geoscolecids (except the chiefly aquatic _Sparganophilus_); this +family stops short in the West Indies where their presence has been +briefly referred to in considering the worms of South America. The +vast majority of the genera enumerated here are only found in the +warmer parts of the North American continent. We have therefore +in this division of the world some nine genera of which at least +one, viz. _Diplocardia_ (and its close ally, hardly perhaps to be +separated, _Zapotecia_), is confined to it, while _Trigaster_ only +extends as far south as to the West Indies. + +Europe and northern Asia, of which Europe alone and certain +limited tracts of Asia are at all known, contain all the genera +of the family Lumbricidae which are (according to Michaelsen) +_Eiseniella_, _Eisenia_, _Helodrilus_, _Octolasium_, _Lumbricus_ +with various sub-genera of _Helodrilus_, such as _Dendrobaena_ and +_Allolobophora_. Besides these forms, which amount to at least 130 +species, we find the genus _Hormogaster_, with two species, the only +genus of the Geoscolecid sub-family Hormogastrinae; there is also +a species apiece of the genera _Criodrilus_ and _Sparganophilus_ +(sub-family Criodrilinae). But as these are at least largely aquatic +they come under another set of rules and are not important in the +present survey of the earthworms of the world. It will be thus seen +that with these two last exceptions the area in question contains but +six genera of which _all_ are peculiar. + +We have now completed the survey of the principal land-masses of +the globe. We shall conclude with a reference to one of the largest +islands of the world, viz. New Zealand, with which will be included +a few outlying islands such as the Aucklands, Snares Island, etc. +The reason for not ignoring these islands as we have ignored certain +other islands of large size, such as Japan, will be apparent from +the peculiarity of the earthworm fauna which they show. From New +Zealand the Moniligastridae, Geoscolecidae, Lumbricidae, and, of +course, the Eudrilidae, are absent, save the ubiquitous _Eudrilus_. +The only family of earthworms which is here represented is that of +the Megascolecidae. Of this family the Acanthodrilinae are very +well represented. We have at any rate seven species of the genus +_Notiodrilus_ of which the bulk are from the small adjacent islands +and not from the mainland. The genus _Rhododrilus_ with nine species +is actually limited to the New Zealand group and so is an allied +genus consisting of two species only, viz. _Leptodrilus_. This +latter genus is confined, so far as present information goes, +to the Auckland and Campbell islands. _Dinodriloides_ with two +species is also limited to New Zealand and to the North Island. +_Maoridrilus_ with some ten species is another native and restricted +genus. So too is the allied _Plagiochaeta_ with numerous setae on +each segment but with the alternating and single nephridial pores of +_Maoridrilus_. There are several species of _Plagiochaeta_ of which +one has been lately regarded by Dr Michaelsen as really falling +within the otherwise Indian genus _Hoplochaetella_, while for another +he has formed the genus _Pereiodrilus_. _Neodrilus_ with but one +species _N. monocystis_ is another peculiar New Zealand genus. The +family Octochaetinae contains only four genera, of which one, viz. +_Dinodrilus_ (with four species), is limited to New Zealand, while +_Octochaetus_ has about five representatives. _Hoplochaetella_ +(if Dr Michaelsen's surmise referred to above be correct) has +one species in New Zealand. The Megascolecinae are less numerous +than the Acanthodrilinae, but there are ten species of the genus +_Megascolides_ (which includes Benham's genus _Tokea_), perhaps seven +species of _Diporochaeta_, and two other species which Michaelsen has +removed from the genus _Diporochaeta_ and placed in _Spenceriella_. +One _Plutellus_ (which however may have been introduced) completes +the New Zealand Megascolecines. We have therefore in this part of the +world fifteen genera including between them some 58 species; eight +of the genera are peculiar to the islands. + +From this brief statement of facts some inferences of interest can +be drawn. It is in the first place plain that every part of the +world except the extreme north and south has a considerable fauna of +earthworms. The one exception would appear to be the northern part +of the North American continent. Here we meet with members of the +family Lumbricidae which are however species that are met with in +the Euro-Asiatic province and are thus to be regarded as possibly +later immigrants introduced probably by man. Thus temperature +short of a constantly frozen condition of the ground is not a bar +to the existence of earthworms. Even a freezing of the ground for +lengthy periods is not a complete obstacle to the existence of those +Annelids; for I have myself received examples of Lumbricidae from +the arctic island of Kolguev. Moreover the temperate regions would +seem to be as fully populated in the way of individuals, and even of +species, as are the tropical regions. Indeed as to individuals it +seems that the temperate regions are more fully supplied than much +of the tropics. This however is not quite the object of the present +section to discuss. We are here concerned with the relative frequency +of genera and species. There are according to a recent estimate of +the Rev. H. Friend some forty species recognisable in Great Britain. +And as already has been stated the earthworms of Europe amount to +perhaps 130,--at any rate well over one hundred. In tropical America +there are hardly more. But in the latter case the number of genera is +very greatly in excess of that of Europe. We cannot however say that +an abundance of generic types is quite characteristic of the tropics. +For the Eastern Archipelago, though rich in species, is but poor +in genera, not possessing more than half a dozen or so. And on the +other hand the temperate climate of New Zealand has produced a very +considerable series of genera, much more than those of the islands +of the East and nearly as many as those of, for instance, Central +America and the West Indies. + +This conclusion is in its turn contradicted by the conditions +observable in Chili and the temperate regions of South America, where +the number of species is large but the number of genera small. In +short no general laws, in the present state of our knowledge, can +be laid down as to the connection between species and genera on the +one hand and climatic conditions on the other. In this department of +our subject we cannot do more than has already been done, _i.e._ to +state the actual facts. One is tempted in comparing the rich fauna +of tropical Africa with the very limited fauna of Madagascar to +associate a richness of types with extent of land surface. In the two +cases cited this conclusion is obvious. It may also be extended--if +we confine ourselves to species and not to genera. For the two great +islands of New Zealand have not between them more than fifty species +of earthworms, while Australia has four or five times that number. It +will be noticed however that we cannot associate poverty of generic +differentiation with limited land masses; for New Zealand has a large +number of generic types, very many more than the huge Euro-Asiatic +tract of continent. + + +THE RANGE OF GENERA. + +We have seen, and shall again refer to the fact, that individual +species of earthworms have not as a rule a range over a great +extent of country, save only in those cases such as _Pheretima +heterochaeta_ which belong to that physiological section of these +worms called 'peregrine' forms; these appear to possess some means +of extending their range by the assistance of man which is denied to +other forms. Apart from these instances, which do not come under the +present category, it is only _Lumbricus_ and its immediate allies, +_Helodrilus_, etc., of which certain species are found to exist over +wide tracts of land. There are however many genera which have a wide +range and which may be contrasted with others in which the range is +very limited. The two extremes are moreover connected by forms with +an intermediate range. There is no doubt whatever that the genus +with the widest range is _Notiodrilus_ of which species are found +throughout the antarctic region, viz., in Patagonia, the islands of +the Antarctic Ocean, the Cape of Good Hope, New Zealand, and also +further to the north, sometimes even to and beyond the tropics in +America, Australia, and Africa. There is no other genus of which the +genuine extension (_i.e._ not in any way due to man) is so great as +this genus _Notiodrilus_. And this fact gains much significance from +the now generally accepted view that in its anatomical structure +_Notiodrilus_ comes near to the original type of earthworm. + +Perhaps the next most widely distributed genus is _Helodrilus_ of +the family Lumbricidae which occupies Europe and Asia to the extreme +east, and is thought also to be indigenous to certain parts of North +America. But this range, though equally wide perhaps in mileage, is +less impressive than that of _Notiodrilus_, since the land areas +inhabited by the genus are continuous--almost so if we accept North +America as its real habitat. Here we have a case precisely the +opposite of that of _Notiodrilus_; for while there are reasons for +regarding _Notiodrilus_ as an ancient form of Lumbricid, there are +equally good reasons for regarding the Lumbricidae as the most modern +family of earthworms. + +To find other instances of widely spread genera we must recur to the +great family Megascolecidae. There are practically no Geoscolecidae +which have a really extensive range. The only instances are +_Criodrilus_ and its ally _Sparganophilus_ which occur in America, +whether North or South, and in Europe; but as these forms are at +least largely aquatic the facts are not quite comparable with those +now under consideration. + +The genus _Dichogaster_ (which includes as synonyms _Benhamia_, +_Millsonia_, _Microdrilus_) is unquestionably indigenous to tropical +Africa and certain parts of America including the West Indies. It has +been also met with in the East; but as the species there occurring, +such as for example the species originally described by myself as +_Microdrilus saliens_, are of small size, an accidental introduction +is quite possible, and it is by no means certain that it has not +occurred. In any case the genus is known to possess species which +are undoubtedly to be reckoned among peregrine forms--such as _D. +bolavi_, which has turned up in Europe. _Gordiodrilus_ and also +_Ocnerodrilus_ with its sub-genera have very much the same range as +has _Dichogaster_. It is to be noted however that these forms are +circumtropical, and that their distribution is thus less continuous +than that of _Notiodrilus_; they do not however show the markedly +discontinuous range of certain other genera of Megascolecidae. For +instance _Octochaetus_ is well known from New Zealand, and, not +occurring in the intermediate tracts, is again met with in India. +_Hoplochaetella_ is believed by Michaelsen to present us with another +precisely similar instance. Then also the genera _Woodwardia_ and +_Notoscolex_ are to be found in Australia and again (absent from the +immense tract lying in between) in