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| author | Roger Frank <rfrank@pglaf.org> | 2025-10-15 05:32:06 -0700 |
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| committer | Roger Frank <rfrank@pglaf.org> | 2025-10-15 05:32:06 -0700 |
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diff --git a/8700-h/old/chap16.html b/8700-h/old/chap16.html new file mode 100644 index 0000000..f71c92b --- /dev/null +++ b/8700-h/old/chap16.html @@ -0,0 +1,972 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> +<!-- saved from url=(0036)http://../Haeckel/The Evolution of Man --> +<html> +<head> +<meta name="generator" content="HTML Tidy, see www.w3.org"> +<title>The Evolution of Man: Title</title> +<meta content="text/html; charset=iso-8859-1" http-equiv="Content-Type"> +<meta content="MSHTML 5.00.2919.6307" name="GENERATOR"> +<link rel="stylesheet" href="haeckel.css" type="text/css"> +</head> +<body> +<center>THE EVOLUTION OF MAN<br> +Volume II<br> +<br> +<hr noshade size="1" align="center" width="10%"> +<br> +C<font size="-2">HAPTER</font> XVI<br> +<br> +<b>STRUCTURE OF THE LANCELET AND THE SEA-SQUIRT</b></center> + +<br> + + +<p class="one">In turning from the embryology to the phylogeny of +man—from the development of the individual to that of the +species—we must bear in mind the direct causal connection +that exists between these two main branches of the science of human +evolution. This important causal nexus finds its simplest +expression in “the fundamental law of organic +development,” the content and purport of which we have fully +considered in the first chapter. According to this biogenetic law, +ontogeny is a brief and condensed recapitulation of phylogeny. If +this compendious reproduction were complete in all cases, it would +be very easy to construct the whole story of evolution on an +embryonic basis. When we wanted to know the ancestors of any higher +organism, and, therefore, of man—to know from what forms the +race as a whole has been evolved we should merely have to follow +the series of forms in the development of the individual from the +ovum; we could then regard each of the successive forms as the +representative of an extinct ancestral form. However, this direct +application of ontogenetic facts to phylogenetic ideas is possible, +without limitations, only in a very small section of the animal +kingdom. There are, it is true, still a number of lower +invertebrates (for instance, some of the Zoophyta and Vermalia) in +which we are justified in recognising at once each embryonic form +as the historical reproduction, or silhouette, as it were, of an +extinct ancestor. But in the great majority of the animals, and in +the case of man, this is impossible, because the embryonic forms +themselves have been modified through the change of the conditions +of existence, and have lost their original character to some +extent. During the immeasurable course of organic history, the many +millions of years during which life was developing on our planet, +secondary changes of the embryonic forms have taken place in most +animals. The young of animals (not only detached larvæ, but +also the embryos enclosed in the womb) may be modified by the +influence of the environment, just as well as the mature organisms +are by adaptation to the conditions of life; even species are +altered during the embryonic development. Moreover, it is an +advantage for all higher organisms (and the advantage is greater +the more advanced they are) to curtail and simplify the original +course of development, and thus to obliterate the traces of their +ancestors. The higher the individual organism is in the animal +kingdom, the less completely does it reproduce in its embryonic +development the series of its ancestors, for reasons that are as +yet only partly known to us. The fact is easily proved by comparing +the different developments of higher and lower animals in any +single stem.</p> + +<p>In order to appreciate this important feature, we have +distributed the embryological phenomena in two groups, <i> +palingenetic</i> and <i>cenogenetic.</i> Under palingenesis we +count those facts of embryology that we can directly regard as a +faithful synopsis of the corresponding stem-history. By cenogenesis +we understand those embryonic processes which we cannot directly +correlate with corresponding evolutionary processes, but must +regard as modifications or falsifications of them. With this +careful discrimination between palingenetic and cenogenetic +phenomena, our biogenetic law assumes the following more precise +shape:—The rapid and brief development of the individual +(ontogeny) is a condensed synopsis of the long and slow history of +the stem (phylogeny): this synopsis is the more faithful and +complete in proportion as the original features have been preserved +by heredity, and modifications have not been introduced by +adaptation.</p> + +<br> +<hr> +<p class="page"><a name="page 180">[ 180 ]</a></p> + +<p> </p> + +<p>In order to distinguish correctly between palingenetic and +cenogenetic phenomena in embryology, and deduce sound conclusions +in connection with stem-history, we must especially make a +comparative study of the former. In doing this it is best to employ +the methods that have long been used by geologists for the purpose +of establishing the succession of the sedimentary rocks in the +crust of the earth. This solid crust, which encloses the glowing +central mass like a thin shell, is composed of different kinds of +rocks: there are, firstly, the volcanic rocks which were formed +directly by the cooling at the surface of the molten mass of the +earth; secondly, there are the sedimentary rocks, that have been +made out of the former by the action of water, and have been laid +in successive strata at the bottom of the sea. Each of these +sedimentary strata was at first a soft layer of mud; but in the +course of thousands of years it condensed into a solid, hard mass +of stone (sandstone, limestone, marl, etc.), and at the same time +permanently preserved the solid and imperishable bodies that had +chanced to fall into the soft mud. Among these bodies, which were +either fossilised or left characteristic impressions of their forms +in the soft slime, we have especially the more solid parts of the +animals and plants that lived and died during the deposit of the +slimy strata.