VERTEBRATA, the name of a great branch or phylum of the Animal Kingdom which comprises those animals having bony “vertebrae ”, or pieces of bone jointed so as to form a spinal column. The first recognition of the group is due to Lamarck (1797), who united the four highest classes of Linnaeus’s system as “animaux à vertèbres,” whilst distinguishing the rest of the animal world as “animaux sans vertèbres.” The same union of the four Linnaean classes had been previously made by Batsch in 1788, who, however, proposed for the great division thus constituted the name “Knochenthiere.” The significance of Lamarck’s classification was materially altered, and the foundation laid of our present attempts to represent by our classifications the pedigree of the animal kingdom, when Cuvier propounded his doctrine of “types,” and recognized the Vertebrata as one of four great types or plans of structure to be distinguished in the animal world (I ).
The Vertebrata of Lamarck and Cuvier included beasts, birds, reptiles, and fishes, and until recently the group was considered as one of the most sharply limited in the animal kingdom. The progress of anatomical studies very soon rendered it clear that all Vertebrata did not possess bony vertebrae; for, besides the commoner sharks and skates, with their purely cartilaginous skeletons, naturalists became acquainted with the structure of fishes, such as the sturgeons and the lampreys, which possess no vertebrae at all, but merely a continuous elastic rod (the notochord) in the place of the jointed spinal column. The muscles and their skeletal septa were seen in these fishes to be arranged in a series of segments attached to the sides of this continuous rod; and hence the structural character of bony vertebrae, as distinguishing the Vertebrata, gave place to the character of segmental arrangement of the muscles of the body-wall, such muscles being supported by a skeletal axis which might be itself unsegmented (notochord), or replaced by segmental cartilaginous or bony vertebrae. The studies of embryologists furnished a sound foundation for this conception by demonstrating that in the embryos of Vertebrata with true vertebrae these structures are preceded by an unsegmented continuous notochord. The inquiry into the structural characteristics of Vertebrata led further to the recognition of several additional points of structure, the combination of which was present only in the group which had been recognized by Lamarck on superficial grounds. It was found that all Vertebrata possess laterally-placed passages leading from the pharynx to the exterior, serving in the aquatic forms as the exits for water taken in by the mouth, and provided with vascular branchial processes, whilst in the embryos of the higher air-breathing classes they appear only as temporary structures. It was further established that the great mass of nervous tissue lying dorsally above the spinal column, and known as the cerebro-spinal nerve-centre or brain and spinal cord, is in all cases a tube, and originates as part of the dorsal surface of the embryo, which becomes depressed in the form of a long groove and finally closed in by the adhesion of its opposite edges, thus forming a tube or canal. The three structures,—notochord, gill-slits, and tubular dorsal nerve-cord,—were more than twenty years ago recognized as characterizing, together with the metameric segmentation of the musculature of the body-wall, all Vertebrata at some one or other period of their existence.
The establishment by Darwin of the doctrine of organic evolution in 1859 led naturalists consciously to make the attempt to determine the genetic affinities and the probable ancestry of the various groups of animals, and enabled them to recognize in the classifications by “type”, and other such conceptions of earlier systematists, the unconscious striving after genealogical representation of the relationships of organic beings. The question naturally arose in regard to the Vertebrata, as in regard to other great divisions of the animal kingdom, What were the characters of the earliest forms, the ancestors of those now living? Then came the further questions as to whether any surviving Vertebrata closely resemble the ancestral form, and whether any animals are still in existence which retain the general characters of those primeval forms which were the common ancestors at once of Vertebrates and of other large and equally well-marked phyla or branches of the animal kingdom, such as the Molluscs, the , &c. This fascinating subject of inquiry received its most important impulse from the embryological investigations of the Russian naturalist Kowalewsky, and has been for nearly a quarter of a century the fertile source of speculation and its indispensable accompaniments, new observation and research. Kowalewsky published in 1866 an account of the embryology of the lowest and simplest of then recognized Vertebrates, the lancelet (Amphioxus lanceolatus), in which he attempted to trace, cell for cell from the fertilized egg-cell, the origin of the characteristic Vertebrate organs of this animal (2 ). This work alone would not have acquired historic importance, although it is the starting-point of what may be called strict cellular embryology, as compared with the less severely histological works of previous students. But it was accompanied by an account (3 ) of the development of Ascidia mamillata, one of the so-called Tunicate Molluscs, in which it was demonstrated by Kowalewsky, not only that this supposed Mollusc possesses when first hatched from its egg-envelope a notochord, pharyngeal gill-slits, and a tubular dorsal nerve-cord and brain, but that these three characteristically Vertebrate features of organization originate from the same cell-layers of the embryo, and in essentially the same way as in Amphioxus, whilst the cell-layers themselves originate from the egg-cell in the two animals by precisely similar movements of cell division and invagination (see figs. 1 and 2). Kowalewsky’s discoveries established once for all that the Ascidian tadpole is identical in three very special and distinct features of structure with the frog’s tadpole. No classification which pretended to set forth the genetic affinities of animals could henceforth separate the Ascidian from the Vertebrata, and with it the Ascidian brought the whole series of Tunicata.
