Encyclopædia Brittanica, vol. 2 (1878)
 Animals, Classification of. The object classification is to bring together those things which are like, and to separate those which are unlike. Each science has its own classification of the objects with which it deals, the kinds of likeness and unlikeness according to which these objects are grouped varying in relation to the special qualities or properties of matter with which the science is concerned. Thus, the physicist classifies bodies according to their mechanical, electrical, thermic, or other physical properties; the chemist, regards their composition; while the zoologist and the botanist group them according to their likenesses and unlikenesses of structure, function, and distribution.
As soon as the labours of anatomists had extended over a sufficiently great variety of animals, it was found that they could be grouped into separate assemblages, the members of each of which, while varying more or less in minor respects, had certain structural features in common, and these common morphological characters became the definition of the group thus formed. The smallest group thus constituted is a Morphological Species. A certain number of species having characters in common, by which they resemble one another and differ from all other species, constitutes a Genus; a group of genera, similarly associated, constitutes a Family; a group of families, an Order; a group of orders, a Class; a group of classes, a Sub-Kingdom; while the latter, agreeing with one another only in the characters in which all animals agree, and in which they differ from all plants, make up the Animal Kingdom.
The formation of a morphological classification is therefore a logical process, the purpose of which is to throw the facts of structure into the smallest possible number of general propositions, which propositions constitute the definitions of the respective groups. A perfect classification will fulfil this end, and, in order to form it, two conditions are necessary: Firstly, we must have a full knowledge of the adult structure of every animal, recent and extinct; secondly, we must know all the modifications of structure through which it has passed, in order to attain the adult condition, or in other words, the mode of development of the animal. For it is the sum of all the structural conditions of an animal which constitutes the totality of its structure; and if two animals, similar in their adult state, were unlike in their development, it is clear that the latter circumstance would have to be taken into account in determining their position in a classification.
Linnæus, living at a time when neither comparative anatomy nor embryology can be said to have existed, based his classification of animals upon such broad resemblances of adult structure and habit as his remarkable sagacity and wide knowledge enabled him to detect. Cuvier and his school devoted themselves to the working out of adult structure, and the Leçons d'Anatomie Comparée and the Règne Animal are wonderful embodiments of the results of such investigations. But the Cuvierian system ignores development; and it was reserved for Von Baer to show the importance of developmental studies, and to inaugurate the marvellous series of researches which, in the course of the last fifty years, have made us acquainted with the manner of development of every important group of animals. The splendid researches of Cuvier gave birth to scientific palæontology, and demonstrated that, in some cases, at any rate, extinct forms of life present characters intermediate between those of groups which are at present widely different. The investigations of Agassiz upon fossil fishes tended in the same direction, and further showed that, in some cases, the older forms preserve, as permanent features, structural characters which are embryonic and transitory in their living congeners. Moreover, Darwin, Owen, and Wallace proved that, in any great area of geographical distribution, the later tertiary extinct forms are clearly related to those which now exist in the area. As Taxonomic investigations increased in accuracy and in extent, the careful examination of large suites of specimens revealed an unexpected amount of variability in species; and Darwin's investigation of the phenomena presented by animals under domestication proved that forms, morphologically as distinct as admitted natural genera, could be produced by selective breeding from a common stock.
Upon the foundation thus furnished, the doctrine of Evolution, first scientifically formulated by Lamarck, has been solidly built up by Darwin, and is now, with various modifications and qualifications, widely accepted. But the acceptance of this doctrine introduced a new element into Taxonomy. If all existing animals are the last terms of a long series of developmental stages, represented by the animals of earlier ages of the earth's history, the starting point of which has been a primordial form of the extremist simplicity consistent with animal life, then every animal has an "ancestral" as well as what may be termed a "personal" embryology; and the same considerations which oblige the Taxonomist to take account of the latter phenomena, compel his attention to the former stages of development. Two animals belong to the same group, when they are similar in structure, personal development, and ancestral development, and not otherwise. Hence it follows that a perfect and final zoological classification cannot be made until we know all that is important concerning 1, the adult structure; 2, the personal development; and 3, the ancestral development of animals. It is hardly necessary to observe that our present knowledge, as regards even the first and second heads, is very imperfect; while, as respects the third, it is utterly fragmentary.