</p> + +<p>Hence each of the sedimentary strata has its characteristic +fossils, the remains of the animals and plants that lived during +that particular period of the earth’s history. When we make a +comparative study of these strata, we can survey the whole series +of such periods. All geologists are now agreed that we can +demonstrate a definite historical succession in the strata, and +that the lowest of them were deposited in very remote, and the +uppermost in comparatively recent, times. However, there is no part +of the earth where we find the series of strata in its entirety, or +even approximately complete. The succession of strata and of +corresponding historical periods generally given in geology is an +ideal construction, formed by piecing together the various partial +discoveries of the succession of strata that have been made at +different points of the earth’s surface (cf. Chapter +XVIII).</p> + +<p>We must act in this way in constructing the phylogeny of man. We +must try to piece together a fairly complete picture of the series +of our ancestors from the various phylogenetic fragments that we +find in the different groups of the animal kingdom. We shall see +that we are really in a position to form an approximate picture of +the evolution of man and the mammals by a proper comparison of the +embryology of very different animals—a picture that we could +never have framed from the ontogeny of the mammals alone. As a +result of the above-mentioned cenogenetic processes—those of +disturbed and curtailed heredity—whole series of lower stages +have dropped out in the embryonic development of man and the other +mammals especially from the earliest periods, or been falsified by +modification. But we find these lower stages in their original +purity in the lower vertebrates and their invertebrate ancestors. +Especially in the lowest of all the vertebrates, the lancelet or +Amphioxus, we have the oldest stem-forms completely preserved in +the embryonic development. We also find important evidence in the +fishes, which stand between the lower and higher vertebrates, and +throw further light on the course of evolution in certain periods. +Next to the fishes come the amphibia, from the embryology of which +we can also draw instructive conclusions. They represent the +transition to the higher vertebrates, in which the middle and older +stages of ancestral development have been either distorted or +curtailed, but in which we find the more recent stages of the +phylogenetic process well preserved in ontogeny. We are thus in a +position to form a fairly complete idea of the past development of +man’s ancestors within the vertebrate stem by putting +together and comparing the embryological developments of the +various groups of vertebrates. And when we go below the lowest +vertebrates and compare their embryology with that of their +invertebrate relatives, we can follow the genealogical tree of our +animal ancestors much farther, down to the very lowest groups of +animals.</p> + +<p>In entering the obscure paths of this phylogenetic labyrinth, +clinging to the Ariadne-thread of the biogenetic law and guided by +the light of comparative anatomy, we will first, in accordance with +the methods we have adopted, discover and arrange those fragments +from the manifold embryonic developments of very different animals +from which the stem-history of man can be composed. I would call +attention particularly to the fact that</p> + +<br> +<hr> +<p class="page"><a name="page 181">[ 181 ]</a></p> + +<p> </p> + +<p class="one">we can employ this method with the same confidence +and right as the geologist. No geologist has ever had ocular proof +that the vast rocks that compose our Carboniferous or Jurassic or +Cretaceous strata were really deposited in water. Yet no one doubts +the fact. Further, no geologist has ever learned by direct +observation that these various sedimentary formations were +deposited in a certain order; yet all are agreed as to this order. +This is because the nature and origin of these rocks cannot be +rationally understood unless we assume that they were so deposited. +These hypotheses are universally received as safe and indispensable +“geological theories,” because they alone give a +rational explanation of the strata.</p> + +<p>Our evolutionary hypotheses can claim the same value, for the +same reasons. In formulating them we are acting on the same +inductive and deductive methods, and with almost equal confidence, +as the geologist. We hold them to be correct, and claim the status +of “biological theories” for them, because we cannot +understand the nature and origin of man and the other organisms +without them, and because they alone satisfy our demand for a +knowledge of causes. And just as the geological hypotheses that +were ridiculed as dreams at the beginning of the nineteenth century +are now universally admitted, so our phylogenetic hypotheses, which +are still regarded as fantastic in certain quarters, will sooner or +later be generally received. It is true that, as will soon appear, +our task is not so simple as that of the geologist. It is just as +much more difficult and complex as man’s organisation is more +elaborate than the structure of the rocks.