The admission of Tunicata as a group of Vertebrata was proposed by the present writer as long ago as 1877 (4 ); but it required the intermediate proposition by Balfour of a group Chordata, to comprise the two divisions Tunicata and Vertebrata, in order to render the final admission of Tunicata to their proper association with the Vertebrata of Cuvier palatable to systematists. As an objection to the simple inclusion of Tunicata in the great phylum Vertebrata it has been urged that Tunicata do not possess vertebrae,—a proposition which is equally true of Amphioxus and of some Fishes. Shifting the objection, some writers have maintained that the vertebration of the Vertebrata may be understood as having reference to the segmentation of the muscles of the body-wall, which is exhibited by all Cuvier’s Vertebrata without exception, inclusive of Amphioxus, though not by Tunicata. To this it may be replied that the Ascidian tadpole, and more clearly the free-swimming Tunicate Appendicularia (see fig. 9), do exhibit a segmentation of the muscles of the hinder part of the body-wall similar to and identical with that of Amphioxus, whilst no such strict application of a name in its original descriptive sense is desirable in systematic nomenclature. All Gastropoda (it has been pointed out) are not gastropodous; all Arthropoda are not arthropodous; and many Echinoderma are not echinodermous. It is, in the present writer’s opinion, better to retain an historic and familiar name for the great branch of the animal pedigree to which it has become necessary to admit forms whose affinities therewith were at one time unsuspected rather than to sacrifice historical significance to a futile striving after etymological accuracy.
The admission of Tunicata to association with Cuvier’s Vertebrata has been followed by a further innovation. The remarkable marine worm Balanoglossus— originally described by Della Chiaje at the end of the 18th century— was shown in 1866 by Kowalewsky (5) to possess a series of pharyngeal gill-slits similar to those of Tunicata and Amphioxus. Later researches by Bateson (ό) have demonstrated that Balanoglossus develops in embryonic life a short notochord, whilst its nerve-cord is, in part at least, tubular, and similar in position and relations to the median epidermal tract by the infolding of which the nerve-tube of Tunicata and the other Vertebrata is formed. Hence it seems impossible to exclude Balanoglossus from a place in the phylum Vertebrata. The possession of pharyngeal gill-slits alone might not justify the association; but, when this is combined with the presence of the notochord, though rudimentary and with a special condition and position of the main nerve-centre, it becomes impossible to ignore Balanoglossus in our conception of the Vertebrate phylum. The recent discoveries of Harmer (7 ) with regard to Cephalodiscus will hereafter render it necessary to associate that form, and in all probability Rhabdopleura also, with Balanoglossus in the Vertebrate phylum. The further consequences of such an association in regard especially to the affinities of Polyzoa and of Gephyraea open up a large field of speculation and of consequent embryological and anatomical research.