The only genus of animals of which we posses a satisfactory, though still not quite complete, ancestral history, is the genus Equus, the development of which in the course of the Tertiary epoch from an Anchitherioid ancestor, through the form of Hipparion, appears to admit of no doubt. And all the facts of geology and palæontology not only tend to show that the knowledge of ancestral development is likely long to remain fragmentary, but lead us to doubt, whether even such fragments as may be vouchsafed to us by the extension of geological inquiry will ever be sufficiently old, in relation to the whole duration of life on the earth, to give us positive evidence of the nature of the earliest forms of animals.
While holding the doctrine of evolution to its fullest extent, and having no doubt that Taxonomy ought to be the expression of ancestral development, or phylogeny, as well as of embryogeny and adult structure, and while conceiving that the attempts at founding a scientific phylogeny, which have been made by Haeckel and others, are of much interest and importance as guides to and suggestors of investigation, the present writer looks upon all such attempts as provisional hypotheses; and he conceives that, at any rate for the present, it is a mistake to introduce considerations of this purely hypothetical kind into classification, which should be based on verifiable data.
In the case of an existing animal, it is possible to determine its adult structure and its development, and therefore to assign its place relatively to other animals, the  structure and development of which are also known; and, in the case of an extinct animal, it is possible to ascertain certain facts of its structure, and sometimes certain facts of its development, which will justify a more or less positive assignment of its place relatively to existing animals. So far, Taxonomy is objective, capable of proof and disproof, and it should leave speculation aside, until speculation has converted itself into demonstration.
In the present rapidly shifting condition of our knowledge of the facts of animals structure and development, however, it is no easy matter to group these facts into general propositions which shall express neither more nor less than is contained in the facts; and no one can be more conscious of the manifold imperfections of the following attempt at such a classification than the author of it.
In certain of the lower animals, the substance of the body is not differentiated into histogenetic elements; that is, into cells1 which, by their metamorphoses, give rise to tissues. In all other animals, on the other hand, the protoplasmic mass, which constitutes the primitive body, is converted into a multitude of cells, which become metamorphosed into tissues of the body.
For the first of these divisions the old name of Protozoa may be retained; for the second, the title of Metazoa, recently proposed by Haeckel, may be conveniently employed.
I. The Protozoa.
Haeckel has shown that, among the Protozoa, there are some which are simpler than the rest, inasmuch as they are devoid of both nuclei and contractile vesicles. To these he applies the name of
1. Monera. Among the members of this group, which are at present known, three series are distinguishable, in all of which multiplication is effected by division, preceded, or not preceded, by the assumption of an encysted condition. In one state, each of these Monera is a myxopod,2 that is, is provided with longer or shorter pseudopodia as locomotive organs, and, in Protamba and Protogenes, the result of the process of division is also two or more myxopods. But, in Protomonas, the myxopod, after becoming encysted, gives rise by division to bodies provided with long flagelliform cilia, by which they are propelled, and which my be termed mastigopods; and in Myxastrum, the encysted body divides into a multitude of oval particles, each enclosed in its own coat. These are set free, and each gives rise to a new myxopod of the same character as the parent.
In Protomyxa, the myxopods coalesce into a reticulated plasmodium; and Vampyrella is parasitic, devouring stalked diatoms, and encysting itself upon the ends of their stalks, the encysted form dividing into new Vampyrella . Most of these interesting Monera have been made known by Haeckel, so that, in all probability, many others remain to be discovered. It is probable the Foraminifera, notwithstanding the complexity of the skeletons, belong to this group, but too little is known of the structure of their soft parts to enable any certain conclusion to be drawn respecting them, and the analogy of Gromia leads to the suspicion that they may belong to the next division.
2. Endoplastica. In these Protozoa a portion of the interior protoplasmic body is separated from the rest as a distinct, more or less rounded, body, which may be termed the endoplast, as a term suggestive of its similarity to the nucleus of a histogenetic cell, without implying its identity therewith. Of such endoplasts there may be one or many, but the protoplasm in which they lie does not give rise to cells, which become metamorphosed into elements of the tissues. Very often they possess one or more vacuoles, which rhythmically dilate and contract, in accordance with the changes in the protoplasm in which they lie, and which are termed contractile vesicles.
