Encyclopædia Britannica, Ninth Edition/Animal Kingdom/II
II. The Metazoa.
The germ becomes differentiated into histogenetic cells, and these cells become arranged into two sets, the one con stituting the outer wall of the body, or ectoderm, while the other, or endoderm, lies internal to the foregoing, and con stitutes the lining of the alimentary cavity, when, as is usually the case, a distinct alimentary cavity exists. In the embryo, the representatives of these two layers are the epiblast and hypoblast.
All the Metazoa, in fact, commence then- existence in the form of an ovum, which is essentially a nucleated cell, sup plemented by more or less nutritive material, or food yelk. The ovum, after impregnation, divides into cleavage masses, or blastomeres, giving rise to a Morula, in the midst of which arises a cavity, the blastoccele (cleavage cavity, " Furchungshohle" of the Germans), which maybe larger or smaller, filled only with fluid, or occupied by food yelk. "\Then it is largest, the blastomeres, disposed in a single layer, form a spheroidal vesicle, enclosing a correspondingly shaped blastocoele. When it is reduced to a minimum, the Morula is an almost solid aggregation of blastomeres, which may be nearly equal in size, or some may be much larger than others, in consequence of having undergone less rapid division. The next stage in the development of the embryo of a Metazoon consists (in all cases except a few parasitic anenterous forms) in the conversion of the Morula into a body having a digestive cavity, or a Gastrula. The animals in which the embryo takes on the form of a Gastrula, may be termed, as Haeckel has proposed, Gastrcece.
The conversion of the Morula into the Gastrula may take place in several ways.
In the simplest, the Morula being composed of equal or nearly equal blastomeres, more or less completely converted into cells, these differentiate themselves into an outer layer, the epiblast, investing the remaining cells, which constitute the hypoblast. The central cells of the hypoblast next diverge and give rise to a space filled with fluid, the alimentary cavity, which opens at one end, and thus gives rise to the Gastrula. This is the process generally observed in Porifera, Ccelenterata, Turbellaria, Trematoda, and Nematoidea.
In a second class of cases, the Morula becomes converted into blastomeres of unequal sizes, a small and a large set. The smaller rapidly become converted into cells, and invest the larger and any remains of the food yelk, as a blastoderm, which at first represents only the epiblast of the former case. The hypoblast arises either from the epiblast thus formed, or from the included larger blastomeres. This is the process observed in certain Turbellaria, in the Ctenophora, in the Oligochoeta and Hirudinea, in the Artliropoda, and in most Vertebrata.
In a third group of instances, the Morula, whether con sisting of equal or unequal blastomeres, becomes spheroidal, and encloses a correspondingly shaped blastoccele. One part of the wall of this vesicular Morula then becomes invaginated, and gives rise to the alimentary cavity, with the hypoblast which limits it. This process has been observed in the Ch&tognatha, Echinodermata, Gephyrea, polychsetous Annelida, Enter opneusta, Brachiopoda ; in most Mollusca ; in Amphioxus ; and, slightly modified, in Petromyzon and in the Amphibia. These various modes in. which the two primary layers of the germ may be developed shade off into one another, and do not affect the essence of the process, which is the segregation of one set of cells to form the external covering of the body, and of another set to constitute the lining of the alimentary canal.
In whatever manner the Gastnda is formed, and what ever be its shape when its alimentary cavity is complete, one of two things happens to it. It becomes provided with many ingestive apertures, distinct from that first formed ; or with only one, which may or may not be distinct from the first aperture. The former division comprises only the Sponges (Porifera or Spongida) in which, as the remark able researches of Haeckel have shown, the walls of the deeply cup-shaped Gastrula become perforated by the numer- rous inhalent ostioles, while the primitive opening serves as the exhalent aperture. These may be termed the Metazoa polystomata.
The latter division includes all the remaining forms, which may be grouped together as Metazoa monostomata. Among these, two primary groups are distinguishable, of which the second exhibits an advance in organisation upon the first. In the first, the aperture of the Gastrula be comes the permanent mouth (Archceostomata). In the second, the permanent mouth is a secondary perforation of the body wall (Deuterostomata).