Ceylon. _Megascolex_ has much the +same range, showing also this marked and remarkable discontinuity. +Stranger still, perhaps, is the range of _Plutellus_ and +_Megascolides_, of which the former, chiefly found in Australia and +Tasmania, not only extends its habitat to Ceylon but also to North +America; it is there represented by Eisen's species _Argilophilus +marmoratus_, referred by him, and not unnaturally, to a distinct +genus, but placed by Michaelsen in _Plutellus_. _Megascolides_ is +Australian and from the North Island of New Zealand, where its +species were regarded by Benham as of a distinct genus, _Tokea_. +There is also one form, _Megascolides americanus_, in the western +region of North America. + +The two genera _Yagansia_ and _Chilota_, closely related to +_Notiodrilus_, have a range which is short of that of _Notiodrilus_, +and we shall see later that there are reasons for regarding these +genera as derived from _Notiodrilus_. They are met with only in the +south of South America, and in the Cape of Good Hope region. + +The range of _Microscolex_ seems to be much the same as that of +_Notiodrilus_; but it is a little uncertain how far the genus is +really autochthonous in the countries where it occurs; and in any +case it differs from _Notiodrilus_ in occurring in Europe, where the +species has been named for a long time _M. phosphoreus_. We do not +positively know whether this is 'peregrine' in Europe or not. + +The range of the antarctic Acanthodrilinae is in a sense continuous; +for they argue the former northward extension of the antarctic +continent and in any case they occupy neighbouring land masses. In +_Octochaetus_ and _Plutellus_ the case is different and one of real +discontinuity. There are however cases of wide range which is also +actually continuous and such is afforded by the genus _Pheretima_. +This genus appears to be possibly indigenous to Australia; in any +case it reaches from the Solomon Islands on the east to India towards +the west, being found in all intermediate continents, while it +reaches Japan on the north side of this large area. + +There are other genera which extend their range over a considerable +area, but which are not so widely distributed as these which we have +just been considering. Thus _Diporochaeta_ is chiefly Australian but +also reaches even the South Island of New Zealand and the southward +lying antarctic islands. _Desmogaster_ and _Eupolygaster_ among the +Moniligastridae range from Burmah in the east to Sumatra and Borneo +further east, though they are not recorded from intermediate islands. +_Perionyx_ is found in Burmah, India, Zanzibar, Sumatra, and Java. +There are other examples of genera which have much the same range +as those enumerated. Finally there are those which are confined to +one land mass and very often to a restricted region of that. Thus +_Kynotus_ is confined to Madagascar, all the genera of Eudrilidae to +tropical Africa, some of them, _e.g._ _Beddardiella_ and _Euscolex_, +to very limited tracts, others to wider or less wide areas in that +continent. _Maoridrilus_ is only found in New Zealand, to the South +Island of which also is confined the genus _Neodrilus_. To the Cape +region of Africa is limited _Microchaetus_; and to a belt running +across the northern part of the tropical region and extending down +the Nile, the remarkable, partly aquatic, _Alma_. + +As a kind of appendix to these facts and conclusions we shall next +deal with certain widely spread forms that have been already referred +to, with the range of different genera over great land masses of the +world, and with the earthworms of oceanic islands. + + + + +CHAPTER VI + +PEREGRINE FORMS + + +Dr Michaelsen has used this term to describe those species which +possess some powers of migration over the sea, denied to the majority +of worms, and probably due to the direct interference of man. Thus +we find in collections of earthworms from various parts of the +world not only examples of forms which do not come from other parts +of the world, but also a few which occur in many or even most of +such collections. It is for example to be actually expected that +a collection of earthworms made in South America, the Philippine +Islands, or Australia will contain examples of the apparently +ubiquitous _Pontoscolex corethrurus_. This is what has actually +happened in cases of which I have personal knowledge, as well as in +many others recorded in the literature of the subject. I have myself +received this worm from the three parts of the world mentioned, and +also from Hawaii. Others have increased its known range to other +parts of the South American continent, to Central America, the West +Indies, the islands of Sumatra, Java, Borneo, Celebes, Mauritius, +and Madagascar, etc. It is in fact found everywhere in the tropics. +With this range may be contrasted that of another genus of the same +family (Geoscolecidae), viz. _Kynotus_, which, though consisting of +many species, is not found outside of the Madagascar district. It +should be added that _Pontoscolex_ does not appear to contain more +than two species, the one not mentioned in the above survey of its +distribution being _P. insignis_ of Kinberg, which is apparently the +same as _P. liljeborgi_ of Eisen, and is limited to certain parts of +America. + +Before attempting to grapple with the remarkable facts implied by the +distribution of this genus, it will be well to survey the whole group +of Oligochaeta and to reduce to as short a space as possible the +total series of facts which are of the same nature. + +A case, even more striking than that of _Pontoscolex_, is afforded +by the Eudrilid genus _Eudrilus_. As with _Pontoscolex_ there are +two species of this genus, one, _E. pallidus_, being confined to +West Africa, the remaining one, _E. eugeniae_, being world-wide in +range. This latter species has received the following names, viz. +_E. decipiens_, _E. lacazii_, _E. peregrinus_, _E. sylvicola_, _E. +boyeri_, _E. jullieni_, _E. erudiens_, and _E. roseus_; they appear +to be all synonyms of the name originally given by Kinberg who +however did not recognise the distinctness of the form as a genus. It +is now known as _Eudrilus eugeniae_. The variety of names given to +supposed different forms (for two of which I am myself responsible) +is due to the fact that in earlier days when nothing was known +about the geographical distribution of this group of animals it was +thought by no means unreasonable that a given genus represented by +several species should range over the globe. This fact coupled with +imperfect description of structural details led to the multiplication +of supposed species, a position which is no longer tenable. This +worm is quite as abundant in gatherings from all parts of the world +as is _Pontoscolex corethrurus_; and in addition to the countries +inhabited by the latter, _Eudrilus eugeniae_ has been met with in New +Caledonia: tropical Africa is probably its original home. + +The two families that have been hitherto considered offer no further +instances, among their many species, of worms with so wide a range +as those just dealt with. There are indeed one or two forms, _e.g._ +_Criodrilus_ and _Glyphidrilus_, which have a considerable range +though not nearly equalling that of _Eudrilus_ and _Pontoscolex_. +These are, however, aquatic forms and the range of aquatic forms is +determined as far as we can see by a different series of causes to +that of terrestrial forms, which are referred to later. + +Among the Moniligastridae we have apparently an instance of a +peregrine form. The genus itself has its headquarters in Ceylon and +extends a little way in other eastern regions; there is, however, +one species, _Moniligaster bahamenis_, described some years since +from the Bahamas which must surely be an example of a peregrine form, +particularly since it is probably identical with _M. japonicus_ whose +name is indicative of its habitat. + +Among the huge family of the Megascolecidae there are a considerable +number of species which apparently possess the same facilities for +making their way in different directions and across seas from the +locality that is thought to be their real home. + +Of the very many genera, however, of which this family is composed, +a comparatively small number are thus peregrine in habit at times. +All the species known which are distributed broadcast, more or less, +over the tropics belong to the genera _Pheretima_, _Microscolex_, +_Dichogaster_, _Megascolex_, _Perionyx_, _Ocnerodrilus_, _Kerria_. +These several genera are placed in order of frequency of exotic +occurrence. Indeed of the two latter genera their frequent life +in fresh water may really remove them from the present category +altogether. In addition to these are some perhaps more questionable +instances, such as the genus _Gordiodrilus_ which, prevalently West +African, has also been found in the West Indies, in East Africa, and +in India and Madagascar. These instances I propose to leave out of +consideration in the present sketch. The most obviously peregrine +genus of all those enumerated is _Pheretima_, which according to +my experience turns up in almost all gatherings of earthworms from +any part of the tropical or even sometimes temperate regions of the +world. It seems to be fairly well settled that this extensive genus +has its real home in the islands of the Eastern Archipelago, perhaps +extending a little in various directions from that centre. But +examples of the genus have been found in almost all other regions. +And what is especially to the point in considering the facts, as will +be pointed out with more emphasis later, the assumedly peregrine +species do not differ from those found in the real district in which +the genus is indigenous. + +Dr Cognetti de Martiis enumerates in the Neotropical region, that +is in South and Central America and the West Indies, the following +species: _Pheretima biserialis_, _P. californica_, _P. capensis_, +_P. elongata_, _P. hawayana_, _P. hesperidum_, _P. heterochaeta_, +_P. houlleti_, _P. posthuma_, _P. rodericensis_, _P. schmardae_ and +_P. violacea._ Of these twelve species it is quite certain that the +last six occur in the East, where they are doubtless indigenous. So +too do the species _P. biserialis_, _P. capensis_, and _P. hawayana._ +The synonymy of the different species of this large genus is not yet +in a completely settled condition. But in the meantime it is in my +opinion quite possible that both _P. hesperidum_ and _P. californica_ +are identical with species also occurring in the East. There remains +the somewhat doubtful _P. elongata_ from Peru which has not been very +fully described. There is thus no convincing evidence of species +really indigenous to and confined to any part of America. Some of +these species also occur in many other parts of the world. For +instance, _P. heterochaeta_ is very widely spread indeed, occurring +as it does in Australia, New Caledonia, Madagascar, and even England +(in hothouses). This species indeed is the most prevalent of all +Pheretimas and seems to be abundant in gatherings of earthworms from +various localities as are _Eudrilus_ and _Pontoscolex_. + +From the island of Madagascar and neighbouring islands the following +species of _Pheretima_ have been obtained and identified by Dr +Michaelsen: viz. _Pheretima