In this division of the Protozoa, three groups the Ambidæ, the Flagellata (or flagellate Infusoria), and the Gregarinidæ closely repeat the forms and mode of reproduction of the Protambidæ, Protomonadidæ, and Myxastridæ among the Monera. Among the rest, the Acinetidæ are distinguished by their pseudopodia being converted into suckers, through which they draw the juices of their prey. In all these, and in the preceding forms, there is a more or less marked distinction of the protoplasm constituting the body into a firmer and denser outer layer, the ectosarc, and a more fluid inner substance, the endosarc; and, in some of the Gregarinidæ, the ectosarc becomes differentiated into muscular fibres. In the Flagellata there is a permanent oral aperture; and in one member of this group, Noctiluca, additional complications of structure, in the form of ridge-like tooth and a tentacle, occur. In the Radiolaria, the body is still more clearly differentiated into an inner substance, surrounded by a capsule, and containing nuclei and even cells, and a vacuolated ectosarc, whence the radiating pseudopodia proceed. Coloured corpuscles, usually yellow, appear in the ectosarc, and have been shown by Haeckel to contain starch and to multiply independently. In the Ciliata (ciliated Infusoria), with which the Catallacta of Haeckel may be included, the differentiation of the protoplasm of the body, without any development of histogenetic cells, goes still further. A permanent mouth and anus may appear, connected by a permanently softer and more fluid region of the protoplasm (as is plainly seen, for example, in Nyctotherus) foreshadowing an intestinal cavity. The ectosarc may be differentiated into a specially modified cortical layer, and well-marked muscular fibres may be developed. Moreover, the endoplast, or "nucleus," becomes an organ of reproduction, the germs of the young being given off by division from it. Very generally, a small body -- the so-called "nucleolus," but which has, admittedly, nothing to do with the structure so named in a true cell, and may be termed the "endoplastula" is to be found close to the nucleus, and there is some ground for supposing it to be a testis. The Infusoria frequently multiply by fission, which may, or may not, be preceded by encystment; and in many of them, as in the Gregarinid, Acinetid, and some Flagellata, conjugation has been observed. It is yet disputed how far the conjugation is a necessary antecedent of the process of endogenous germ formation.
Ehrenberg concluded, from those remarkable researches which first gave a clear insight into the structure of the ciliated Infusoria, that they were animals of complex structure, possessing, on a minute scale, all the organs characteristic of the higher forms of animal life. In opposition to this view, Dujardin started the conception that they are little more than masses of sarcode (= protoplasm); and Von Siebold, modifying this view in accordance with the cell theory, regards them as the equivalents of single cells of the tissues of the higher animals. The result of the long controversy which has been carried on on this subject seems to be, on the one hand, that Ehrenberg was quite right in vindicating for the Infusoria a far greater complexity of structure than they had been supposed to possess. It is certain that an Infusorium may possess a distinct integumentary layer, muscles, a permanent sophagus, a permanent anal area, and, in some cases, a persistent tract  of the body substance, more permeable to alimentary matters than the rest, which might be fairly termed a permanent alimentary tract. Moreover, there is much reason for regarding the endoplast and endoplastula as generative organs, while there is, sometimes, a rather complex persistent system of water vessels. But, on the other hand, this complexity of organisation is different from that observed in the higher animals, inasmuch as the various structures enumerated do not result from the metamorphosis of histogenetic cells, but arise by immediate differentiation of the finely granular protoplasm of which the body is composed. And, so far, Von Siebold appears to have been fully justified in regarding a ciliated Infusorium as the homologue of a single cell. This is a view which will present no difficulty to those who are familiar with the morphology of the lower plants. The complicated mycelium of Mucor Mucedo, for example, is, while young, nothing but a single cell; and, in Caulerpa, a single undivided cell grows, without division, into an organism which simulates one of the higher algæ in the diversity of its parts.
II. The Metazoa.
The germ becomes differentiated into histogenetic cells, and these cells become arranged into two sets, the one constituting the outer wall of the body, or ectoderm, while the other, or endoderm, lies internal to the foregoing, and constitutes 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 their existence in the form of an ovum, which is essentially a nucleated cell, supplemented 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 blastocle (cleavage cavity, "Furchungshöhle" of the Germans), which may be larger or smaller, filled only with fluid, or occupied by food yelk. When it is largest, the blastomeres, disposed in a single layer, from a spheroidal vesicle, enclosing a correspondingly shaped blastocle. 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 anentorous 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 form of a Gastrula, may be termed, as Haeckel has proposed, Gastræ.
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, Clenterata, 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 blastomeres. This is the process observed in certain Turbellaria, in the Ctenophora, in the Oligochæta and Hirudinea, in the Arthropoda, and in most Vertebrata.