The Cuvierian Vertebrata, Amphioxus, Tunicata, and Balanoglossus being thus indisputably connected by a remarkable combination of structural points, which admit of no explanation consistent with the principles of evolutional morphology except that of the genetic relationship of the forms thus enumerated, we are at once confronted by those questions as to the ancestral history of Vertebrata which have been already mentioned above as stimulated by Kowalewsky’s discoveries. Undoubtedly Amphioxus is lower and simpler in structure than any Fish, Tunicata as low as or lower than Amphioxus, and Balanoglossus, in some respects, more archaic than either Amphioxus or the Ascidian tadpole. The first tendency arising from the discovery of the affinities of these simpler forms with the Cuvierian Vertebrata was to see in them the representatives of the ancestors of all Vertebrata. Amphioxus has been pointed to by authorities in morphology as the living presentation of our common Vertebrate ancestor; a similar position corresponding to an earlier stage of development has been admitted by no less an authority than Darwin for the Ascidian. It appears, nevertheless, that all such simple solutions of the problem of Vertebrate ancestry are without warrant. They arise from a very common tendency of the mind, against which the naturalist has to guard himself,—a tendency which finds expression in the very widespread notion that the existing anthropoid apes, and more especially the gorilla, must be looked upon as the ancestors of mankind, if once the doctrine of the descent of man from ape-like forefathers is admitted. A little reflexion suffices to show that any given living form, such as the gorilla, cannot possibly be the ancestral form from which man was derived, since ex hypothesi that ancestral form underwent modification and development, and in so doing ceased to exist. The same considerations apply to the question of the ancestry of Vertebrata. Probably no existing low form of Vertebrate closely represents the ancestral form by the modification of which higher forms have been developed. We have no justification for assuming that such low forms do more than present to us a collateral branch of the family, and that collateral branch must, in all probability, have experienced its own special series of modifications of structure. Not only this, but we have no sufficient ground for assuming that, even in respect of the simplicity of their structure, any given animal forms at present existing exhibit a mere survival of a corresponding degree of simplicity in their remote ancestors. Such an assumption was almost universally made, until a more correct view was pressed on the attention of naturalists by Dr Anton Dohrn, the founder of the zoological station of Naples (8 ). So far from its being the case that simplicity of organization necessarily implies the continuous hereditary transmission of a low stage of structural development from remote ancestors, there are numerous instances in which it is certain that the existing simplicity of structure is due to a process of degeneration, and that an existing form of simple structure is thus descended from ancestors of far higher complexity of organization than itself. Such are various parasitic worms and Crustacea. The evidence in favour of the occurrence of progressive simplification of structure or degeneration, in place of progressive elaboration, depends (1) upon the comparison of the adult structure of the degenerate organism with that of its nearest allies, by which it is often rendered clear that the ensemble of the organization of the simpler organism cannot be explained on the hypothesis that it represents an ancestral or archaic condition common to it and its more elaborate congeners, and (2) on the direct evidence of individual development or life-history. The latter evidence is conclusive, when we find, as in the case of Cirrhipede Crustaceans and of Ascidian Tunicates, that the embryo on its way to the adult condition passes through-stages of development presenting a higher degree of organization than that ultimately reached, so that, as in the Cirrhipede larva and the Ascidian tadpole, the young form resembles allied organisms of a higher stage of development, and subsequently degenerates from the point of progressive elaboration to which it had attained, and becomes greatly simplified in the final stage of its growth. Conclusive as such evidence is, there is no law of development which necessitates its preservation. If it be an advantage to the organism, the full force of heredity has play, and what are called the “recapitulative phases” of ancestral development are passed through by the individual in the course of development from the egg. But with remorseless thoroughness all such hereditary tendencies may be removed when such removal is an advantage to the organism, and the development from the egg may proceed directly to the adult degenerate form. Such is the case with many Tunicata, the young of which never exhibit notochord and tadpole form; indeed, were it not for the preservation of a few exceptional cases, like that of the Ascidian section of the group, we should have no direct evidence of the degeneration of Tunicata from tadpole-like ancestry.
The general result of the considerations which have been urged with regard to degeneration is this, that it is prima facie as legitimate an hypothesis, that any existing animal has developed by progressive simplification from more elaborate ancestors, as it is that such an animal has developed by a continuous and unbroken progress in elaboration from simpler ancestors; and we are specially called upon to apply the hypothesis of degeneration where the animal under consideration is likely from its mode of life to have undergone that process. Such modes of life, tending to degeneration, are parasitism, sessile or adherent habit, burrowing in the sea-bottom, and diffuse feeding. The animal which pursues living prey, and contends with other organisms for the dominion of the regions of earth and water that are flooded with light and richly supplied with oxygen gas, is the animal which represents the outcome of a longer or shorter period of progressive elaboration. It is worth while noting in parenthesis that in all cases the “whirligig of time” has probably brought its revenges, and that the ancestry of a form evolved through a long period of progressive elaboration was at an antecedent period subject to simplification and degeneration, whilst in the past records of the present exemplars of the latter process there must certainly have been long stretches of continuous elaboration.