pentacystis_, _P. peregrinus_, _P. +heterochaeta_, _P. biserialis_, _P. rodericensis_, _P. houlleti_, +_P. robusta_, _P. mauritiana_, _P. taprobanae_, and _P. voeltzkovi_. +It will be noticed that the majority of these are also included in +the list from South America, and that many of them are also found +in other parts of the world, and nearly all of them in the East. +There remain a few which are doubtful. It is quite possible that _P. +mauritiana_ is the same as _P. hawayana_ and _P. bermudensis_, in +which case it has a world-wide range. _P. taprobanae_ is well known +as a Ceylon species. _P. robusta_ also occurs in the East Indian +islands. There remain _P. pentacystis_, _P. peregrinus_, and _P. +voeltzkovi_. _P. peregrinus_ is known from Australia and also from +Sumatra, so that that species need not concern us in enumerating +those which are possibly endemic. In fact it is only _P. pentacystis_ +and _P. voeltzkovi_ which may be really Mascarene. + +Another peregrine genus belonging to the sub-family Acanthodrilinae +is _Microscolex_. But the limits of this genus may be regarded as +at present rather uncertain. And this difficulty somewhat affects +the bearing of the facts to be related, though it hardly affects the +value of the facts themselves. Dr Michaelsen referred to the genus +in his great work seven well-defined species, and four others not so +plainly distinct. Of these eleven, two are confined to New Zealand, +four to North and Central America, one to Hawaii, one to Madeira, +one to Algeria, while the remaining two are found pretty well over +the whole surface of the world. More recently the same authority +has somewhat extended his view of the generic characters, so as to +include a number of forms found in Patagonia, Cape of Good Hope, and +the antarctic region generally, while he has lumped together into two +species only, viz. _M. phosphoreus_ and _M. dubius_, the eleven forms +just mentioned, which species therefore are absolutely world-wide in +range, and thus form an excellent example of a peregrine form. These +species moreover differ from _Pontoscolex_ and some others in that +they have been able to establish themselves in Europe. Dr Michaelsen +also relates that in the cultivated lands of South West Australia, +_Microscolex dubius_ and _Helodrilus caliginosus_ are actually the +commonest species; and he calculates that they form together quite +90% of the earthworms gathered in any locality belonging to this +region. + +Some of the other Megascolecid peregrine forms will be referred +to later. There is no doubt that the family Lumbricidae offers by +far the greatest number of peregrine forms and that these are most +abundant in collections from extra-European countries, where the +collector has searched in cultivated lands. There are at least eight +or nine species which are common in many parts of the world though +their original home is undoubtedly Europe. + +This is a brief review of the facts, more detailed in some cases than +in others. It remains to review and compare the results arrived at. + +The first general statement that may be made is that this faculty +of extending their range beyond the limits assigned by nature is +not confined to any one family. For all the chief sub-divisions of +the terrestrial Oligochaeta seem to possess it, though in unequal +degrees. But the inequality may be more apparent than real. For if +it be recollected that the species of the family Megascolecidae are +very much more numerous than those of the Eudrilidae or even the +Geoscolecidae, the fact that there are more peregrine Megascolecidae +will lose some of its importance. With the Lumbricidae the case seems +to me to be different. Here the preponderance, not only in species +(relatively speaking) but in individuals, is much above that of other +families. This preponderance I should be disposed to assign to the +newness of the family coupled with the vigour seen in new races. That +this is a possible explanation is borne out by the fact that the +'Perichaetidae' (_i.e._ the genus _Pheretima_) is the most salient +race of peregrine Megascolecidae, and it is now generally held that +this group is the most modern of that enormous family. + +Another general statement may be made with even more confidence, +viz. that it appears to be an undoubted fact that some species are +more capable of extending themselves than others. Thus _Eudrilus +eugeniae_ occurs everywhere on the great land masses of the globe, +except in Europe; it is in fact circummundane in the tropical zone, +as is also _Pontoscolex_. _Dichogaster bolavi_ is again a trifle more +restricted in its range, having been recorded from tropical Africa, +South America, West Indies, Madagascar, and India. Its occurrence +near Hamburg in Europe is also to be noted. A little more restricted +still is _Nematogenia panamaensis_ whose range is in Central America, +tropical West Africa, and Ceylon. Lastly there are cases such as +_Pheretima taprobanae_ which, a native of Ceylon, is also found in +Madagascar. + +It may be asserted in the third place that there are no peculiarities +of structure shared by all of these peregrine forms which might +account for their physiological similarity, except indeed the +somewhat negative feature which they have in common, that is of +being of small or moderate size. _Eudrilus_ and _Pontoscolex_ are +not isolated types in their respective families; nor do they seem +to approach each other in any respect. Nor can it fairly be said +that these peregrine species are marked by any great variability of +structure as compared with other forms, which might allow for their +suiting themselves to various climates and conditions. It is true +that _Eudrilus eugeniae_ has received many names which might at first +argue some variability. But these names have been perhaps given by +persons rather under the influence of the idea that remote habitat +implied specific difference, and who were thus inclined to see minute +differences, and who perhaps were furthermore led astray in the +matter by imperfectly accurate descriptions on the part of others. +Certainly some of the peregrine species of _Pheretima_ are subject to +some variation, particularly in the number and arrangement of their +genital pupillae. But this feature is by no means confined to those +species and cannot be utilised as in any way an adaptation to wide +distribution. + +But while we can lay down no general explanation of the phenomenon, +it is possible to furnish some explanation of particular cases. Thus +the genus _Microscolex_ is the only exotic genus which appears to +have established itself in Europe, from which country indeed it was +early known as an apparently indigenous inhabitant. We must put this +and some similar cases down to ability to do without great heat. +It is probable in fact that the original home of _Microscolex_ is +the antarctic half of the globe; and this of itself would allow of +its establishing a new home in the northern hemisphere, did other +circumstances allow of it. + +It might be urged that this genus has been able to establish itself +in Europe because it has in fact had the chance denied to other +species. There are a good many, however, which would in that case be +in the same category. Some years ago I received from time to time a +very large number of earthworms from the Royal Gardens at Kew which +had been accidentally imported thither from many quarters of the +globe, among which I described some eighteen or twenty new species +including, for instance, the African genus _Gordiodrilus_. There are +plenty of facts of a similar nature and Dr Michaelsen has pointed out +that botanical gardens act as centres of dispersion for accidentally +introduced Oligochaeta. We must therefore come to the conclusion +that temperature is at least one of the causes of a difference in +the capability of extending their range shown by the Oligochaeta, a +cause which doubtless operates as a check upon extension of range in +non-peregrine forms also, and prevents for instance the dispersion of +the tropical African Eudrilidae into the region of the Cape. + +We may, as it appears to me, confidently look upon indifference +to varying temperature as a condition of ability to colonise new +countries. But it is obvious that this is not of itself a sufficient +cause to explain the facts. Otherwise this country and N. Europe +would contain many antarctic earthworms; the only one that has been +recorded to my knowledge is _Microscolex_. + +Though an inability to endure a temperate climate may have rendered +the movements of peregrine species more limited, the same or rather +the exactly opposite cause does not seem to have played any important +part in this direction. For it is above all the Lumbricidae, normally +dwellers in temperate climates, that are so remarkable for their +wide range over the world. Nor can it be convincingly asserted that +the extra-Palaearctic Lumbricids are real indigenes of those--often +tropical--countries. For if so we should expect them to be at +least of different species. Lumbricids however from South America, +Australia, etc., are specifically identical with European forms. + +There is no doubt that wherever land has been at all long under +cultivation in any part of the world that land will be found to +produce species of the European genera _Lumbricus_, _Helodrilus_, +_Eisenia_, etc. More than this the recently imported European forms +will be found to have largely or almost entirely driven out the +native species, which have retired more into the interior of the +country. There is thus here no barrier placed by temperature. It +should be remarked, however, that while these earthworms are most +abundant in the less tropical regions, they occur in such tropical +districts as Peru, though in less striking numbers. Whether those +of North America are really indigenes or not remains perhaps a +matter for discussion; but it is at least noteworthy that the vast +preponderance of species occurring there are also European and even +British. In this particular case, which is on the whole the most +emphatic of all the cases of peregrine earthworms, some explanations +are possible, or at least have been offered. In the first place +it would appear that earthworms are more abundant as individuals +in northern countries where the soil is rarely dry for prolonged +periods. And as has been already pointed out there is a close +relation between earthworms and agriculture. Dunghills are fertile +gathering grounds for some species, and ploughed fields and gardens +always swarm with several species. In the tropics these animals are +not so evident; and the strong rays of the sun appear to drive them +further underground and into marshes; this obviously lessens the +chance of their accidental transference by man. Furthermore Dr Eisen +has pointed out that the European species are apt to have clitella +and to be fertile all the year round, which is not always the case +with other genera. That naturalist has added to this observation the +fact that in rich cultivated soils in California it is impossible to +find anything but imported European species, since cultivation itself +appears actually to drive away the native forms. + + + + +CHAPTER VII + +THE EARTHWORMS OF OCEANIC ISLANDS + + +Oceanic islands are islands that have always been islands, a +definition that seems tautological until we compare it with some +other land masses that may be termed 'islands.' Geology teaches us +in fact that from the point of view of their origin islands may be +divided into two quite sharply contrasted classes. There are those +detached land masses usually lying near to or comparatively near to +some continent, which have been in the course of time detached by the +action of the waves from that continent, such as for instance the +British Isles, which undoubtedly represent a portion of the European +continent which was once quite continuous with Europe. On the other +hand we have the Hawaiian archipelago, St Helena, Fernando Noronha, +and other similar islands, which are more remote in their position +from continents and concerning which it seems clear that they have +originated _de novo_ by the action of submarine volcanos or of the +growth of coral, combined with subsidence, following elevation, or +from several of the causes combined. In any case the islands which +are termed oceanic islands have never formed part of a continent. +They are not relics of previously existing continents. It becomes +a matter of great interest to compare the earthworms which are to +be found upon oceanic islands with those which inhabit continental +islands. Fortunately there are a good many facts at our disposal for +this purpose; and we shall compare the earthworms of the Hawaiian +archipelago with those which are found upon certain small islands +lying to the south of New Zealand, viz. Campbell and Auckland islands +and the more southern Macquarie islands. + +The earthworms of the Hawaiian archipelago have been studied by a +good many persons, and altogether a number of species have been +described from that group of islands of which the following is a +list: _Pheretima hawayana_, _P. heterochaeta_, _P. peregrina_, +_P. schmardae_, _P. hesperidum_, _P. morrisi_, _P. perkinsi_, +_P. biserialis_ (= _P. elongata_), _Allolobophora putris_ (= +Kinberg's _Hypogaeon havaicum_), _A. foetida_, _A. caliginosa_, _A. +nordenskiöldi_, _A. limicola_, _A. rosea_, and finally the well-known +_Pontoscolex corethrurus_. Of these species there is only one which +is even possibly a form limited to the Sandwich Islands, and that is +_Pheretima perkinsi_, a species which I myself at first described as +a new form, but which was afterwards regarded as identical with _P. +heterochaeta_ by Michaelsen, and later still resuscitated by Ude. All +the others are found in many parts of the world and not only in the +nearest mainland to the archipelago which we are now considering. +I have had already occasion to speak of some of them as peregrine +forms, especially of _Pontoscolex corethrurus_ which occurs all over +the world. + +The conditions which have been recently revealed by an exploration +of the antarctic islands mentioned above are totally different. Dr +Benham has enumerated the following species from those islands, +viz. _Notiodrilus haplocystis_, _N. fallax_, _N. aucklandicus_, +_N. campbellianus_, _N. macquariensis_, _Plagiochaeta plunketi_, +_Rhododrilus cockayni_, _Leptodrilus leptomerus_, _L. magneticus_, +_Plutellus aucklandicus_, _Diporochaeta heterochaeta_, _D. +brachysoma_, _D. helophila_, _D. perionychopsis_ among the +Megascolecidae, besides _Phreodrilus campbellianus_, _Pelodrilus +tuberculatus_, _P. aucklandicus_ and the Lumbricid _Helodrilus +constrictus_. There were also four species of purely aquatic +Oligochaeta which we shall leave aside from the present enumeration, +as their range in space is a matter requiring a different explanation +from that of the terrestrial forms. Here we have a series of worms, +all of which, save the widely spread Lumbricid, are apparently +absolutely indigenous to the islands mentioned since they are all +different as _species_ from those found elsewhere. Indeed there is +a whole genus _Leptodrilus_, consisting, it is true, of but two +species, which is a native of the Campbell and Auckland islands and +of those only. The other genera are found in the antarctic region, +while _Pelodrilus_ is still more widely spread. + +These facts as will be observed contrast about as strongly as they +can with those supplied by the fauna of Honolulu and its adjacent +islands. Not only are the worms of the antarctic islands different +species from those found elsewhere, but the majority of them do not +consist of widely ranging peregrine forms. It appears therefore most +probable that these islands are not oceanic islands but a portion of +the former existing northern portion of the antarctic continent. Were +the species _identical_ with those of New Zealand this conclusion +would have of course to be reconsidered. The barriers to migration +(see chap. VIII) explain the contrast recorded in the foregoing +pages. + + + + +CHAPTER VIII + +MOVEMENT AND MIGRATION AMONG EARTHWORMS + + +That earthworms can move upon the surface of the ground at a rapid +pace is probably well enough known to everyone, and that they can +also burrow with considerable celerity. Multiplying the inches of +progress in minutes of time by centuries with the resulting miles, it +is quite clear that there is no reason to suppose that an individual +earthworm might not enormously extend its range under favourable +circumstances. Their powers of locomotion are such that they could +in the course of comparatively few centuries people a continent. As +a matter of fact these animals are frequently very widely spread +upon a given land surface; but on the other hand they are sometimes +equally limited. It behoves us therefore to enquire the reasons +for the possibility of extended migration and the causes which +have led to its restriction. We are now, it must be borne in mind, +considering these animals as purely terrestrial animals moving over +the surface of the land by their own unaided efforts. We leave out +of consideration any possible assistance in crossing water, whether +fresh or salt. We have to consider in fact in the present section +the earthworm inhabitants of larger and smaller tracts of continuous +land such as the African continent, which will serve as an excellent +example wherewith to test the facts and inferences. + +And as a 'control' we can compare this continent with the very +different continent of Europe. + +As an excellent instance, because of the certitude of specific and +in most cases of generic distinctions, we may take the Eudrilidae as +illustrative of the facts that are to be considered in the present +section. That family consists, as will be remembered, of 33 genera +at most, which have the following more exact range on the African +continent. The genus _Eudriloides_ occurs in British and German East +Africa and has been met with as far south as Mosambique and even +Durban, in which latter locality it has been thought that it is +really an accidentally introduced stranger. _Platydrilus_ is limited +to eastern equatorial Africa, thus not having quite the range of +_Eudriloides_. + +The small genera (that is small in numbers of species) +_Reithrodrilus_, _Bogertia_, _Megachaetina_, _Metadrilus_, _Notykus_ +have the same limitation of range as the last genus. _Metschaina_ +has a wider range from tropical North East to lake Tanganyika. +_Stuhlmannia_ has a wider range still being found as it is in the +Tanganyika district, in tropical North East Africa, and in British +and German East Africa near the coast. _Pareudrilus_ reaches still +further north while _Nemertodrilus_ is limited to the Mosambique +region and to the Orange River district further south. The only +remaining genus of this sub-family of the Eudrilidae is _Libyodrilus_ +which is purely West African and equatorial. + +Of the remaining genera which are usually grouped together +into a second sub-family, five, viz. _Malodrilus_, _Kaffania_, +_Gardullaria_, _Teleudrilus_ and _Teleutoreutus_, are confined to +tropical North East Africa. _Eminoscolex_ occurs in the same district +but also to the south in the great lake region. The most remarkable +fact about this genus is that one species _E. steindachneri_ comes +from the Cameroons, and another _E. congicus_ from the Congo, +and thus the range of the genus is right across the continent. +_Neumanniella_ has much the same range. _Polytoreutus_ is a purely +equatorial East and Central genus, reaching from the coast to the +lakes. _Bettonia_ known by three species is from British East Africa. + +The remaining genera, viz. _Hyperiodrilus_, _Heliodrilus_, _Alvania_, +_Iridodrilus_, _Rosadrilus_, _Euscolex_, _Parascolex_, _Preussiella_, +_Buttneriodrilus_, _Beddardiella_, _Metascolex_, are all West African +and the vast majority equatorial. We thus see that with one exception +the genera of East Africa are totally different from those of West +Africa and that the family as a whole is restricted in its range to +a comparatively small part of the vast African continent. It also +obviously follows, and it is advisable to state this fact however +obvious, that no species are common to the two sides of the continent +except indeed the ubiquitous _Eudrilus_, whose range over the world +has been more than once referred to in this book. + +On the other hand the genus _Dichogaster_ offers quite different +facts, which are in contradiction to those just enumerated. This +genus as already said is very characteristic of tropical Africa, +and a large preponderance of the known species are confined to that +continent. Although there is some variation in structural characters +among the many species which compose this genus, there is but little +doubt that they are all rightly referred to one genus with perhaps +some doubtful, though not very striking, exceptions. In any case the +utmost divergence of structure between worms usually placed together +in this genus is nowhere near to that which separates the genera of +Eudrilidae from each other. Of the African members of the genus the +species are pretty evenly divided between the eastern and western +halves of the continent; they are, like the Eudrilidae, tropical in +range, not occurring to the southward, where their place is taken by +the Acanthodrilinae and Geoscolecidae. There are it is true a few +species, such as _D. gracilis_ and _D. bolavi_, which are common to +the two sides of Africa; but in these cases we clearly have to do +with those rather mysterious species which can apparently unduly +extend their range and which are known as peregrine forms; for they +also occur in other parts of the world besides Africa. We have +therefore in _Dichogaster_ the case of a genus which ranges all over +the tropical parts of Africa, but whose species are not common to the +Atlantic and Indian shores of that continent. + +We will now contrast these conditions, which exemplify certain +facts shown by the characteristic Oligochaeta of tropical Africa, +with those which obtain in Europe. In this region of the world the +prevalent and practically the only genera which need be taken into +consideration in surveying the Oligochaetous fauna from the present +point of view, are _Lumbricus_ and the genus _Allolobophora_ of +Eisen which has been variously rearranged into genera and sub-genera +known by the names of _Helodrilus_, _Bimastos_, _Octolasium_, etc. +The structural differences which divide these genera and sub-genera +are not great; in any case they do not exhibit such a wide range +of variation from each other as do two such Eudrilid genera as +_Stuhlmannia_ and _Hyperiodrilus_. We find the genera mentioned not +only in Europe but extending themselves over more or less of Asia, +even occurring in Japan; while the North American continent contains +also representatives of the same. Not only do we find this community +of genera over vast extents of country greater in diameter than +the African continent, but there are also many species which range +as widely or nearly as widely as the case may be as the genus to +which they belong. Thus the species of _Allolobophora_ (we do not +trouble about the newer sub-divisions as they hardly affect the +facts to be emphasised), _A. caliginosa_, _A. longa_, _A. rubida_, +_A. chlorotica_, _A. octaedra_, _A. constricta_, _A. beddardi_, +_Lumbricus terrestris_, _L. castaneus_, have an enormously wide range +over what is generally termed the Palaearctic region, extending +also in some cases into the Nearctic. It is true no doubt that the +majority, indeed perhaps all, of these are, like certain species of +_Dichogaster_ mentioned above, among those forms termed peregrine +which have the capability of living in every quarter of the globe to +which they have apparently been conveyed by man. But there remain +many species which have a very extended habitat in the northern +hemisphere, and in any case the genera and the species are there +truly indigenous and widely spread. + +It would thus appear that the capability for independent migration +varies greatly among earthworms. Of the types selected for +consideration the Eudrilidae are the slowest movers; the genus +_Dichogaster_ comes next, while the power of migration possessed by +the genera _Allolobophora_ and _Lumbricus_ is very much greater. +Assuming for the moment the correctness of this inference it is +clear that it will influence many other propositions connected +with the relative age of the families of these worms and with many +problems of geographical distribution. It appears to us that this +simple explanation is the correct one. But to show this it will be +necessary to eliminate other possible explanations. It might be +urged that the wider range of the genus _Dichogaster_ and the still +wider range of the genus _Allolobophora_ (shown by community of +species in widely distant localities) was evidence merely of relative +age, that the older groups have had more time to travel and that +the newer groups have not had so long a time to spread themselves +over their habitat. On this hypothesis the genera of Eudrilidae +would be geologically much newer than the genus _Dichogaster_ and +similar statements might be made for the other forms here under +consideration. As already explained we cannot attempt to answer this +question in the only way in which it can be really satisfactorily +answered, by a reference to fossil forms; for there are no fossils +to refer to. So far as comparative anatomy enables us to arrive +towards a solution of the question, it would appear that the genus +_Dichogaster_ belongs to a more ancient race than either of the other +two groups considered, and that of these latter the Lumbricidae are +the most modern. Moreover we associate not only a wide, but also +a discontinuous, distribution with an archaic race; and for this +reason also we should place the genus _Dichogaster_ in the position +of being the most ancient of these Oligochaeta. For the genus occurs +in Central America and in certain parts of the East as well as in +Africa. So that we can fairly dismiss the view that the Lumbricids by +virtue of their greater range over a given area are the most ancient +type and that their range is associated merely with their antiquity. +Nor does it appear that geographical or meteorological consideration +can have had effect in the present instances. For conditions +favourable to earthworms prevail in tropical Africa, as in Europe and +much of North Asia. + + +CLIMATE AS AFFECTING MIGRATION. + +That excessively rigorous climatic conditions affect the range of +earthworms as well as fresh-water forms is quite clear from the +conditions which obtain in the most northern climes. At any rate +in those regions where physical conditions render it impossible +for these Annelids to have their being. A perpetual mantle of snow +and a temperature far below freezing point are absolute barriers +to the extension of range. And yet there are some few Oligochaeta +which do not in the least suffer from a somewhat milder taste of +such conditions. Thus species of Enchytraeidae have been met with +on glaciers and even found in frozen water, while a few earthworms +have been brought from the island of Kolguev. These however are +quite exceptions to the general sterility as regards earthworms of +the excessively cold regions. We have already seen that there are +no general facts to be deduced as concerning the relative abundance +of terrestrial worms in the tropics and in more temperate climes. +Tropical Africa is, it is true, rich in genera and species; but on +the other hand tropical East Indies have but few genera inhabiting +their numerous islands. Temperate England has very few genera and not +a large number of species; temperate New Zealand has a considerable +number of different indigenous genera. When however we leave this +general aspect of the question and consider separate families and +genera, there seems to be some little relation between climate and +distribution and thus some effect of climate in acting as a barrier +to migration. For example, though continuity of land surface permits +of the tropical African Eudrilidae ranging southwards as far as the +Cape they are not met with so far as we know in the most southern +parts of Africa; nor are the South American Geoscolecidae found in +Patagonia or northward beyond Central America. These instances do +really look like an influence of climate upon range. On the other +hand we must be careful to eliminate the possibility of another +explanation and that is the impossibility of successful migration +owing to the previous occupation of the ground with abundant other +forms. The very same countries would appear to show that this +explanation is unnecessary. For the prevalent genus of the southern +tracts of South America _Notiodrilus_ extends its way northward as +does the same genus from temperate to tropical Africa and Madagascar. + +It looks very much, therefore, as if certain Oligochaeta are +dependent upon climate for their range, and as if others were at +least more independent of climatic conditions. And there are other +facts which support this view. The same opinion is supported by +the phenomena of involuntary migration, a subject which has been +considered also separately under the head of 'Peregrine forms.' The +great prevalence of Lumbricidae accidentally imported into many +parts of the world shows that temperature is no real bar to their +voluntary migration. On the other hand the fact that specimens of the +East Indian genus _Pheretima_ though commonly imported accidentally +into the warmer regions of the world have not been able to make good +a footing in Europe, save in greenhouses, shows that this genus is +affected in its range by questions of climate. These facts suggest +another inference of great interest which can only be mentioned +tentatively, and not put forward as a demonstrated conclusion. Seeing +that _Lumbricus_ (_sensu lato_) can comfortably take up its home +in warm extra-European countries, but yet that it has evidently +not spread to those countries in the course of nature but by man's +interference, it seems possible that time alone has prevented this; +and that therefore this family Lumbricidae is one of the most +recently evolved families of Oligochaeta. Certain structural features +support this way of looking at the matter. The same arguments +precisely apply to the genus _Pheretima_, which is also regarded by +most systematists as a recently developed race of earthworms. Anyhow +the conclusion which the facts seem to warrant is that the effects +of climate in influencing distribution are seen to have an unequal +effect upon earthworms, some genera being debarred by climatic +conditions while others are indifferent to the same. + + +MOUNTAIN RANGES AND THE MIGRATION OF EARTHWORMS. + +In many groups of animals the interposition of a lofty chain of +mountains presents an insuperable barrier to migration. The barrier +is effective for more than one reason. Lack of vegetation and a +differing climate are among the more obvious causes which render +Alpine chains important as affecting distribution. There is plenty +of evidence in the way of positive fact that mountains are not +necessarily barriers to the spread of earthworms. The recent +explorations of the Ruwenzori chain of mountains in Africa have +resulted in the collection of a considerable number of species, some +of which come from great altitudes (_e.g._ 4000 metres and slightly +upwards), and one species, viz. _Dichogaster duwonica_, which Dr +Cognetti de Martiis described from the foot of the glacier Elena. I +have in my temporary possession a number of examples of the eastern +genus _Pheretima_, some of which are new species from lofty areas in +the Philippine Islands. There are plenty of other examples pointing +to a like conclusion. It is noteworthy that these forms which have +been met with at lofty heights are not essentially different from +the plain living forms. One cannot exactly speak, at any rate in the +present state of our knowledge, of anything like an Alpine fauna. + +It is in fact clear enough that whatever may prove to be the +case with regard to particular species, a mountain range is not +necessarily a barrier to the dispersal of generic types. + + +THE OCEAN AS A BARRIER TO MIGRATION. + +It is very possible that further investigations into the Oligochaeta +will prove that there are more marine forms than those which are +enumerated in another chapter. Particularly is this likely to be +the case among the family Tubificidae and Naididae. For up to the +present those forms belonging to those families which are known to be +positively marine in their habit show no great difference from allies +inhabiting fresh water, and are in one case indeed (_Paranais_) +common to fresh brackish and saline waters. As to earthworms, +the number is also extremely limited, and _Pontodrilus_ is up to +the present the only genus which is known to inhabit a marine +situation almost exclusively. It has, moreover, been shown that both +earthworms and their cocoons are susceptible to salt water and are +killed thereby. Thus the facilities which these animals possess of +crossing tracts of ocean are limited by this fact alone, besides +other impediments offered by tracts of water as such. We may in fact +entirely discount the possibility of earthworms floating across +arms of the sea--of any extent at any rate. For they do not swim or +float, but sink in water. Possibly when the alimentary tract was +entirely empty of earth the worms might float; but it is always full +and even if evacuated during their passage to the bottom waters the +body thus freed would hardly rise. However the noxious qualities of +sea water to earthworms is a sufficient barrier to their traversing +even narrow straits. On the other hand it might be suggested that +torn up trees especially with the roots and clinging earth still +attached might harbour worms and thus transmit them to foreign +shores. It