In a third group of instances, the Morula, whether consisting of equal or unequal blastomeres, becomes spheroidal, and encloses a correspondingly shaped blastocle. 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 Chtognatha, Echinodermata, Gephyrea, polychætus Annelida, Enteropneusta, 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 Gastrula is formed, and whatever 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 remarkable researches of Haeckel have shown, the walls of the deeply cup-shaped Gastrula become perforated by the numerous 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 becomes the permanent mouth (Archostomata). 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 the Gastrula becomes the oral aperture of the adult in the Clenterata, which group includes animals differing much in grade of organisation, from the simple Hydra to the complex Ctenophore, but all manifestly exhibiting variations of one fundamental type. Parallel with these may be ranged an assemblage composed of the Turbellaria, Rotifera, 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 Clenterata. The lower Rhabdocla come very close to the Infusoria (as close as the multicellular to the unicellular Algæ), and are but little superior to Hydra in the degree of their organic differentiation; while, in the Trematoda, the land Planari, and the Nemertidæ, we have animals which attain a considerable complexity , and in the case of many Trematoda and of Lineus (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 Oligochta and the Hirudinea also belong to this division of Scolecimorpha, but they differ from the foregoing in the development of a segmented mesoblast.
In the Clenterata, Nematoidea, Turbellaria, Trematoda, and Rotifera, the mode of origin of the cells which lie between the epiblast and the hypoblast, constitute the mesoblast, 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 Oligochta and the Hirudinea, after the epiblast and hypoblast are differentiated, 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 mesoblast. This becomes marked out by transverse constriction into segments, and, in each segment, gives rise to all the tissues which lie between the epiblast (epidermis) and hypoblast (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 Rhabdocle 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 together. The like takes place in a great many Clenterata, 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 "enterocle" 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 Oligochta 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 Annelida polychæta; 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 of 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 perivisceral cavity, or vascular canals, or both combined, they arise from the splitting of excavation of the mesoblast itself, and are not prolongations of the alimentary cavity. Hence the may be termed Schizocla.
But, in certain other deuterostomatous Metazoa, the mesoblast becomes excavated, and a "perivisceral cavity" and vessels are formed in quite another fashion.
Thus in the Chætognatha, represented by the strange and apparently anomalous Sagitta, Kowalewsky's researches show, that the vitellus undergoes complete segmentation, and is converted into a vesicular Morula, on one side of which involution takes place, and gives rise to the primitive 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 cæcal 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 primitive alimentary cavity (which opens at its anterior end, and becomes the permanent alimentary canal), and are converted 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 Sagitta is temporarily clenterate, but the two gastrovascular sacs, each enclosing an enterocle, 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 enterocle 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 Müller, confirmed in their general features by all subsequent observers, proved, firstly, that the ciliated embryonic Gastrula (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 Clenterata; secondly, that the embryo thus provided 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 apertures; and fourthly, that, within this bilaterally symmetrical larva or Echinopdium, as it may be called, the more or less completely radiate Echinoderm is developed by a process of internal modification.
Müller 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 pseudhæmal vessels, nor of the perivisceral cavity. Müller's conclusions remained unchallenged until 1864, 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 commence as two primitively symmetrical diverticula of the stomach (the "würstformige Körper of Müller), one of which becomes connected with the exterior by an opening (the dorsal pore observed by Müller, 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 Echinopdium, 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 pseudhæmal 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 diverticula. Moreover, his observations on other Echinodermata show that essentially the same process of development of the peritoneal cavity occurs in Ophiurid, Echinid, and Holothurid.
The precise mode of origin of the pseudhæmal system, or so-called blood-vessels, of the Echinoderms is not yet made out. But it is known that the cavity of these vessels 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 Metschnikoff justly insist upon the correspondence in development of the lateral gastric diverticula of the Echinopdium with the gastrovascular canal system of the Ctenophora; and, on the ground of this resemblance, the former refers the Echinoderms to the Radiata, retaining under that Cuvierian denomination the Acalephæ (Clenterata) 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 clenterate 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 Echinopdium 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 symmetrical. In fact, the larva of Balanoglossus (the sole representative of the Enteropneusta), was originally described by Müller, 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 perivisceral 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 Chætognatha and Echinodermata, 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 Schizocla, in which the perivisceral cavity is produced by a splitting of the mesoblast, they may be said to be Enterocla.