</p> + +<p>When we approach this task, we find an auxiliary of the utmost +importance in the comparative anatomy and embryology of two lower +animal-forms. One of these animals is the lancelet +(<i>Amphioxus</i>), the other the sea-squirt (<i>Ascidia</i>). Both +of these animals are very instructive. Both are at the border +between the two chief divisions of the animal kingdom—the +vertebrates and invertebrates. The vertebrates comprise the already +mentioned classes, from the Amphioxus to man (acrania, lampreys, +fishes, dipneusts, amphibia, reptiles, birds, and mammals). +Following the example of Lamarck, it is usual to put all the other +animals together under the head of invertebrates. But, as I have +often mentioned already, the group is composed of a number of very +different stems. Of these we have no interest just now in the +echinoderms, molluscs, and articulates, as they are independent +branches of the animal-tree, and have nothing to do with the +vertebrates. On the other hand, we are greatly concerned with a +very interesting group that has only recently been carefully +studied, and that has a most important relation to the ancestral +tree of the vertebrates. This is the stem of the Tunicates. One +member of this group, the sea-squirt, very closely approaches the +lowest vertebrate, the Amphioxus, in its essential internal +structure and embryonic development. Until 1866 no one had any idea +of the close connection of these apparently very different animals; +it was a very fortunate accident that the embryology of these +related forms was discovered just at the time when the question of +the descent of the vertebrates from the invertebrates came to the +front. In order to understand it properly, we must first consider +these remarkable animals in their fully-developed forms and compare +their anatomy.</p> + +<p>We begin with the lancelet—after man the most important +and interesting of all animals. Man is at the highest summit, the +lancelet at the lowest root, of the vertebrate stem.</p> + +<p>It lives on the flat, sandy parts of the Mediterranean coast, +partly buried in the sand, and is apparently found in a number of +seas.<sup>1</sup> It has been found in the North Sea (on the +British and Scandinavian coasts and in Heligoland), and at various +places on the Mediterranean (for instance, at Nice, Naples, and +Messina). It is also found on the coast of Brazil and in the most +distant parts of the Pacific Ocean (the coast of Peru, Borneo, +China, Australia, etc.). Recently eight to ten species of the +amphioxus have been determined, distributed in two or three +genera.</p> + +<p>Johannes Müller classed the lancelet with the fishes, +although he pointed out that the differences between this simple +vertebrate and the lowest fishes are much greater than between the +fishes and the amphibia. But this was far from expressing the real +significance of the animal. We may confidently lay down the +following principle: The Amphioxus differs more from the fishes +than the fishes do from</p> + +<p class="fnote">1. See the ample monograph by Arthur Willey, <i> +Amphioxus and the Ancestry of the Vertebrates</i>; Boston, +1894.</p> + +<br> +<hr> +<p class="page"><a name="page 182">[ 182 ]</a></p> + +<p> </p> + +<center> +<table class="capt" width="367" summary= +"Fig. 210. The lancelet (Amphioxus lanceolatus), left view. Fig. 211. Transverse section of the head of the Amphioxus."> +<tr> +<td align="justify"><img src="images3/fig210.GIF" width="367" +height="473" alt= +"Fig. 210. The lancelet (Amphioxus lanceolatus), left view. Fig. 211. Transverse section of the head of the Amphioxus."> +<br> +<a name="Fig. 210">Fig. 210</a>—<b>The lancelet</b> +(<i>Amphioxus lanceolatus</i>), left view. The long axis is +vertical; the mouth-end is above, the tail-end below; <i>a</i> +mouth, surrounded by threads of beard; <i>b</i> anus, <i>c</i> +gill-opening (<i>porus branchialis</i>), <i>d</i> gill-crate, <i> +e</i> stomach, <i>f</i> liver, <i>g</i> small intestine, <i>h</i> +branchial cavity, <i>i</i> chorda (axial rod), underneath it the +aorta; <i>k</i> aortic arches, <i>l</i> trunk of the branchial +artery, <i>m</i> swellings on its branches, <i>n</i> vena cava, <i> +o</i> visceral vein.<br> +<a name="Fig. 211">Fig. 211</a>—<b>Transverse section of the +head of the Amphioxus.</b> (From <i>Boveri.</i>) Above the +branchial gut (<i>kd</i>) is the chorda, above this the neural tube +(in which we can distinguish the inner grey and the outer white +matter); above again is the dorsal fin (<i>fh</i>). To the right +and left above (in the episoma) are the thick muscular plates +(<i>m</i>); below (in the hyposoma) the gonads (<i>g</i>). <i> +ao</i> aorta (here double), <i>c</i> corium, <i>ec</i> endostyl, +<i>f</i> fascie, <i>gl</i> glomerulus of the kidneys, <i>k</i> +branchial vessel, <i>ld</i> partition between the cœloma +(<i>sc</i>) and atrium (<i>p</i>), <i>mt</i> transverse ventral +muscle, <i>n</i> renal canals, of upper and <i>uf</i> lower canals +in the mantle-folds, <i>p</i> peribranchial cavity, (atrium), <i> +sc</i> cœloma (subchordal body-cavity), <i>si</i> principal +(or subintestinal) vein, <i>sk</i> perichorda (skeletal +layer).</td> +</tr> +</table> +</center> + +<p class="one">man and the other vertebrates. As a matter of fact, +it is so different from all the other vertebrates in its whole +organisation that the laws of logical classification compel us to +distinguish two divisions of this stem: 1, the Acrania (Amphioxus +and its extinct relatives); and 2, the Craniota (man and the other +vertebrates). The first and lower division comprises the +vertebrates that have no vertebræ or skull</p> + +<br> +<hr> +<p class="page"><a name="page 183">[ 183 ]</a></p> + +<p> </p> + +<p class="one">(<i>cranium</i>). Of these the only living +representatives are the Amphioxus and Paramphioxus, though there +must have been a number of different species at an early period of +the earth’s history.</p> + +<p>Opposed to the Acrania is the second division of the +vertebrates, which comprises all the other members of the stem, +from the fishes up to man. All these vertebrates have a head quite +distinct from the trunk, with a skull (<i>cranium</i>) and brain; +all have a centralised heart, fully-formed kidneys, etc. Hence they +are called the <i>Craniota.</i> These Craniotes are, however, +without a skull in their earlier period. As we already know from +embryology, even man, like every other mammal, passes in the +earlier course of his development through the important stage which +we call the chordula; at this lower stage the animal has neither +vertebræ nor skull nor limbs <a href="chap10.html#Fig. 83"> +(Figs. 83–86).</a> And even after the formation of the +primitive vertebræ has begun, the segmented fœtus of +the amniotes still has for a long time the simple form of a +lyre-shaped disk or a sandal, without limbs or extremities. When we +compare this embryonic condition, the sandal-shaped fœtus, +with the developed lancelet, we may say that the amphioxus is, in a +certain sense, a permanent sandal-embryo, or a permanent embryonic +form of the Acrania; it never rises above a low grade of +development which we have long since passed.