has been suggested that in this or in some similar way +the species of _Notiodrilus_ have been wafted from shore to shore +of those lands which are washed by the Antarctic Ocean. Dr Benham, +however, in criticising this, calls attention to the violent gales +and disturbances of the ocean surface which are so prevalent in those +stormy regions, and doubts much whether these animals could retain +a safe hold upon some travelling tree trunk. Moreover it is only +in this antarctic region where the earthworm fauna of the various +continents and islands are so very similar. + + +FACILITIES OF MIGRATION. + +The above brief account of physical features which affect the +range in space of the terrestrial Oligochaeta seem to show that +the only really important barrier is the ocean; and even a narrow +tract of sea water would, as it appears, act fatally in preventing +the successful immigration of a race inhabiting one shore to the +opposite shore. On the other hand we do undoubtedly find in different +countries--even when separated by a large expanse of ocean--closely +related forms. The most striking instance of this is that afforded +by a consideration of the antarctic species of _Notiodrilus_ and +_Chilota_. Can this interchange of Oligochaetous faunas be explained +by any means which earthworms possess of crossing tracts of sea by +the aid of living carriers such as birds? It has been definitely +shown that these creatures actually do convey such small animals as +Mollusca attached to their feet. Is anything of the kind likely in +the case of earthworms? In the first place it may be safely asserted +that if it be possible it has not been actually proved. This however +might be perhaps put down to the lack of sufficient observation of +actual birds and the contents of such masses of soil as are found +attached to their feet. A consideration of the habits of earthworms +seems to imply that such a mode of transference from country to +country is unlikely. In the first place we remark that the general +behaviour of earthworms renders this unlikely. Even the smaller +kinds, whose bulk would allow of their being carried, are too active +in their habits to permit of a safe transference. When disturbed +they wriggle and progress with activity. It is not conceivable that +they would remain quiescent for sufficient time to allow of a long +voyage. But while the bodily transference of adult earthworms seems +highly improbable it is conceivable at the first view that their +cocoons might be so transferred. We require to know rather more +about the cocoons of earthworms before we can accept this view as a +possibility; as far as our present knowledge goes it is not likely +that these animals can be assisted to emigrate in this way. + +For the cocoons are rather bulky for this kind of porterage. +Moreover they are apt to be deposited rather deep down and among the +roots of grasses, and in situations where they are not so likely to +become entangled in the feet of drinking birds. Assuming, however, +that these difficulties can be got over there remains another +difficulty. A single cocoon among the terrestrial Oligochaeta does +not contain a large number of embryos, as has been pointed out on a +previous page. It is true that _Allolobophora foetida_ has six within +one cocoon, but most of our indigenous forms have but from one to +three embryos in a single cocoon. Thus, if successfully imported, it +is hardly likely that the developed embryos scattered after their +emergence would come together for breeding purposes; and in cocoons +with but one embryo the accidental importation in this way would have +to be very frequent to produce any result. + +The case here is exactly the reverse of that afforded by the aquatic +families (or many of them). In these Annelids the attachment of the +cocoon to water plants, which are liable to be entangled in the feet +of shore-frequenting birds, would tend to favour migration. And in +addition to this the cocoons are naturally smaller and often contain +a considerable number of embryos. We are to note that the aquatic +forms are on the whole distinctly wider in their range than are the +earthworms. + + + + +CHAPTER IX + +THE GEOGRAPHICAL DISTRIBUTION OF EARTHWORMS + + +The facts referred to and considered in the last chapter lead to +further observations upon the geographical distribution of this group +of animals and suggest problems for solution. + +It is not the place here to give a general sketch of the division of +Biology termed Zoogeography; but a few general conclusions must be +laid before the reader in order to render what follows intelligible. +It is universally agreed that the range in space (and in time also) +of a given species of animal (or plant) is as much a part of its +scientific definition as are its anatomical characters. A description +for instance of _Acanthodrilus ungulatus_ is incomplete without a +reference to the fact that it occurs in, and is confined to, the +island of New Caledonia. + +Each continent or island or part of a continent and part of an +island has its own peculiar inhabitants as well as some others +which range beyond its confines. Thus as we have seen the genus +_Hyperiodrilus_ is confined to the tropical West of Africa while the +genus _Dichogaster_ also found in that region is also met with in +other parts of Africa as well as in certain parts of America and of +the East. In this way the entire globe may be mapped out into regions +characterised by their inhabitants and these regions may also be +further subdivided. The commonly accepted regions were originally +devised by Mr Sclater and are known as the Palaearctic, Nearctic, +Neotropic, Ethiopian, Oriental (Mr Sclater's name was 'Indian'), +and Australian. These regions were originally formed to convey the +facts relative to the distribution of Passerine birds only; but +it is generally held that they apply also to the distribution of +vertebrates generally. The science of zoogeography does not however +end with the display of maps conveying graphically the mere facts of +distribution of this group and that. Its business is also to enquire +into the causes of the affinities between the faunas of different +regions or the varying degree of remoteness which those faunas may +show. On the one hand the varying powers of dispersal and the means +of extending their range possessed by different animals have to be +considered, and on the other hand geological changes in the relative +position of land masses have to be taken into account. + +The specific identity between the earthworms of Great Britain and +the adjacent part of the continent of Europe would be very difficult +to understand were we only acquainted with the fact that salt water +is fatal to these animals. But we also know from geology that it +was only at a very recent date that England was cut off from union +with the continent. Thus an identity of fauna was to be expected. +On the other hand we are confronted with a very great difference +between the earthworms of eastern tropical Africa and of the adjacent +island of Madagascar. In the latter we have as a prevalent form the +genus _Kynotus_; in the former continent many Geoscolecidae but no +_Kynotus_. It is believed that the separation of Madagascar from +the mainland was at an earlier date than that of Great Britain from +Europe. We must however be cautious before slipping into what might +seem a case of arguing in a circle. It will however probably not be +disputed that Madagascar was severed earlier than England. + +We will now attempt to map out the world into a series of regions +characterised by their earthworm inhabitants and see how far these +regions agree with those rendered necessary by the distribution of +some other animals. + +We can to begin with accept the Palaearctic region. The region +however will be a little different from that usually accepted. For +we must probably exclude Japan, whose earthworm fauna contains the +characteristically Eastern genus _Pheretima_. Otherwise we have a +region characterised by the family Lumbricidae, which is really +limited to it, and by just a few traces of other genera such as +_Hormogaster_ among the Geoscolecidae and _Sparganophilus_ which +however is possibly an accidental immigrant. This region is certainly +quite clear. Now according to some persons such as Prof. Heilprin the +northern part of America should be joined with Europe and Asia to +form an Holarctic region; while by most authors, the separate name of +Nearctic is given to the north of the New World. With regard to the +terrestrial Oligochaeta it appears to me that this part of the world +is possibly to be excluded altogether as possessing no indigenous +worms. + +In considering the distribution of the Mammalia Sir Ray Lankester +excluded New Zealand from his view as never having possessed any +indigenous mammalian fauna, and termed this part of the world +Atheriogaea. In the same way it is possible that the northern part +of the United States and Canada, whose earthworm fauna consists of +species of Lumbricidae identical with those of Europe, may possibly +be also a region to be excluded in the present survey and spoken of +as 'Ascolecogaea.' In the southern part of the United States we shall +find genera which will be considered presently. On the other hand it +is equally conceivable that this part of the world lost its earthworm +fauna through excessive glaciation in the ice age, the forms having +been driven south and are now only gradually making their way +northwards again. In this case the modern earthworm population which +appears to be absent from large tracts of Canada will be simply due +to involuntary migration. These two views must be left for further +development. + +In any case the southern parts of the United States seem to +be separable as a distinct region from South America and to +be characterised by the sub-family Diplocardiinae, the genus +_Diplocardia_ extending as far northwards as the state of Illinois. +The distinctness of such a region however from Central America and +the West Indies is marred by the abundance of _Ocnerodrilus_ of +which Dr Eisen has described so many forms. On the other hand the +West Indies are closely allied in their earthworm fauna to tropical +South America, sharing with that region several forms of Geoscolecids +belonging in both cases invariably to the sub-family Geoscolecinae. +The bulk of the latter are undoubtedly tropical South American in +range and there is no doubt whatever about the distinctness of +this part of the world as a separate region. There is moreover a +further puzzle which confronts us who are trying to delimit an +American region or regions. In North America are species of the +genus _Argilophilus_ which is referred by Michaelsen to the genus +_Plutellus_ which comes from the East and at least one species of +_Megascolides_, also an Eastern genus. + +There is at present no doubt to be thrown upon the indigeneity +of _Plutellus_. The species according to Dr Eisen show every sign +of being genuine inhabitants of California and like certain New +Zealand species such as the _Tokea esculenta_ of Benham (referred by +Michaelsen to the genus _Megascolides_) were eaten by the natives. If +these genera were forms restricted to North America, that is not only +with reference to the rest of America but to the world generally, +there would be as I think no