1. It is now well established that the aperture of tho Gastrula becomes the oral aperture of the adult in the Coslenterata, which group includes animals differing much in grade of organisation, from the simple Hydra to the complex Ctenophore, but all manifestly exhibiting varia tions of one fundamental type. Parallel with these may be ranged an assemblage composed of the Turbellaria, Kotifera, and Trematoda, which are associated together by the closest resemblances of structure, and which present an even greater range in grade of organisation than the Coslenterata. The lower Rhabdoc&la come very close to the Infusoria (as close as the multicellular to the unicellular Algae), and are but little superior to Hydra in the degree of their organic differentiation ; while, in the Trematoda, the laud Planarice, and the Nemertidce, we have animals which attain a considerable complexity, and in the case of many Trematoda and of Linens (Pilidium) undergo remarkable metamorphoses. As a cognate group, the Nematoidea may be enumerated, extremely simple in their lowest forms, considerably differentiated in the higher, and connected with the Turbellaria by such forms as Polygordius. The Oligochaia and the Hirudinea also belong to this division of Scolecimorpha, but they differ from the foregoing in the development of a segmented rnesoblast.
In the Ccelenterata, Nematoidea, Turbellaria, Trematoda, and Rotifera, the mode of origin of the cells which lie ) between the epiblast and the hypoblast, constitute the meso- / blast, and give rise to the connective tissue and muscles of the body wall and of that of the intestine, is not precisely known. They may take their origin in the epiblast, or in the hypoblast, or in both. But in the Oligoch&ta and the Hirudinea, after the epiblast and hypoblast are difierentiated, the cells of the latter give rise by division to two bands of cells, which lie, one on each side of the long axis of the ventral face of the worm, and constitute the meso- blast. This becomes marked out by transverse constric tion into segments, and, in each segment, gives rise to all the tissues which lie between the epiblast (epidermis) and lypoblast (epithelium of the alimentary canal). The mouth corresponds with the primitive involution of the Morula ; the anal aperture is a new formation. In the Nematoidea and in the lower Rhabdoccele Turbellaria, the intestinal canal is a simple tube or sac. But in some Turbellaria and Trematoda, the alimentary canal gives off diverticula, which ramify through the mesoblast and even unite to gether. The like takes place in a great many Ccelenterata, and the " gastrovascular apparatus," as it has been well termed, which is thus formed, is highly characteristic of them. The animals just referred to, therefore, have an " entero- ccele" more or less distinct from the proper digestive cavity, but connected with it, and ramifying through the mesoblast.
2. In the remaining members of the animal kingdom, the -embryo develops a secondary mouth as a perforation of the body wall, the primary aperture sometimes becoming the anus, and sometimes disappearing. Of these Metazoa deuterostomata, there are some which follow the mode of development of the Oli jochceta and Hirudinea very closely, so far as the formation and segmentation of the mesoblast is concerned, though the question, whether this segmented mesoblast arises from the epiblast or the hypoblast, has not been exhaustively worked out. These are the Anne lida polychceta ; and there is the closest resemblance in development between them and the lower Arthropoda (Crustacea, Arachnida, lower Insecta), while, in the higher Arthropods, the process is complicated by the development 3f an amnion, and by some other special peculiarities which need not be considered in detail. In all these Metazoa, whatever cavities are developed in the mesoblast, whether a wide peri visceral cavity, or vascular canals, or both combined, they arise from the splitting or excavation of tho mesoblast itself, and are not prolongations of the alimentary cavity. Hence they may be termed Schizoccela.
But, in certain other deuterostomatous Metazoa, the meso blast becomes excavated, and a " perivisceral cavity " and vessels are formed in quite another fashion.
Thus in the Chcetognatha, represented by the strange and apparently anomalous Sagitta, Kowalewsky s researches show, that the vitellus undergoes complete segmentation, and is converted into a vesicular Morida, on one side of which involution takes place, and gives rise to the primi tive alimentary canal, of which the opening of involution becomes the permanent anus, the mouth being formed by perforation at the opposite end of the body. Before the mouth is formed, however, the primitive alimentary cavity throws out, on each side, a csecal pouch, which extends as far forwards as its central continuation does, and grows backwards behind the anus. The two sacs, thus prolonged posteriorly, meet, but remain divided from one another by their applied walls in the median line. These lateral sacs now become shut off from the median portion of the primi tive alimentary cavity (which opens at its anterior end, and becomes the permanent alimentary canal), and are con verted into shut sacs, the cavity of each of which forms one-half of the perivisceral cavity. The inner wall of each sac, applied to the hypoblast, gives rise to the muscular wall of the intestine ; and the outer wall, applied to the epiblast, becomes the muscular wall of the body, and from it the generative organs are evolved. The great ganglia and nerves are developed from the cells of the epiblast. Thus tSagitta is temporarily ccelenterate, but the two gastrovas cular sacs, each enclosing an enteroccele, become shut off from the alimentary canal and metamorphosed into the walls of the perivisceral cavity. But it is not altogether clear whether the cells of the enterocoele give rise only to the lining of the perivisceral cavity, and whether the muscles and connective tissue are otherwise derived or not. Kowalewsky s evidence, however, is in favour of the origin of the muscles directly from the cells of the mesoblastic diverticula.