</p> + +<p>The fully-developed lancelet (Fig. 210) is about two inches +long, is colourless or of a light red tint, and has the shape of a +narrow lancet-formed leaf. The body is pointed at both ends, but +much compressed at the sides. There is no trace of limbs. The outer +skin is very thin and delicate, naked, transparent, and composed of +two different layers, a simple external stratum of cells, the +epidermis, and a thin underlying cutis-layer. Along the middle line +of the back runs a narrow fin-fringe which expands behind into an +oval tail-fin, and is continued below in a short anus-fin. The +fin-fringe is supported by a number of square elastic +fin-plates.</p> + +<p>In the middle of the body we find a thin string of cartilage, +which goes the whole length of the body from front to back, and is +pointed at both ends (Fig. 210 <i>i</i>). This straight, +cylindrical rod (somewhat compressed for a time) is the axial rod +or the <i>chorda dorsalis</i>; in the lancelet this is the only +trace of a vertebral column. The chorda develops no further, but +retains its original simplicity throughout life. It is enclosed by +a firm membrane, the chorda-sheath or <i>perichorda.</i> The real +features of this and of its dependent formations are best seen in +the transverse section of the Amphioxus (Fig. 211). The perichorda +forms a cylindrical tube immediately over the chorda, and the +central nervous system, the medullary tube, is enclosed in it. This +important psychic organ also remains in its simplest shape +throughout life, as a cylindrical tube, terminating with almost +equal plainness at either end, and enclosing a narrow canal in its +thick wall. However, the fore end is a little rounder, and contains +a small, almost imperceptible bulbous swelling of the canal. This +must be regarded as the beginning of a rudimentary brain. At the +foremost end of it there is a small black pigment-spot, a +rudimentary eye; and a narrow canal leads to a superficial +sense-organ. In the vicinity of this optic spot we find at the left +side a small ciliated depression, the single olfactory organ. There +is no organ of hearing. This defective development of the higher +sense-organs is probably, in the main, not an original feature, but +a result of degeneration.</p> + +<p>Underneath the axial rod or chorda runs a very simple alimentary +canal, a tube that opens on the ventral side of the animal by a +mouth in front and anus behind. The oval mouth is surrounded by a +ring of cartilage, on which there are twenty to thirty +cartilaginous threads (organs of touch, Fig. 210 <i>a</i>). The +alimentary canal divides into sections of about equal length by a +constriction in the middle. The fore section, or head-gut, serves +for respiration; the hind section, or trunk-gut, for digestion. The +limit of the two alimentary regions is also the limit of the two +parts of the body, the head and the trunk. The head-gut or +branchial gut forms a broad gill-crate, the grilled wall of which +is pierced by numbers of gill-clefts (Fig. 210 <i>d</i>). The fine +bars of the gill-crate between the clefts are strengthened with +firm parallel rods, and these are connected in pairs by cross-rods. +The water that enters the mouth of the Amphioxus passes through +these clefts into the large surrounding branchial cavity or <i> +atrium,</i> and then pours out behind through a hole in it, the +respiratory pore (<i>porus branchialis,</i> Fig. 210 <i>c</i>). +Below, on the ventral side of the gill-crate, there is in the +middle</p> + +<br> +<hr> +<p class="page"><a name="page 184">[ 184 ]</a></p> + +<p> </p> + +<p class="one">line a ciliated groove with a glandular wall (the +hypobranchial groove), which is also found in the Ascidia and the +larvæ of the Cyclostoma. It is interesting because the +thyroid gland in the larynx of the higher vertebrates (underneath +the “Adam’s apple”) has been developed from +it.</p> + +<p>Behind the respiratory part of the gut we have the digestive +section, the trunk or liver (hepatic) gut. The small particles that +the Amphioxus takes in with the water—infusoria, diatoms, +particles of decomposed plants and animals, etc.—pass from +the gill-crate into the digestive part of the canal, and are used +up as food. From a somewhat enlarged portion, that corresponds to +the stomach (Fig. 210 <i>e</i>), a long, pouch-like blind sac +proceeds straight forward (<i>f</i>); it lies underneath on the +left side of the gill-crate, and ends blindly about the middle of +it. This is the liver of the Amphioxus, the simplest kind of liver +that we meet in any vertebrate. In man also the liver develops, as +we shall see, in the shape of a pouch-like blind sac, that forms +out of the alimentary canal behind the stomach.</p> + +<br> + + +<table class="capt" summary= +"Fig. 212. Transverse section of an Amphioxus-larva, with five gill-clefts, through the middle of the body. Fig. 213. Diagram of the preceding."> +<tr> +<td><img src="images3/fig212.GIF" width="252" height="170" alt= +"Fig. 212. Transverse section of an Amphioxus-larva, with five gill-clefts, through the middle of the body. Fig. 213. Diagram of the preceding."> +</td> +<td align="left" valign="bottom"><a name="Fig. 212">Fig. +212</a>—<b>Transverse section of an Amphioxus-larva,</b> with +five gill-clefts, through the middle of the body.<br> +Fig. 213—<b>Diagram of the preceding.</b> (From <i> +Hatschek.</i>) <i>A</i> epidermis, <i>B</i> medullary tube, <i> +C</i> chorda, <i>C</i><sub>1</sub> inner chorda-sheath, <i>D</i> +visceral epithelium, <i>E</i> sub-intestinal vein. <i>1</i> cutis, +<i>2</i> muscle-plate (myotome), <i>3</i> skeletal plate +(sclerotome), <i>4</i> cœloseptum (partition between dorsal +and ventral cœloma), <i>5</i> skin-fibre layer, <i>6</i> +gut-fibre layer, <i>I</i> myocœl (dorsal body-cavity), <i> +II</i> splanchnocœl (ventral body-cavity).)