If we endeavor to determine the place of the three remaining great groups of animals, the Mollusca, the Tunicata, and the Vertebrata, obstacles arise, firstly, from a want of sufficiently exact knowledge respecting the Mollusca; and secondly, from the difficulty of interpreting certain well-ascertained facts in the Vertebrata.
That the Mollusca, including under that name the Polyzoa and Brachiopoda, as well as the higher Mollusks, are closely allied to the Annelida, is readily demonstrated. The known forms of Brachiopoda, Lamellibranch, Pteropod, and Gastropod larvæ all have their parallels among Annelidan larvæ. The Polyzoa 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 conclusion. In the Brachiopoda, there is some ground for thinking that the perivisceral cavity is formed in the same way as in Sagitta and the Echinodermata; while, in the Lamellibranchiata and Odontophora, there is every reason to believe that the perivisceral cavity is formed by splitting of the mesoblast, or that they are schizoclous.
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 blastocle, and the "house" in which these singular animals shelter themselves is a cuticular secretion, representing the cellulose coat of the higher ascidians. The Appendiculariæ have no atrium, or at most only rudiments of it, hence the branchial clefts open directly on the hæmal aspect of the body, which corresponds with the ventral face of a vertebrate animal. In all other Tunicata, an atrial cavity is formed by involution of the ectoderm, which 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 invaginated 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; and the space between the two (the blastocle) 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 developed, 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 epithelium of the alimentary canal, which, to the same extent, correspond with the splanchnopleure 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 enteroclous nor schizoclous, but what, at first sight, resembles a perivisceral cavity is formed within the body by involution, and the ascidian may therefore by said to be epiclous. 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 pleuroperitoneal3 cavity appears to be formed by the splitting of the mesoblast into two layers, a splanchnopleure and a somatopleure, and, therefore, 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 embyronic of known Vertebrata, Amphioxus, throws very great doubt upon this interpretation of the facts. One of the most singular of the many peculiarities of Amphioxus is the fact 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 Amphioxus 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 pleuroperitoneal cavity. Thus the lining of the cavity, like the atrial tunic of the ascidian, is a derivative of the epiblast; and Amphioxus is epiclous. As it can hardly be doubted that the somatopleure of Amphioxus is the homologue of the somatopleure in the higher Vertebrata, it becomes  highly probably that the apparent splitting of the mesoblast in the latter, after all, represents the mode of development of the pleuroperitoneal cavity which obtains in the former, and, thus, that the Vertebrata are not schizoclous, but epiclous. Whether this suggestion will turn out to be well based or not, must be decided by the embryological 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 pleuroperitoneal 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 between 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 Gastr. In some members of this division, however, the alimentary canal may be rudimentary, as in sundry male Rotifera 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 apparatus 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 parasites have not descended from Gastr, but have passed into their present condition directly from a Morula-like form of Metazoon. In the latter case they will form a division of Agastr, apart from the other Metazoa.
The subjoined synopsis indicates the general relations of the different groups of the Animal kingdom, in accordance with the views which have been put forward in the preceding pages.
Those who are familiar with the existing condition of our knowledge of animal morphology, will be aware that any such scheme must needs, at present, be tentative and subject to extensive revision, in correspondence with the advance of knowledge. Nor will they regard it as any objection to the scheme of classification proposed, that the divisions sketched out may be incapable of sharp definition -- the constant tendency of modern investigations being to break through all boundaries of groups, and to fill up the gaps between them by the discovery of transitional forms. In the place of assemblages of distinctly definable groups, which it has hitherto been the object of the Taxonomist to define an co-ordinate in precise logical categories, we are gradually learning to substitute series, in which all the modifications by which a fundamental form passes from lower to higher degrees of organic complication, are summed up.
1 The term "cell" here in its broadest sense, as equivalent to a nucleated mass of protoplasm. 2 The term "Rhizopod" is already employed in a limited and special sense. 3 It must be recollected that the pericardium is also originally a part of this cavity, and that in some fishes, e.g., the Rays, it never becomes completely shut off from it.
1 The term "cell" here in its broadest sense, as equivalent to a nucleated mass of protoplasm.
2 The term "Rhizopod" is already employed in a limited and special sense.
3 It must be recollected that the pericardium is also originally a part of this cavity, and that in some fishes, e.g., the Rays, it never becomes completely shut off from it.
C. Blinderman & D. Joyce