In the Echinodermata, the brilliant investigations of Johannes Miiller, confirmed in their general features by all subsequent observers, proved, firstly, that the ciliated embryonic Gastnda (the primitive alimentary canal of which is formed by involution of a vesicular blastoderm), to> which the egg of all ordinary Echinoderms gives rise, acquires a mouth, by the formation of an aperture in the body wall, distinct from the primitive aperture of the Gastrula, so that, in this respect, it differs from the embryo of all Ccelenterata; secondly, that the embryo thus pro vided with mouth, stomach, intestine, and anus acquires a complete bilateral symmetry ; thirdly, that the cilia, with which it is primitively covered, are ultimately restricted to one or more series, some of which encircle the axis of the body, or a line drawn from the oral to the anal aper tures ; and fourthly, that, within this bilaterally sym metrical larva or Echinopaidiiim, as it may be called, the more or less completely radiate Echinoderm is developed by a process of internal modification.
Miiller believed that the first step in this process was the ingrowth of a diverticulum of the integument, as a hollow process, which became converted into the ambulacral vascular system of the Echinoderm. He did not attempt to explain the origin of the so-called blood-vascular system, or pseudhoemal vessels, nor of the perivisceral cavity. Miiller s conclusions remained unchallenged until 1SG4, when Prof. Alexander Agassiz took up the question afresh, and, in a remarkable paper on the development of the genus Asteracanthion, detailed the observations which led him to believe that the ambulacral vessels do not arise by involution of the external integument, but that they com mence as two primitively symmetrical diverticula of the stomach (the " wiirstf ormige Korper" of Miiller), one of which becomes connected with the exterior by an opening (the dorsal pore observed by Miiller, and considered by him to be the origin of the ambulacral vessels), and gives- rise to the ambulacral vessels, the ambulacral region of the body of the Echinoderm being modelled upon it; while upon the other gastric sac, the antambulacral wall of the starfish body is similarly modelled. Both gastric sacs early become completely separated from the stomach of the Echinopocdium, and open into one another, so as to form a single horseshoe-shaped sac, connected with the exterior by a tube which is converted into the madreporic canal. Agassiz does not explain the mode of formation of the perivisceral cavity of the starfish, and has nothing to say respecting the origin of the pscudhsenial vessels.
Recently, Metschnikoff has confirmed the observations of Agassiz, so far as the development of the ambulacral system from one of the diverticula of the alimentary canal of the starfish larva is concerned, and he has added the important discovery that the perivisceral cavity of the Echinoderm is the product of what remains of these diver ticula. Moreover, his observations on other Echinodermata. show that essentially the same process of development of the peritoneal cavity occurs in Ophiuridce, Echinidce, and Holotkuridce.
The precise mode of origin of the pseudhsemal system, or so-called blood-vessels, of the Echinoderms is not yet made out. But it is known that the cavity of these vessel* contains corpuscles similar to those which are found in. the perivisceral cavity and in the ambulacral vessels, and that all three communicate.