</td> +</tr> +</table> + +<br> + + +<p>The formation of the circulatory system in this animal is not +less interesting. All the other vertebrates have a compressed, +thick, pouch-shaped heart, which develops from the wall of the gut +at the throat, and from which the blood-vessels proceed; in the +Amphioxus there is no special centralised heart, driving the blood +by its pulsations. This movement is effected, as in the annelids, +by the thin blood-vessels themselves, which discharge the function +of the heart, contracting and pulsating in their whole length, and +thus driving the colourless blood through the entire body. On the +under-side of the gill-crate, in the middle line, there is the +trunk of a large vessel that corresponds to the heart of the other +vertebrates and the trunk of the branchial artery that proceeds +from it; this drives the blood into the gills (Fig. 210 <i>l</i>). +A number of small vascular arches arise on each side from this +branchial artery, and form little heart-shaped swellings or <i> +bulbilla</i> (<i>m</i>) at their points of departure; they advance +along the branchial arches, between the gill-clefts and the +fore-gut, and unite, as branchial veins, above the gill-crate in a +large trunk blood-vessel that runs under the chorda dorsalis. This +is the principal artery or primitive aorta (Fig. 214 <i>D</i>). The +branches which it gives off to all parts of the body unite again in +a larger venous vessel at the underside of the gut, called the +subintestinal vein (Figs. 210 <i>o,</i> 212 <i>E</i>). This single +main vessel of the Amphioxus goes like a closed circular +water-conduit along the alimentary canal through the whole body, +and pulsates in its whole length above and below. When the upper +tube contracts the lower one is filled with blood, and <i>vice +versa.</i> In the upper tube the blood flows from front to rear, +then back from rear to front in the lower vessel. The whole of the +long tube that runs along the ventral side of the alimentary canal +and contains venous blood may be called the “principal +vein,” and may be compared to the ventral vessel in the +worms. On the other hand, the long</p> + +<br> +<hr> +<p class="page"><a name="page 185">[ 185 ]</a></p> + +<p> </p> + +<p class="one">straight vessel that runs along the dorsal line of +the gut above, between it and the chorda, and contains arterial +blood, is clearly identical with the aorta or principal artery of +the other vertebrates; and on the other side it may be compared to +the dorsal vessel in the worms.</p> + +<p>The cœloma or body-cavity has some very important and +distinctive features in the Amphioxus. The embryology of it is most +instructive in connection with the stem-history of the body-cavity +in man and the other vertebrates. As we have already seen (Chapter +X), in these the two cœlom-pouches are divided at an early +stage by transverse constrictions into a double row of primitive +segments <a href="chap13.html#Fig. 124">(Fig. 124),</a> and each of +these subdivides, by a frontal or lateral constriction, into an +upper (dorsal) and lower (ventral) pouch.</p> + +<br> + + +<center> +<table class="capt" width="325" summary= +"Fig. 214. Transverse section of a young Amphioxus, immediately after metamorphosis. Fig. 215. Diagram of preceding."> +<tr> +<td align="justify"><img src="images3/fig214.GIF" width="325" +height="241" alt= +"Fig. 214. Transverse section of a young Amphioxus, immediately after metamorphosis. Fig. 215. Diagram of preceding."> +<br> +<a name="Fig. 214">Fig. 214</a>—<b>Transverse section of a +young Amphioxus,</b> immediately after metamorphosis, through the +hindermost third (between the atrium-cavity and the anus).<br> +Fig. 215—<b>Diagram of preceding.</b> (From <i>Hatschek.</i>) +<i>A</i> epidermis, <i>B</i> medullary tube, <i>C</i> chorda, <i> +D</i> aorta, <i>E</i> visceral epithelium, <i>F</i> subintestinal +vein. <i>1</i> corium-plate, <i>2</i> muscle-plate, <i>3</i> +fascie-plate, <i>4</i> outer chorda-sheath, <i>5</i> myoseptum, <i> +6</i> skin-fibre plate, <i>7</i> gut-fibre plate, <i>I</i> +myocœl, <i>II</i> splanchnocœl, <i>I</i><sub>1</sub> +dorsal fin, <i>I</i><sub>2</sub> anus-fin.)</td> +</tr> +</table> +</center> + +<br> + + +<p>These important structures are seen very clearly in the trunk of +the amphioxus (the latter third, Figs. 212–215), but it is +otherwise in the head, the foremost third (Fig. 216). Here we find +a number of complicated structures that cannot be understood until +we have studied them on the embryological side in the next chapter +(cf. Fig. 81). The branchial gut lies free in a spacious cavity +filled with water, which was wrongly thought formerly to be the +body-cavity (Fig. 216 <i>A</i>). As a matter of fact, this atrium +(commonly called the peribranchial cavity) is a secondary structure +formed by the development of a couple of lateral mantle-folds or +gill-covers (<i>M</i><sub>1</sub>, <i>U</i>). The real body-cavity +(<i>Lh</i>) is very narrow and entirely closed, lined with +epithelium. The peribranchial cavity (<i>A</i>) is full of water, +and its walls are lined with the skin-sense layer; it opens +outwards in the rear through the respiratory pore (Fig. 210 <i> +c</i>).</p> + +<p>On the inner surface of these mantle-folds +(<i>M</i><sub>1</sub>), in the ventral half of the wide mantle +cavity (atrium), we find the sex-organs of the Amphioxus. At each +side of the branchial gut there are between twenty and thirty +roundish four-cornered sacs, which can clearly be seen from without +with the naked eye, as they shine through the thin transparent +body-wall. These sacs are the sexual glands they are the same size +and shape in both sexes, only differing in contents. In the female +they contain a quantity of simple ova (Fig. 219 <i>g</i>); in the +male a number of much smaller cells that change into mobile +ciliated cells (sperm-cells). Both sacs lie on the inner wall of +the atrium, and have no special outlets. When the ova of the female +and the sperm of the male are ripe, they fall into the atrium, pass +through the gill-clefts into the</p> + +<br> +<hr> +<p class="page"><a name="page 186">[ 186 ]</a></p> + +<p> </p> + +<center> +<table class="capt" summary= +"Fig. 216. Transverse section of the lancelet, in the fore half."> +<tr> +<td><img src="images3/fig216.GIF" width="298" height="466" alt= +"Transverse section of the lancelet, in the fore half."></td> +<td align="left" valign="bottom"><a name="Fig. 216">Fig. +216</a>—<b>Transverse section of the lancelet,</b> in the +fore half. (From <i>Ralph.