doubt about the practicability of +making a Nearctic region. As it is, it seems to me to suit the +facts of distribution better to regard the whole of the land under +consideration as forming one great Neogaean region with three +sub-regions, the North American, Central American and West Indian, +and tropical South American. This region however will not as I take +it include the southernmost extremity of South America. Here in +Patagonia and in neighbouring islands we have a different earthworm +fauna. It is in fact characterised by the sub-family Acanthodrilinae +of which it is true some members of the genus _Notiodrilus_ extend +further north. I shall however defer this part of the subject until +the more easy delimitations of regions are disposed of. + +Tropical Africa is evidently to be included in a third region +which will be defined by the Eudrilidae, Microchaetinae among the +Geoscolecidae, and by the great prevalence of _Dichogaster_, a genus +whose occurrence in other parts of the tropics is perhaps not yet +explained satisfactorily. Also we may record as characteristic of +this Ethiopian region a few peculiar genera such as _Nannodrilus_ and +_Gordiodrilus_. _Alma_ being a partly aquatic genus is perhaps less +distinctive and as a matter of fact it strays into the Palaearctic +region, being found in the lower waters of the Nile. It will be +observed that with this exception the limits of the Ethiopian region +according to earthworms agrees with that delimitation afforded by a +consideration of other groups since it stops short at the Sahara, +leaving northern Africa to be referred to the Palaearctic region. +At the same time we have an analogy with South America as concerns +the southern extremity of the African continent; here we meet with +_Notiodrilus_ and allied Acanthodrilinae just as in Patagonia and--as +also in that quarter of the world--these forms just stray into the +Ethiopian region above--specimens of _Notiodrilus_ being met with +in Madagascar as well as in tropical Africa. This bit of Africa as +it appears to me must also be cut off from the Ethiopian region and +included in an Antarctic region. Madagascar offers a further problem. +Are we to include this in Ethiopia or speak of a Malagasy region? +Apart from a few forms which are at least possibly to be looked upon +as accidental immigrants, such as members of the genera _Pheretima_ +and _Gordiodrilus_, the fauna of Madagascar consists mainly of +many species of _Kynotus_. This genus, a member of the sub-family +Microchaetinae, of the family Geoscolecidae, affines Madagascar to +Ethiopia and leads me to place both in the same region though we may +doubtless speak of a Malagasy sub-region. + +We have now to consider the eastern region of the world comprising +the two regions known generally to zoogeographers as the Oriental +and Australian. Taking a large view of the range of sub-families +and genera, and endeavouring to make the great regions of the globe +more or less equal, it seems difficult to divide further a region +which shall include all of this vast territory, and which may +therefore be termed Indo-Australian. For we find as characteristic +of the entire stretch of country the great majority of the genera +of the huge family Megascolecidae. Indeed the largest sub-family of +this family, _i.e._ the Megascolecinae, is, save for the mysterious +occurrence of the genera _Plutellus_ and _Megascolides_ in America, +absolutely limited to this area. Another sub-family, that of the +Octochaetinae, is limited to it. So far as concerns the others of +the sub-families of Megascolecidae it is only the Trigastrinae +which occur here (the genus _Eudichogaster_ and a few possibly +introduced species of _Dichogaster_) and a scattered species or two +of _Notiodrilus_ of the sub-family Acanthodrilinae. Again there are a +few and probably introduced species of the sub-family Ocnerodrilinae. +More important still this region has confined to itself the family +Moniligastridae; for a species described some years ago by myself +from the Bahamas is doubtless an introduced form. We have a complete +absence of indigenous Lumbricidae and Geoscolecidae excepting the +aquatic _Glyphidrilus_ of the sub-family Microchaetinae. It is true +that by taking isolated tracts, even large tracts, of this great +regional expanse a sub-division into well characterised regions can +be apparently recognised. But in taking such a step we shall be +confronted with the curious fact that it is rather neighbouring than +widely remote sub-divisions which present the greater differences. + +If we compare for example India and New Zealand we find in common +such striking genera as _Octochaetus_, _Hoplochaetella_ and +_Diporochaeta_; whereas these genera are absent from the intervening +islands of the great Malay archipelago. On the other hand Australia +differs from the comparatively neighbouring islands of Borneo +and others by the absence in those islands of the characteristic +Australian genera such as _Megascolex_, _Notoscolex_, _Plutellus_ +etc. which are in their turn found in India. It is facts like these +which render very difficult the apportioning of the tracts of country +forming the eastern hemisphere into separate regions. + +There is no doubt that the Malay archipelago and the adjacent coasts +of Asia up to Japan differ from both India and Australia by the +almost entire limitation of the genus _Pheretima_ to them; but we +cannot intercalate a region in the middle of another geographical +area in this fashion! + +The limitations of this great Indo-Australian region now demand +consideration. The chief difficulty is offered by the islands of New +Zealand and by some of the smaller islands lying far from but still +in the neighbourhood of New Zealand. Are we to include New Zealand +in this region? There is no doubt that the northern island of New +Zealand is much nearer to Australia in its earthworm fauna than +is the southern island. There are, it is true, a number of genera +peculiar to New Zealand, which are _Rhododrilus_, _Leptodrilus_, +_Maoridrilus_, _Neodrilus_, _Plagiochaeta_, _Pereiodrilus_, +_Dinodrilus_, _Dinodriloides_, but these do not represent the whole +of any family or even sub-family and they have all of them near +relations in other parts of the region as has been pointed out--even +to the peninsula of India itself. Again New Zealand contains members +of the genus _Notiodrilus_, that characteristic Antarctic form. In +fact New Zealand would appear to be a transitional zone between an +Indo-Australian and an Antarctic region. + +The last region into which the world can be divided according to its +fauna of earthworms is an Antarctic. I am of distinct opinion that +this region is quite necessary in spite of the views of some others. +Although the genus _Notiodrilus_ certainly, and _Microscolex_ +possibly, extend into the tropical regions of America, Africa, and +Australia, these species are but few, and the bulk of the species +and of the allied genus _Chilota_ are restricted to the antarctic +quarter of the globe; they also extend all over it, that is to say +in the southernmost parts of South America, in the Cape region of +Africa, in Kerguelen and the Crozet Islands, and in New Zealand, as +well as in the Auckland Islands and other neighbouring islands. It is +true that I have excluded New Zealand from this region on the grounds +that it forms a debateable ground between it and the Indo-Australian. +But apart from this part of the world the rest of the territories +mentioned should be combined to form the antarctic region. + +Having therefore arrived at a mapping out of the world into regions +in accord with its earthworm fauna, it is desirable to ascertain +what light the facts throw upon the geological and evolutionary +questions with which the study of zoogeography deals. The existence +of an antarctic region binding together such distant points as South +Georgia, the Cape of Good Hope and Kerguelen Island, seems to argue +strongly for the former extensions northwards of the antarctic +continent so far north as to embrace these several regions of that +hemisphere. In view of the facts relating to the danger of sea water +to earthworms, to their lack of facilities for migration, other than +unassisted locomotion, points which have been dealt with earlier, it +is difficult to explain their range in the antarctic hemisphere on +other grounds. The very fact that the actual earthworm fauna of New +Zealand has led us on the whole to assign it to the Indo-Australian +regions shows the inherent uselessness of the current view of +zoogeography. For were we to leave the matter here the relationship +of New Zealand to the regions of the world which lie to the south of +it would not be apparent. However, here as in so many cases there is +an antagonism between cut and dried systems and the indications of +evolution. + +This assumed existence of a former antarctic continent which +connected Southern Africa and Southern America as well as various +islands has perhaps a further justification in the distribution of +the Geoscolecidae. This family is divisible into two well-marked +sub-families of which one as has already been mentioned is limited +to South America and another practically to Africa (the exceptions +being species of the largely aquatic _Glyphidrilus_), while a third +sub-family the Criodrilinae is more widely distributed--again in +accordance, one may perhaps assume, with its largely aquatic mode of +life. It is also conceivable that the genus _Dichogaster_ is another +example pointing the same way. The arguments for regarding this genus +as an indigene of the East are not strong. But there is on the +other hand no doubt that the Indian _Eudichogaster_ is very closely +allied to it. But it is by no means excluded from this argument to +suppose that these Trigastrinae owe their likeness to convergence. +At any rate there are examples of equally marked convergence which +seem to be as nearly proved as can be in another though allied +group. The New Zealand _Neodrilus_ is to all intents and purposes a +_Maoridrilus_ in which one of the two pairs of spermiducal glands +and spermathecae has disappeared. It retains the characteristic +alternation in the position of the nephridia of _Maoridrilus_, and +other structural similarities unite the two genera. In the same way +species of _Microscolex_ seem as easily derivable from _Notiodrilus_. +_Microscolex_ and _Neodrilus_ are so near that had we no such hint of +their origin it would be reasonable to place them in the same genus. +They at least show a marked convergence. + +It will be noticed therefore that the facts of their distribution +agree, as it would appear, with the structure of the terrestrial +Oligochaeta. The primitive characters of the genus _Notiodrilus_ are +to be seen in the double spermaries and glands appended to the duct, +and the corresponding spermatheca, in the absence, or very slight +development, of the papillae, so frequent in more specialised genera +such as _Pheretima_, and in the general simplicity of many organs of +the body which are more complicated elsewhere. As one would expect +with an archaic form this genus is widely ranging, being found in all +the principal land masses of the globe except in the Euro-Asiatic +continent. + +Furthermore geographical facts would at least be not contradictory to +the view that this