Both Agassiz and Metschuikoff justly insist upon the correspondence in development of the lateral gastric diverticula of the Echinopaedium with the gastrovascular canal system of the Ctenophora ; and, on the ground of this resemblance, the former refers the Echinoderms to the Itadiata, retaining under that Cuvierian denomination the Acalephce (Coelenterata) and the Echinodermata. But this arrangement ignores the real value of his own discovery, which shows that the Echinoderms have made a great and remarkable step, in passing from their primarily ccelen- terate stage of organisation to their adult condition. And it further ignores the unquestionable fact, admirably brought out by the same excellent observer s recent investigations into the development of Balanoglossus, that the Echino- piedium is almost identical in structure with the young of animals, such as the Gephyrea and Enteropneusta, which are in no sense radiate, but are, eminently, bilaterally symmetri cal. In fact, the larva of Balanoglossus (the sole repre sentative of the Enteropneusta), was originally described by Miiller, under the name of Tornaria, as an Echinoderm larva; and was subsequently more fully examined by Prof. Alex. Agassiz, who also regarded it as an unquestionable Echinoderm larva ; and it is only recently that it has been proved, partly by Metschnikoff and partly by Agassiz himself, to be the larval form of Balanoglossus. In Balanoglossus, as in the Echinoderms, it appears that saccular outgrowths of the intestine give rise to the perivis- ceral cavity and its walls ; and, if such be the case, the mesoblast will be chiefly, if not wholly, represented by diverticula of the alimentary canal. Thus in the Chcetog- natha and Eckinodermata, and possibly in the Enteropneusta, the perivisceral cavity is a portion of the alimentary cavity shut off from the rest ; and, in contradistinction to the Schizoccela, in which the perivisceral cavity is produced by a splitting of the mesoblast, they may be said to be En- ierocoela.
If we endeavour to determine the place of the three remaining great groups of animals, the Mollnsca, the Tuni- cata, and the Vertebrata, obstacles arise, firstly, from a want of sufficiently exact knowledge respecting the Mol- lusca; and secondly, from the difficulty of interpreting certain well-ascertained facts in the Vertebrata.
That the Mollnsca, including under that name the Poly-oa and Brachiopoda, as well as the higher Mollusks, are closely allied to the Annelida, is readily demonstrated. The known forms of Brachiopod, Lamellibranch, Pteropod, and Gasteropod larvae all have their parallels among Annelidan larva?. The Folyzoa are closely allied with the Gephyrea and Rotifera; and a Mollusk may be said to be a few-segmented annelid with a mantle. But whether the perivisceral cavity is developed in the annelidan or in the echinoderm fashion is not yet clear. In the Polyzoa, the evidence is at present insufficient to justify any conclu sion. In the Brachiopoda, there is some ground for think ing that the perivisceral cavity is formed in the same way as in Sagitta and the Echinodermata ; while, in the Lamelli- branchiata and Odontophora, there is every reason to believe that the perivisceral cavity is formed by splitting of the mesoblast, or that they are schizocoelous.
In the lowest Tunicata, represented by Appendicularia, the recent investigations of Fol have shown, that, in the adult, the body proper is formed almost exclusively by an ectoderm and endoderm, which proceed directly from the epiblast and hypoblast of the embryo. It is only in the caudal appendage that a distinct mesoblast is represented by the notochord and the muscles. The blood channels correspond with the blastococle, and the
- house" in which these singular animals shelter themselves
is a cuticular secretion, representing the cellulose coat of the higher ascidians. The Appendicularice have no atrium, or at most only rudiments of it, hence the branchial clefta open directly on the haemal aspect of the body, which corresponds with the ventral face of a vertebrate animal. In all other Tunicata, an atrial cavity is formed by involu tion of the ectoderm, which thus gives rise to a cavity on each side of the branchial sac, into which the branchial clefts of the adult open ; and a thick cellulose cuticula, into which cells from the ectoderm usually wander, invests the exterior of the body. The "atrial tunic," or invagi- nated layer of the ectoderm, is reflected, as a visceral layer, over more or less of the outer surface of the alimentary canal, and, as a parietal layer, over more or less of the inner surface of the body wall j and the space between the two (the blastocoele) becomes converted into the blood passages. Thus, such an ascidian resembles a vertebrated animal, not only in the manner in which its nervous centre is deve loped, but in the fact that it possesses an atrial cavity, which singularly resembles the pleuroperitoneal chamber of a vertebrate. For this cavity is bounded externally by the atrial tunic and the integument, which correspond with the somatopleural layer of the mesoblast and the epiblast of a vertebrate embryo ; and it is bounded, internally, by the atrial tunic and the epithelium of the alimentary canal, which, to the same extent, correspond with the splanch- nopleure and the hypoblast. The primitively double atrial aperture has its parallel in the peritoneal openings which persist in many Vertebrata.