</i>) The outer covering is the simple +cell-layer of the epidermis (<i>E</i>). Under this is the thin +corium, the subcutaneous tissue of which is thickened; it sends +connective-tissue partitions between the muscles +(<i>M</i><sub>1</sub>) and to the chorda-sheath. (<i>N</i> +medullary tube, <i>Ch</i> chorda, <i>Lh</i> body-cavity, <i>A</i> +atrium, <i>L</i> upper wall of same, <i>E</i><sub>1</sub> inner +wall, <i>E</i><sub>2</sub> outer wall, <i>Lh</i><sub>1</sub> +ventral remnant of same, <i>Kst</i> gill-reds, <i>M</i> ventral +muscles, <i>R</i> seam of the joining of the ventral folds +(gill-covers), <i>G</i> sexual glands.</td> +</tr> +</table> + +<br> + + +<p class="one">fore-gut, and are ejected through the mouth.</p> + +<p>Above the sexual glands, at the dorsal angle of the atrium, we +find the kidneys. These important excretory organs could not be +found in the Amphioxus for a long time, on account of their remote +position and their smallness; they were discovered in 1890 by +Theodor Boveri (Fig. 217 <i>x</i>). They are short segmented +canals; corresponding to the primitive kidneys of the other +vertebrates (Fig. 218 <i>B</i>). Their internal aperture (Fig. 217 +<i>B</i>) opens into the body-cavity; their outer aperture into the +atrium (<i>C</i>). The prorenal canals lie in the middle of the +line of the head, outwards from the uppermost section of the +gill-arches, and have important relations to the branchial vessels +(<i>H</i>). For this reason, and in their whole arrangement, the +primitive kidneys of the Amphioxus</p> + +<br> +<hr> +<p class="page"><a name="page 187">[ 187 ]</a></p> + +<p> </p> + +<p class="one">show clearly that they are equivalent to the +prorenal canals of the Craniotes (Fig. 218 <i>B</i>). The prorenal +duct of the latter (Fig. 218 <i>C</i>) corresponds to the branchial +cavity or atrium of the former (Fig. 217 <i>C</i>).</p> + +<br> + + +<center> +<table class="capt" width="434" summary= +"Fig. 217. Transverse section through the middle of the Amphioxus. Fig. 218. Transverse section of a primitive fish embryo."> +<tr> +<td align="justify"><img src="images3/fig217.GIF" width="434" +height="357" alt= +"Fig. 217. Transverse section through the middle of the Amphioxus. Fig. 218. Transverse section of a primitive fish embryo."> +<br> +<a name="Fig. 217">Fig. 217</a>—<b>Transverse section through +the middle of the Amphioxus.</b> (From <i>Boveri.</i>) On the left +a gill-rod has been struck, and on the right a gill-cleft; +consequently on the left we see the whole of a prorenal canal +(<i>x</i>), on the right only the section of its fore-leg. <i>A</i> +genital chamber (ventral section of the gonocœl), <i>x</i> +pronephridium, <i>B</i> its cœlom-aperture, <i>C</i> atrium, +<i>D</i> body-cavity, <i>E</i> visceral cavity, <i>F</i> +subintestinal vein, <i>G</i> aorta (the left branch connected by a +branchial vessel with the subintestinal vein), <i>H</i> renal +vessel.<br> +Fig. 218—<b>Transverse section of a primitive fish embryo</b> +(Selachii-embryo, from <i>Boveri.</i>). To the left pronephridia +(<i>B</i>), the right primitive kidneys (<i>A</i>). The dotted +lines on the right indicate the later opening of the primitive +kidney canals (<i>A</i>) into the prorenal duct (<i>C</i>). <i> +D</i> body-cavity, <i>E</i> visceral cavity, <i>F</i> subintestinal +vein, <i>G</i> aorta, <i>H</i> renal vessel.</td> +</tr> +</table> +</center> + +<br> + + +<p>If we sum up the results of our anatomic study of the Amphioxus, +and compare them with the familiar organisation of man, we shall +find an immense distance between the two. As a fact, the highest +summit of the vertebrate organisation which man represents is in +every respect so far above the lowest stage, at which the lancelet +remains, that one would at first scarcely believe it possible to +class both animals in the same division of the animal kingdom. +Nevertheless, this classification is indisputably just. Man is only +a more advanced stage of the vertebral type that we find +unmistakably in the Amphioxus in its characteristic features. We +need only recall the picture of the ideal Primitive Vertebrate +given in a former chapter, and compare it with the lower stages of +human embryonic development, to convince ourselves of our close +relationship to the lancelet. (Cf. Chapter XI)</p> + +<p>It is true that the Amphioxus is far below all other living +vertebrates. It is true that it has no separate head, no developed +brain or skull, the characteristic feature of the other +vertebrates.</p> + +<br> +<hr> +<p class="page"><a name="page 188">[ 188 ]</a></p> + +<p> </p> + +<p class="one">It is (probably as a result of degeneration) without +the auscultory organ and the centralised heart that all the others +have; and it has no fully-formed kidneys. Every single organ in it +is simpler and less advanced than in any of the others. Yet the +characteristic connection and arrangement of all the organs is just +the same as in the other vertebrates. All these, moreover, pass, +during their embryonic development, through a stage in which their +whole organisation is no higher than that of the Amphioxus, but is +substantially identical with it.</p> + +<table class="capt" width="211" align="left" summary= +"Fig. 219. Transverse section of the head of the Amphioxus."> +<tr> +<td><img src="images3/fig219.GIF" width="211" height="296" alt= +"Transverse section of the head of the Amphioxus."> +<a name="Fig. 219">Fig. +219</a>—<b>Transverse section of the head of the +Amphioxus</b> (at the limit of the first and second third of the +body). (From <i>Boveri</i>) <i>a</i> aorta (here double), <i>b</i> +atrium, <i>c</i> chorda, <i>co</i> umlaut cœloma +(body-cavity), <i>e</i> endostyl (hypobranchial groove), <i>g</i> +gonads (ovaries), <i>kb</i> gill-arches, <i>kd</i> branchial gut, +<i>l</i> liver-tube (on the right, one-sided), <i>m</i> muscles, +<i>n</i> renal canals, <i>r</i> spinal cord, <i>sn</i> spinal +nerves, <i>sp</i> gill-clefts.