genus, and therefore the terrestrial Oligochaeta +generally, originated in the Antarctic hemisphere and that in pushing +northwards it has given off various descendants which survive in the +various regions of the world. Basing our views of the possibilities +of range among earthworms on the actual facts already dealt with, it +would seem that the peopling of America from Africa or of Africa from +America, if it has occurred, has not taken place through Europe and +the north generally. For otherwise we should expect traces of the +passage. It is true that we actually have _Hormogaster_ as a possible +sign that the Geoscolecidae have passed this way. But that is an +isolated case and may be referred to the extension northwards of this +particular genus rather than as an indication of a whole migration +through those territories. Another conclusion which a collocation of +the various facts brought together in this book appears to lead to is +that the group of the terrestrial Oligochaeta is relatively speaking +a modern one. + +Convinced as we must be of the fact that range is only possible by +unaided locomotion through continuous land areas, the fact that but +few gaps occur in the range of a particular sub-family or lesser +group seems to indicate that no great time has elapsed since the +specialisation of these different forms. The dependence of earthworms +upon vegetable mould also points in the same direction and furnishes +an argument for the belief that these animals only greatly increased +on the advent of abundant dicotyledonous plants, and perhaps indeed +were actually contemporaneous with them. + + + + +LIST OF LITERATURE REFERRING TO EARTHWORMS + + +In the list given below I am only able to mention a few of the larger +works relating to this group. To give anything like a complete list +would demand many pages of titles. From the works selected the reader +can, if it be desired, find his way to the remaining literature of +the group. + + +A. GENERAL WORKS + + Vejdovsky. System und Morphologie der Oligochaeten. Prag, 1884. + + Beddard. A Monograph of the Oligochaeta. Oxford, 1895. + + Michaelsen. Oligochaeten in 'Das Thierreich.' Berlin, 1900. + + Michaelsen. Die Geographische Verbreitung der Oligochaeten, 1903. + + Vaillant. Annelès in Suites à Buffon. Paris, 1886. + + +B. EARTHWORMS OF (1) AUSTRALIA + + Fletcher. A series of papers in Journ. Linn. Soc. New South + Wales, 1886-90. + + Spencer. A series of papers in Proc. Roy. Soc. Victoria, 1892-5. + + Michaelsen. In Die Fauna Südwest-Australiens. Jena, 1907. + + +(2) NEW ZEALAND AND ANTARCTIC ISLANDS + + Benham. Report on Oligochaeta of the Subarctic Islands of New + Zealand. Wellington, N. Z., 1909. + + Benham. A series of papers in Quart. Journ. Micr. Sci. 1904, + Proc. Zool. Soc. 1904, 1905 and Trans. N. Z. Inst., 1901-10. + + Beddard. In Trans. Roy. Soc. Edinb. 1891 and Proc. Zool. Soc. + 1889. + + +(3) ASIA + + Michaelsen. The Oligochaeta of India etc. in Memoirs Indian Mus., + 1909. + + +(4) EUROPE + + Rosa. Revisione dei Lumbricidi. Mem. Acc. Torino, 1893. + + +(5) AFRICA + + Michaelsen. A series of papers in Mitth. Naturhist. Museum. + Hamburg, 1891-1911. + + Beddard. Quart. Journ. Micr. Sci., 1890-95. + + +(6) AMERICA + + Eisen. Mem. Calif. Acad., 1894-96. + + Cognetti de Martiis. Mem. Acc. Torino, 1905-6. + + Rosa. Ibid., 1895. + + Beddard. In Hamburg. Magalh. Reise, 1895 and Nachtrag to same by + Michaelsen. + +Also numerous other works by the above-named authors and by Perrier, +Horst, Ude, Lankester, Stole, Pierantoni, Friend, Stephenson, +Southern, Goodrich, etc., etc. + + + + +INDEX + + + Aeanthodrilinae, 14, 78, 80, 82, 84, 87, 102 + + Acanthodrilus, 129 + + Aeolosoma, 34-36 + + Aeolosomatidae, 33, 34 + + African earthworms, 74 + + Agriodrilus, 31 + + Allolobophora, 62, 85, 111, 117-119, 128 + + Alluroides, 40, 41 + + Alluroididae, 40 + + Allurus, 45 + + Alma, 24, 49, 50, 60, 61, 95, 135 + + Alpine earthworms, 123 + + Alvania, 75, 115 + + American earthworms, 71 + + Amphichaeta, 37 + + Anachaeta, 42 + + Anatomy of earthworms, 1 + + Andiodrilus, 72, 74 + + Andiorrhinus, 72 + + Anteoides, 72 + + Anteus, 72 + + Antarctic earthworms, 86, 138 + + Aptodrilus, 72 + + Aquatic earthworms, 44 + + Aquatic families of worms, 30 + + Arctic earthworms, 88 + + Argilophilus, 93, 133 + + Asiatic earthworms, 80 + + Astacopsidrilus, 37, 38 + + Athecospermia, 39 + + Aulophorus, 36 + + Aurantina, 39 + + Australian earthworms, 83 + + + Barriers to migration, 124 + + Beddardiella, 76, 95, 115 + + Benhamia, 72, 92 + + Bettonia, 76, 115 + + Bimastos, 117 + + Bogertia, 75, 114 + + Books and Memoirs upon earthworms, 144 + + Bothrioneuron, 38, 39 + + Branchiura, 38, 39, 70 + + Branchiodrilus, 36 + + Buttneriodrilus, 75, 115 + + + Callidrilus, 48, 77 + + Cape of Good Hope, worms of, 76 + + Chaetogaster, 37 + + Chilota, 14, 72, 76, 77, 93, 126, 139 + + Claparedilla, 39 + + Climate and distribution, 120 + + Clitellio, 38, 39, 53 + + Cocoons of worms, 62, 127 + + Criodrilinae, 24, 62, 85, 140 + + Criodrilus, 24, 48, 49, 62, 72, 85, 92, 98 + + + Dendrobaena, 85 + + Dero, 36, 37 + + Desmogaster, 81, 94 + + _Diachaeta_, 73 + + Diaphorodrilus, 76 + + Dichogaster, 15, 22, 28, 56, 72, 74, 76, 80, 82, 83, 84, 92, 99, + 104, 116-120, 129, 134, 140 + + Didymogaster, 84 + + Digaster, 84 + + Dinodriloides, 87, 138 + + Dinodrilus, 16, 55, 87, 138 + + Diplocardia, 85, 133 + + Diplotrema, 83 + + Diporochaeta, 19, 80, 84, 87, 94, 111, 137 + + Dispersal of earthworms, 113 + + Drawida, 57, 81 + + + Eclipidrilus, 39 + + Eisenia, 85, 108 + + Eiseniella, 41, 45, 46, 49, 66, 85 + + Eminoscolex, 75, 115 + + Enantiodrilus, 72 + + Enchytraeidae, 31, 41 + + Environment, 59 + + Eudichogaster, 80, 136, 141 + + Eudrilidae, 25, 26, 28, 29, 61, 66, 75, 78, 86, 116, 134 + + Eudrilus, 66, 75, 78, 79, 82, 83, 86, 97, 98, 101, 104, 105 + + Eudriloides, 27, 75, 114 + + Eupolygaster, 57, 81, 94 + + Eupolytoreutus, 76 + + Euscolex, 76, 95, 115 + + Eutyphoeus, 16, 17, 80 + + + Families of worms, 14 + + Fimoscolex, 72 + + Fletcherodrilus, 26, 84 + + Fresh-water worms, 30, 44 + + + Gardullaria, 76, 115 + + Genera, range of, 90 + + Geographical distribution, 129 + + Geoscolecidae, 20, 22, 23, 46, 59, 61, 71, 76, 79, 86, 104, 132, + 134, 136, 137 + + Geoscolecinae, 22, 23, 71 + + Geoscolex (= Glossoscolex) + + Glossoscolex, 23, 72-74 + + Glyphidrilus, 48, 49, 77, 79, 83, 98, 137, 140 + + Gordiodrilus, 76, 78, 81, 92, 99, 106, 135 + + + Haplodrilus, 73 + + Haplotaxidae, 42 + + Haplotaxis, 31, 42 + + Heliodrilus, 75, 115 + + Helodrilus, 79, 85, 90, 91, 103, 108, 112, 117 + + Henlea, 41 + + Hesperodrilus, 38 + + Hesperoscolex, 23, 58, 72, 73 + + Holoscolex, 72, 76 + + Hoplochaetella, 80, 87, 93, 137 + + Hormogaster, 24, 85, 131, 142 + + Hormogastrinae, 24, 85 + + Howascolex, 78 + + Hyperiodrilus, 30, 75, 115, 117, 129 + + Hypogaeon, 111 + + + Ilyodrilus, 38 + + Ilyogenia, 22, 73 + + Iridodrilus, 76, 115 + + India, earthworms of, 79 + + + Japan, earthworms of, 131 + + + Kerguelen, earthworms of, 139 + + Kaffania, 75, 115 + + Kerria, 18, 73, 99 + + Kynotus, 23, 24, 29, 78, 95, 97, 131, 136 + + + Lampito, 78, 80 + + Lamprodrilus, 39, 40 + + Leptodrilus, 86, 111, 112, 138 + + Libyodrilus, 28, 75, 115 + + Light, influence of, 67 + + Limicolae, 30, 47, 58 + + Limnodriloides, 31, 38 + + Limnodrilus, 38, 44 + + Liodrilus, 73 + + Lophochaeta, 38 + + Lumbricidae, 38, 57, 61, 62, 83-86, 90, 103, 104, 107 + + Lumbriculidae, 37, 41, 48 + + Lumbriculus, 39 + + Lumbricus, 85, 90, 108, 120, 122 + + Lycodrilus, 38 + + + Macrochaetina, 37 + + Madagascar, earthworms of, 77 + + Maheina, 14, 78 + + Malay region, earthworms of, 82, 137 + + Malodrilus, 76, 115 + + Man, his influence in distribution, 106, 123 + + Maoridrilus, 11, 14, 87, 95, 138, 141 + + Marine worms, 50 + + Megachaetina, 75, 114 + + Megadrili, 44, 50 + + Megascolecidae, 14, 22, 29, 61, 75, 76, 79, 82, 87, 92, 99, 103, 136 + + Megascolecinae, 19, 22, 80, 82, 84, 87, 136 + + Megascolex, 19, 55, 62, 64, 80, 84, 93, 99, 137 + + Megascolides, 80, 84, 87, 93, 133, 134, 136 + + Mesoporodrilus, 39 + + Metadrilus, 75, 114 + + Metascolex, 76, 115 + + Metschaina, 75, 114 + + Microchaetinae, 24, 48, 76, 134, 136, 137 + + Microchaetus, 23, 77, 95 + + Microdrili, 31 + + Microdrilus, 92 + + Microscolex, 14, 72, 74, 83, 84, 94, 99, 102, 103, 106, 107, 139, 141 + + Migration of earthworms, 113 + + Millsonia, 92 + + Moniligaster, 57, 81, 99 + + Moniligastridae, 29, 41, 57, 61, 81, 83, 86, 94, 98, 137 + + + Naididae, 33-35 + + Naidium, 37 + + Nais, 37 + + Nannodrilus, 18, 76, 135 + + Nemertodrilus, 75, 115 + + Nematogenia, 76, 81, 104 + + Neodrilus, 87, 95, 138, 141 + + Neumanniella, 76, 115 + + New Zealand, earthworms of, 86 + + Nile, worm of, 50 + + Notiodrilus, 14-16, 20, 22, 37, 47, 52, 72, 74, 76, 77, 83, 84, + 86, 91-94, 111, 122, 126, 134-139, 141 + + Notoscolex, 80, 84, 93, 137 + + Notykus, 75, 114 + + + Ocnerodrilinae, 16, 32, 73, 76, 81, 136 + + Ocnerodrilus, 18, 22, 73, 76, 78, 81, 84, 92, 99, 133 + + Octochaetinae, 16, 80, 84 + + Octochaetus, 16, 62, 80, 87, 93, 94, 137 + + Octolasium, 85, 117 + + Oceanic islands, earthworms of, 109 + + Onychochaeta, 23, 72, 73 + + Opisthodrilus, 72 + + + Papillae, 64 + + Paranais, 36, 125 + + Parascolex, 75, 115 + + Pareudrilus, 75, 115 + + Patagonia, worms of, 139 + + Pelodrilus, 42, 111, 112 + + Peloryctes, 39 + + Peregrine species, 96 + + Pereiodrilus, 87, 138 + + Perichaetidae, 104 + + Perionyx, 19, 22, 82, 84, 95, 99 + + Perionychella, 84 + + Periscolex, 23, 56, 72, 73 + + Perissogaster, 84 + + Pheretima, 19, 23, 55, 60-64, 73, 78, 80, 82, 84, 90, 94, 99, 102, + 104, 105, 110, 111, 122-124, 131, 135, 138, 141 + + Phreatothrix, 39 + + Phreodrilidae, 37 + + Phreodrilus, 37, 38, 70, 111 + + Phreodriloides, 38 + + Phreoryctes (= Haplotaxis) + + Plagiochaeta, 15, 55, 87, 111, 138 + + Platydrilus, 75, 114 + + Pleurophleps, 35 + + Plionogaster, 19, 29, 82 + + Plutellus, 80, 84, 87, 93, 95, 111, 133, 134, 136, 137 + + Polytoreutus, 25, 26, 58, 76 + + Pontodrilus, 50, 51, 125 + + Pontoscolex, 20, 22, 23, 66, 72-74, 78, 79, 82, 83, 96, 97, 98, + 101, 102, 104, 105, 111 + + Preussiella, 75, 115 + + Pristina, 37 + + Psammoryctes, 38 + + Pygmæodrilus, 76 + + + Reithrodrilus, 75, 114 + + Rhinodrilus, 72, 73 + + Rhizodrilus, 39 + + Rhododrilus, 86, 111, 138 + + Rhynchelmis, 39 + + Ripistes, 86 + + Rosadrilus, 75, 115 + + + Sea-shore, worms of, 50 + + Sea as a barrier, 124 + + Schmardaella, 36 + + Senses of worms, 65 + + Sparganophilus, 24, 47, 85, 92, 132 + + Spenceriella, 80, 84, 87 + + Structure (_see_ Anatomy) + + Stuhlmannia, 75, 114, 117 + + Stylaria, 37 + + Stylodrilus, 39 + + Sutroa, 39 + + Systematic arrangement, 14 + + + Teleudrilus, 76, 115 + + Teleutoscolex, 39, 40 + + Teleutoreutus, 76, 115 + + Telmatodrilus, 38 + + Temperature and distribution, 120 + + Thamnodrilus (_see_ Rhinodrilus) + + Titanus, 23 + + Tokea, 87, 134 + + Trichochaeta, 23, 53 + + Trichodrilus, 39 + + Trigaster, 80, 84, 85 + + Trigastrinae, 15, 72, 80, 82, 84, 136 + + Tritogenia, 77 + + Tubifex, 38, 44 + + Tubificidae, 33, 37, 38, 48 + + Tykonus, 72 + + + Urobenus, 72 + + Urochaeta, 20 + + + Vejdovskyella, 37 + + Vermiculus, 39 + + + Woodwardia, 80, 82, 84, 93 + + + Yagansia, 14, 72, 74, 76, 93 + + + Zapotecia, 85 + +CAMBRIDGE: PRINTED BY JOHN CLAY, M.A. AT THE UNIVERSITY PRESS + + + + + +End of Project Gutenberg's Earthworms and their Allies, by Frank E. Beddard + +*** END OF THE PROJECT GUTENBERG EBOOK 43363 *** |