Applying these considerations to the construction of the genealogical tree of Vertebrata, we find that the task is by no means simplified. We cannot with the earliest evolutionists adopt a scale or ladder-like series, placing the simplest form on the lowest step ; nor can we be satisfied with a tree-like arrangement, in which the forms at the ends of the branches are always more elaborate than those nearer the trunk. Our genealogical tree will more strictly conform to that of a parvenu human family, if we take worldly prosperity in the latter case as corresponding to elaboration of structure in the former. The strict family genealogist will include in the successive ramifications of the tree the five sons of the founder of the family, one of whom remained an agricultural labourer, whilst two became brewers and two emigrated. The cousins in the next generation will be set forth in place, the sons of one brewer becoming paupers, whilst those of the other advance to the position of Government employes, and one to the peerage. Thus in successive branchings of the family history there may be alternate progress and degeneration. And so it must be in the genealogical trees constructed by the naturalist: the fact that a branch is later in origin will not imply that it is higher in elaboration than those below it, and accordingly we must not expect to draw our tree so as to be able to trace all simpler forms to lower off-sets of the tree.
The structural features of those animals which must be admitted to the Vertebrate phylum in consequence of possessing notochord, pharyngeal gill-slits, and dorsal nerveplate, tubular or unrolled, are such as enable us very readily to group them in four great divisions, which appear to be equally distinct from one another. As to what may be the genetic relations to one another of these four groups we will inquire subsequently; for the present we term these groups “branches.” They are as follows :— [table]
The Vertebrata thus limited may be defined as Ccelomate Enterozoa (Metazoa) with well-developed coelom. In all, with the exception of the more degenerate members of Urochorda, an elastic skeletal rod—the notochord—is developed dorsally by an outgrowth of cells forming the wall of the primitive archenteron; the notochord may or may not persist in adult life. Pharyngeal gill-slits, which may or may not persist in adult life, are developed in all Vertebrates. In all, except in certain Urochorda, the chief nerve-centre has the form of a dorsal, median, elongate tract, derived from the epiblast, which becomes sunk below the surface and invaginated so as to form a tube. In all there is a tendency to metameric repetition of parts, which may find its expression in a strongly-marked segmentation of the musculature of the body-wall and its skeleton, or may be recognizable only in a limited degree, as exhibited by the successive gill-slits or successive gonads.
We shall now examine the distinctive features of each of these large groups, and form an estimate of their relations to one another, and of their probable ancestry, this being the task to which we must limit ourselves in the brief space here afforded.
The Craniata.
The Craniata are Vertebrata in which the tubular cerebro-spinal nerve-mass is swollen anteriorly to form a brain, consisting primarily of three successive vesicles, in connexion with the anterior of which the special nerves of the olfactory organs and of the eyes originate. The notochord, whilst extending posteriorly to the extremity of the body, does not reach quite so far forward anteriorly as the termination of the nerve-tube. A cartilaginous cranium or brain-case develops round the anterior extremity of the nerve-cord, and rises up laterally so as to enclose and protect the brain (hence Craniata). Cartilage is developed in other parts of the body as a skeletal substance, though it may be subsequently replaced in the cranium, as elsewhere, by bone. The longitudinal muscles of the body-wall are divided by transverse fibrous septa into a series of segments, varying in the adult from ten to one hundred or more in number. Cartilaginous neural arches, corresponding in number and position to the fibrous septa, and resting on the notochord, are developed so as to protect the nerve-cord. Cartilaginous bars also pass outwards, with a direction at first horizontal and then ventral, from the sides of the notochord into the intermuscular fibrous septa. Very generally, but not always, a tubular cartilaginous sheath forms round the notochord; this sheath with rare exceptions becomes segmented to form a series of vertebral bodies, which lie in the planes of the fibrous intersegmental septa, and, increasing in thickness by encroaching upon the substance of the notochord, finally obliterate it almost entirely.