</td> +</tr> +</table> + +<p>In order to see this quite clearly, it is particularly useful to +compare the Amphioxus with the youthful forms of those vertebrates +that are classified next to it. This is the class of the +Cyclostoma. There are to-day only a few species of this once +extensive class, and these may be distributed in two groups. One +group comprises the hag-fishes or Myxinoides. The other group are +the Petromyzontes, or lampreys, which are a familiar delicacy in +their marine form. These Cyclostoma are usually classified with the +fishes. But they are far below the true fishes, and form a very +interesting connecting-group between them and the lancelet. One can +see how closely they approach the latter by comparing a young +lamprey with the Amphioxus. The chorda is of the same simple +character in both; also the medullary tube, that lies above the +chorda, and the alimentary canal below it. However, in the lamprey +the spinal cord swells in front into a simple pear-shaped cerebral +vesicle, and at each side of it there are a very simple eye and a +rudimentary auditory vesicle. The nose is a single pit, as in the +Amphioxus. The two sections of the gut are also just the same and +very rudimentary in the lamprey. On the other hand, we see a great +advance in the structure of the heart, which is found underneath +the gills in the shape of a centralised muscular tube, and is +divided into an auricle and a ventricle. Later on the lamprey +advances still further, and gets a skull, five cerebral vesicles, a +series of independent gill-pouches, etc. This makes all the more +interesting the striking resemblance of its immature larva to the +developed and sexually mature Amphioxus.</p> + +<p>While the Amphioxus is thus connected through the Cyclostoma +with the fishes, and so with the series of the higher vertebrates, +it is, on the other hand, very closely related to a lowly +invertebrate marine animal, from which it seems to be entirely +remote at first glance. This remarkable animal is the sea-squirt or +Ascidia, which was formerly thought to be closely related to the +mussel, and so classed in the molluscs. But since the remarkable +embryology of these animals was discovered in 1866, there can be no +question that they have nothing to do with the molluscs. To the +great astonishment of zoologists, they were found, in their whole +individual development, to be closely related to the vertebrates. +When fully developed the Ascidiæ are shapeless lumps that +would not, at first sight, be taken for animals at all. The oval +body, frequently studded with knobs or uneven and lumpy, in which +we can discover no special external organs, is attached at one end +to marine plants, rocks, or the floor of the sea. Many species look +like potatoes, others like melon-cacti, others like prunes. Many of +the Ascidiæ form transparent crusts or</p> + +<br> +<hr> +<p class="page"><a name="page 189">[ 189 ]</a></p> + +<p> </p> + +<p class="one">deposits on stones and marine plants. Some of the +larger species are eaten like oysters. Fishermen, who know them +very well, think they are not animals, but plants. They are sold in +the fish markets of many of the Italian coast-towns with other +lower marine animals under the name of “sea-fruit” +(<i>frutti di mare</i>). There is nothing about them to show that +they are animals. When they are taken out of the water with the net +the most one can perceive is a slight contraction of the body that +causes water to spout out in two places. The bulk of the +Ascidiæ are very small, at the most a few inches long. A few +species are a foot or more in length. There are many species of +them, and they are found in every sea. As in the case of the +Acrania, we have no fossilised remains of the class, because they +have no hard and fossilisable parts. However, they must be of great +antiquity, and must go back to the primordial epoch.</p> + +<p>The name of “Tunicates” is given to the whole class +to which the Ascidiæ belong, because the body is enclosed in +a thick and stiff covering like a mantle (<i>tunica</i>). This +mantle—sometimes soft like jelly, sometimes as tough as +leather, and sometimes as stiff as cartilage—has a number of +peculiarities. The most remarkable of them is that it consists of a +woody matter, cellulose—the same vegetal substance that forms +the stiff envelopes of the plant-cells, the substance of the wood. +The tunicates are the only class of animals that have a real +cellulose or woody coat. Sometimes the cellulose mantle is brightly +coloured, at other times colourless. Not infrequently it is set +with needles or hairs, like a cactus. Often we find a mass of +foreign bodies—stone, sand, fragments of mussel-shells, +etc.—worked into the mantle. This has earned for the Ascidia +the name of “the microcosm.”</p> + + +<table class="capt" width="192" align="left" summary= +"Fig. 220. Organisation of an Ascidia (left view)."> +<tr> +<td><img src="images3/fig220.GIF" width="192" height="352" alt= +"Organisation of an Ascidia (left view)."> +<a name="Fig. 220">Fig. +220</a>—<b>Organisation of an Ascidia</b> (left view); the +dorsal side is turned to the right and the ventral side to the +left, the mouth (<i>o</i>) above; the ascidia is attached at the +tail end. The branchial gut (<i>br</i>), which is pierced by a +number of clefts, continues below in the visceral gut. The rectum +opens through the anus (<i>a</i>) into the atrium (<i>cl</i>), from +which the excrements are ejected with the respiratory water through +the mantle-hole or cloaca (<i>a</i>); <i>m</i> mantle. (From <i> +Gegenbaur.