Thus the ascidian has no " perivisceral cavity" formed by splitting of the mesoblast, nor has it any "perivisceral cavity " formed by diverticula from the alimentary canal. It is neither enterocoelous nor schizocoelous, but what, at first sight, resembles a perivisceral cavity is formed within the body by involution, and the ascidian may therefore be said to be epiccelous. If the alate prolongations of the body which lie at the sides of the branchial apertures, in Balanoglossus, were to enlarge and unite round the anus so as to leave but a relatively small opening between their edges, the cavity so formed would answer to the atrial chamber of an ascidian.
In the higher Vertebrata, the pleuroperitoneal cavity appears to be formed by the splitting of the mesoblast into two layers, a splanchnopleure and a somatopleure, and, there fore, seems at first to correspond with the perivisceral cavity of the Annelids and Arthropods. But what is now known of the structure and development of the lowest and most embryonic of known Vertebrata, Ampkioxus, throws very great doubt upon this interpretation of the facts. One of the most singular of the many peculiarities of Ampliioxus is the fact that the branchial clefts open, not on the exterior of the body, as in all other Vertebrata, but into a chamber with a single external aperture, which, on the one hand, curiously resembles the atrium of an ascidian ; while, on the other, it is undoubtedly homologous with the pleuroperitoneal cavity of the higher Vertebrata. Now Kowalewsky s investigations have shown, that, at first, the branchial apertures of the embryo Ampkioxus open upon the exterior of the body, but that, after a time, a process of the wall of the body, on the dorsal side of the branchial apertures, grows down over them, and, uniting with its fellow in the median ventral line of the body at all points, except at the abdominal pore, gives rise to the outer wall of the pleuro peritoneal cavity. Thus the lining of that cavity, like the atrial tunic of the ascidian, is a derivative of the epiblast ; and A mph ioxus is epicoelous. As it can hardly be doubted that the somatopleure of Amphioxus is the homo- logue of the somatopleure in the higher Vertebrata,\i becomes highly probable that the apparent splitting of the meso- blast in the latter, after all, represents the mode of develop ment of the pleuroperitoneal cavity which obtains in the former, and, thus, that the Vertebrata are not schizoccelous, but epiccelous. Whether this suggestion will turn out to be well based or not, must be decided by the embryologi- cal investigations specially directed to this point : but that there should be any essential difference between Amphioxus and other Vertebrata, in the manner in which the pleuro peritoneal cavity is formed, is highly improbable.
The distance between Amphioxus and other vertebrate animals, which has hitherto been generally supposed to exist, has been greatly diminished by recent investigations. So far from being devoid of a brain and of a skull, the regions of the cerebro-spinal axis and of the neural canal, which answer to those organs in the higher Vertebrata, are, in proportion, extremely long in Amphioxus, as they are in all vertebrate embryos. But, in Amphioxus, the head retains throughout life a segmentation comparable to that of the rest of the body, while, in the higher Vertebrata, almost all traces of these distinct segments are very early lost. Moreover, in Amphioxus, the renal apparatus, so far from being absent, is represented by a comparatively large structure, and nothing is wanted to equip it with all the organs found in a young Marsipobranch, but auditory sacs, which, however, it must be remembered, make their appearance late in the Lamprey. With all this, the gap be tween Amphioxus and the Marsipobranchii is undoubtedly more considerable than that between the Marsipobranchii and other fishes, and it may represent a primary division of the class Pisces, which, from the segmentation of the skull, may be termed the Entomocrania, as opposed to the rest, in which the primary segmentation of the skull is almost completely effaced, and which may therefore be designated Holocrania.
It has been stated above that the great majority of the Metazoa pass through the Gastrula condition, and belong to the division of the Gastrcece. In some members of this division, however, the alimentary canal may be rudimentary, as in sundry male Eotifera and in the Gordiacei among the Nematoidea, and yet these are so closely allied to other forms possessing fully developed digestive canals, that it is reasonable to regard their rudimentary alimentary appa ratus as absorbed. In two groups, however, the Cestoidea and the Acanthocephala, there is no trace of an alimentary canal either in the embryo or in the adult.
From the point of view of phylogeny, this fact may be interpreted in two ways. Either the alimentary canal which once existed has aborted, and the Cestoidea and Acanthocephala are modified Scolecimorpha, or these para sites have not descended from Gastrcece, but have passed into their present condition directly from a J/orw/a-like form of Metazoon. In the latter case they will form a division of Agastnece, apart from the other Metazoa.