The pharyngeal slits follow closely upon the mouth, and in existing Craniata never number more than eight pairs (see fig. 3). They are separated from one another and their apertures strengthened by a series of cartilaginous hoops, the first of which, that between the mouth and the first gill-slit, forms the primitive upper and lower jaw in all but the small and degenerate group Cyclostoma. The gill-slits when functional are generally protected by an opercular fold of the body-wall, which overhangs them and corresponds to the epipleural fold of Amphioxus, the collar of Balanoglossus, and doubtfully to the wall of the atrial chamber of Urochorda. The extension of this fold along the sides of the middle third of the body (between the pharyngeal region and the anus) acquired in ancestral Craniata the function of a continuous right and left lateral fin (see fig. 4). At the same time a continuous median fin, corre sponding to the dorsal, caudal, and anal fins of existing Fishes, was developed. In both lateral and median fins a cartilaginous skeleton was developed, consisting of a basal longitudinal bar, supporting a number of rods like the teeth of a comb. The primitive form of fin skeleton is retained in the median fins of some sharks; the primitive lateral fin has in all cases either entirely disappeared (Cyclostoma) (as has their anterior extension, the operculum, in many cases) or it has become, together with its skeletal elements, concentrated in two regions—forming the pectoral and the pelvic paired appendages or limbs, with their respective girdles.
The cerebro-spinal nerve-centre and the disposition of the nerves issuing from it present a remarkable complexity, and at the same time uniformity, of structure in all Craniata (see fig. 5). The foremost of the three primary cerebral vesicles gives rise to paired anterior outgrowths, the prosencephala, to a median dorsal outgrowth, the stalk of the pineal eye (rudimentary in all existing Craniata), and to a median ventral outgrowth, which is met by an invagination of the epidermis of the oral cavity forming the pituitary body, further, to a pair of lateral outgrowths, which become the right and left optic nerves and retinae respectively. The modifications of the hindmost of the three primary vesicles are also extremely definite and persistent throughout the group: its anterior dorsal surface enlarges and becomes the cerebellum (the metencephalon), whilst the cavity of its hinder part (the medulla oblongata or the epencephalon) becomes comparatively wide, and is covered dorsally by a thin membrane only, in which nervous tissue does not take a part. The intermediate primary cerebral vesicle (the mesencephalon) does not give rise to outgrowths.
In all Craniata nerves are given off from the cerebro-spinal cord or tube with great regularity, one right and left in each successive myomere or segment of the body-wall. Each nerve has two roots, a dorsal (sensory) and a ventral (motor). A commissure between each successive pair of deep or intestinal branches of the spinal nerves forms the so-called sympathetic nerve-cords, one on each side of the vertebral column. Nerves similar to the spinal nerves, but not identical with them, are given off from the brain, and perforate the cranial box right and left. In all Craniata there are ten pairs of nerves which originate thus, and in the higher forms two more pairs (elsewhere spinal) are included amongst those which thus perforate the cranium (spinal accessory and hypoglossal). The order and character of the cranial nerves are the same in all Craniata. The first (olfactory) and the second (optic) are unlike spinal nerves in both distribution and origin. As we pass backwards along the series, the cranial nerves are found to resemble more and more the ordinary spinal nerves. Hence it has been inferred that the cranial region consisted at one time of a number of distinct myomeres (as many as nine), which have become fused and modified to form the typical craniate “head.” The oculo-motor or 3d nerve indicates the first of these segments, the trochlear or 4th nerve the second, the abducens or 6th the third, the facial and auditory (7th and 8th) the fourth, the glosso-pharyngeal or 9th the fifth, and the vagus or 10th, with certain of its branches supposed to have been originally distinct nerves, the sixth, seventh, eighth, and ninth. It is probable that in Craniata the metamerism of the gill-slits does not correspond to the metamerism of the body-wall. The mouth and each successive gill-slit are related to a bifurcate branch of a cranial nerve (furcal nerve) in lower Craniata (see fig. 3); these furcal nerves do not correspond, so far as we can at present judge, with cranial myomeres.
The lateral pair of eyes (as opposed to the rudimentary pineal or parietal eye) present a striking uniformity of origin and structure throughout Craniata. Not only are they uniformly developed from three elements, viz., the retinal cup which grows out from the anterior of the primary cerebral vesicles, the epidermal lens which grows inwards from the surface of the skin, and the connective tissue between these two, but we find that the muscles attached to the eyeball are identical throughout the series: that is, the superior, inferior, and internal rectus, and the inferior oblique muscles represent the first cranial myomere, the superior oblique represent the second, and the external rectus represent the third cranial myomere.
The olfactory sacs are paired in all except Cyclostoma, in which they are represented by a single sac which may or may not be archaic in its azygos character. The auditory sacs are paired organs which develop as invaginations from the surface,—the orifice of invagination closing up,—at the hinder part of the cranial region. They present a gradually increasing complexity of form as we pass from aquatic to terrestrial forms, but are identical in essential structure throughout.