</i></td> +</tr> +</table> + +<p>The hind end, which corresponds to the tail of the Amphioxus, is +usually attached, often by means of regular roots. The dorsal and +ventral sides differ a good deal internally, but frequently cannot +be distinguished externally. If we open the thick tunic or mantle +in order to examine the internal organisation, we first find a +spacious cavity filled with water—the mantle-cavity or +respiratory cavity (Fig. 220 <i>cl</i>). It is also called the +branchial cavity and the cloaca, because it receives the excrements +and sexual products as well as the respiratory water. The greater +part of the respiratory cavity is occupied by the large grated +branchial sac (<i>br</i>). This is so like the gill-crate of the +Amphioxus in its whole arrangement that the resemblance was pointed +out by the English naturalist Goodsir, years ago, before anything +was known of the relationship of the two animals. As a fact, even +in the Ascidia the mouth (<i>o</i>) opens first into this wide +branchial sac. The respiratory water passes through the +lattice-work of the branchial sac into the branchial cavity, and is +ejected from this by the respiratory pore (<i>a</i>′). Along +the ventral side of the branchial sac runs a ciliated +groove—the hypobranchial groove which we have previously +found at the same spot in the Amphioxus. The food of the Ascidia +also</p> + +<br> +<hr> +<p class="page"><a name="page 190">[ 190 ]</a></p> + +<p> </p> + +<p class="one">consists of tiny organisms, infusoria, diatoms, +parts of decomposed marine plants and animals; etc. These pass with +the water into the gill-crate and the digestive part of the gut at +the end of it, at first into an enlargement of it that represents +the stomach. The adjoining small intestine usually forms a loop, +bends forward, and opens by an anus (Fig. 220 <i>a</i>), not +directly outwards, but first into the mantle cavity; from this the +excrements are ejected by a common outlet (<i>a</i>′) +together with the used-up water and the sexual products. The outlet +is sometimes called the branchial pore, and sometimes the cloaca or +ejection-aperture. In many of the Ascidiæ a glandular mass +opens into the gut, and this represents the liver. In some there is +another gland besides the liver, and this is taken to represent the +kidneys. The body-cavity proper, or cœloma, which is filled +with blood and encloses the hepatic gut, is very narrow in the +Ascidia, as in the Amphioxus, and is here also usually confounded +with the wide atrium, or peribranchial cavity, full of water.</p> + + + +<table class="capt" width="151" align="left" summary= +"Organisation of an Ascidia (as in Fig. 220, seen from the left)."> +<tr> +<td><img src="images3/fig221.GIF" width="151" height="299" alt= +"Organisation of an Ascidia (as in Fig. 220, seen from the left)."> +<a name="Fig. 221">Fig. 221</a>—<b>Organisation of +an Ascidia</b> (as in Fig. 220, seen from the left). <i>sb</i> +branchial sac, <i>v</i> stomach, <i>i</i> small intestine, <i>c</i> +heart, <i>t</i> testicle, <i>vd</i> sperm-duct, <i>o</i> ovary, <i> +o</i>′ ripe ova in the branchial cavity. The two small arrows +indicate the entrance and exit of the water through the openings of +the mantle. (From <i>Milne-Edwards.</i>)</td> +</tr> +</table> + + +<p>There is no trace in the fully-developed Ascidia of a chorda +dorsalis, or internal axial skeleton. It is the more interesting +that the young animal that emerges from the ovum <i>has</i> a +chorda, and that there is a rudimentary medullary tube above it. +The latter is wholly atrophied in the developed Ascidia, and looks +like a small nerve-ganglion in front above the gill-crate. It +corresponds to the upper “gullet-ganglion” or +“primitive brain” in other vermalia. Special +sense-organs are either wanting altogether or are only found in a +very rudimentary form, as simple optic spots and touch-corpuscles +or tentacles that surround the mouth. The muscular system is very +slightly and irregularly developed. Immediately under the thin +corium, and closely connected with it, we find a thin muscle tube, +as in the worms. On the other hand, the Ascidia has a centralised +heart, and in this respect it seems to be more advanced than the +Amphioxus. On the ventral side of the gut, some distance behind the +gill-crate, there is a spindle-shaped heart. It retains permanently +the simple tubular form that we find temporarily as the first +structure of the heart in the vertebrates. This simple heart of the +Ascidia has, however, a remarkable peculiarity. It contracts in +alternate directions. In all other animals the beat of the heart is +always in the same direction (generally from rear to front); it +changes in the Ascidia to the reverse direction. The heart +contracts first from the rear to the front, stands still for a +minute, and then begins to beat the opposite way, now driving the +blood from front to rear; the two large vessels that start from +either end of the heart act alternately as arteries and veins. This +feature is found in the Tunicates alone.</p> + +<p>Of the other chief organs we have still to mention the sexual +glands, which lie right behind in the body-cavity. All the +Ascidiæ are hermaphrodites. Each individual has a male and a +female gland, and so is able to fertilise itself. The ripe ova +(Fig. 221 <i>o</i>′) fall directly from the ovary (<i>o</i>) +into the mantle-cavity. The male sperm is conducted into this +cavity from the testicle (<i>t</i>) by a special duct (<i>vd</i>). +Fertilisation is accomplished here, and in many of the +Ascidiæ developed embryos are found. These are then ejected</p> + +<br> +<hr> +<p class="page"><a name="page 191">[ 191 ]</a></p> + +<p> </p> + +<p class="one"> +with the breathing-water through the cloaca (<i>q</i>), and so +“born alive.”</p> + +<p>If we now glance at the entire structure of the simple Ascidia +(especially <i>Phallusia, Cynthia,</i> etc.) and compare it with +that of the Amphioxus, we shall find that the two have few points +of contact. It is true that the fully-developed Ascidia resembles +the Amphioxus in several important features of its internal +structure, and especially in the peculiar character of the +gill-crate and gut. But in most other features of organisation it +is so far removed from it, and is so unlike it in external +appearance, that the really close relationship of the two was not +discovered until their embryology was studied. We will now compare +the embryonic development of the two animals, and find to our great +astonishment that the same embryonic form develops from the ovum of +the Amphioxus as from that of the Ascidia—a typical <i> +chordula.</i></p> + +<br> + + +<hr noshade align="left" size="1" width="20%"> +<p class="ref"><a href="Title.html">Title and Contents</a><br> +<a href="glossary.html">Glossary</a><br> +<a href="chap15.html">Chapter XV</a><br> +<a href="title2.html">Vol. II Title and Contents</a><br> +<a href="Title.html#Illustrations">Figs. 1–209</a><br> +<a href="title2.html#Illustrations">Figs. 210–408</a></p> +</center> +</body> +</html> + |