All Craniata, except some Fishes, possess a muscular process on the floor of the oral cavity which may carry teeth, or act as a licking organ, or assist in suction. This is the tongue.
All Craniata, with degradational exceptions, possess an outgrowth, single or paired, of the post-pharyngeal region of the alimentary canal, which is filled with gas. In many Fishes this becomes shut off from the gut; in others it remains in communication with the gut by an open duct. In Fishes it functions as a hydrostatic apparatus. In terrestrial Craniata it is subservient to the gas-exchange of the blood and becomes the lungs.
All Craniata have a large and compact liver; and a pancreas is also uniformly present, except in Cyclostoma, some bony Fishes, and the lower Amphibia.
All Craniata have a thick-walled muscular heart, which appears first as an “atrium,” receiving the great veins, attached to a “ventricle” by which the blood received from the atrium is propelled through a number of arteries, right and left, corresponding in number to the pharyngeal gill-slits between which they pass. A praeatrial chamber (the sinus) and an extra-ventricular chamber (the conus) are added to the primitive chambers; but the most important modifications arise in consequence of the development of pulmonary respiration and the gradual separation of the cavities of the heart by median septa into a double series, a right and a left. The plan of the great arteries in all Craniata is in origin the same, and is determined by the primitive existence of a branchial circulation in the gill-slits, which is obliterated in higher forms. Similarly the plan of the great veins is identical, the primitive posterior vertebral veins of lower Craniata, though persistent in higher members of the group, having their function gradually usurped by the excessive development of the renal vein, and of renal-portal and ultimately of iliac veins.
All Craniata have a lymphatic system or series of channels by which the exudation from the capillary blood-vessels is returned to the vascular system. It includes in its space-system the coelom and a variety of irregular and canalicular spaces in the connective tissues. Masses of spongy tissue (adenoid tissue, lymphatic glands) exist, through which the lymph filters, and there acquires corpuscular elements as well as chemical elaboration. At various points in various Craniata pulsating or simple communications are established between the lymphatic system and the veins. A special and characteristic communication is established in the spleen, an organ which is found in all Craniata, either as a single mass or as scattered masses of spongy tissue in which blood-vessels and lymphatics unite.
The renal organs of Craniata are primitively a series of nephridia corresponding in number to the myotomes of the mid-region of the body in which they exist. They are connected in the simplest Craniata by a right and a left archinephric duct, which appear to be in origin lateral grooves of the epidermal surface. This primitive renal system has been modified in some lower forms (Cyclostoma and Teleostean Fishes) by the atrophy of its anterior portion. But in all other Craniata it acquires relations to the gonads or ovary and testes, so that an anterior portion of the archinephros and a corresponding longitudinal tract of the duct become separated to serve as oviduct, a middle portion to serve as sperm-duct, while a posterior portion retains exclusively or shares with the middle portion the function of urinary excretion. The male and female gonads are, with the rarest exceptions, developed in distinct individuals, though the rudiments of the suppressed gonad may in some cases (Amphibia) be traceable in either sex.
The group of Craniate Vertebrata thus anatomically described, whilst retaining the essential unity indicated, presents an immense variety of modifications. The chief modifications are distinctly traceable to and accounted for by mechanical and physiological adaptation to a terrestrial and air-breathing life, as opposed to the earlier aquatic and branchial condition. The existing forms of Craniata have been arrested at several points, in the progress towards the most extreme adaptation to terrestrial conditions, which is presented by those forms that can not only breath air and live on dry ground but fly habitually in the air. The organs most obviously affected by this progressive adaptation are the skin, the skeleton, especially of the limbs, the pharyngeal gills, and the air-bladder. This fact will appear most clearly in the subjoined classification of Craniata; for space does not permit us to pursue further the history of these modifications.
[table]
If we now briefly consider what must have been the common ancestral form from which these Craniata have proceeded, making use of such internal evidence as their structure affords, we find that we get no further back than such an animal as would fit the description given above, with the exception that we should be warranted in substituting in the ancestor a pair of continuous lateral fins, with comb-like cartilaginous skeleton, in place of the two pairs of fins, or their total defect, seen in living Craniata. We get no clear suggestion from the study of Craniata themselves as to the meaning of the curious shape of the brain and its outgrowths (though the pineal outgrowth has recently been explained as an eye), nor as to the original genesis of the notochord. We should, however, be justified in representing that region which now corresponds to the hinder part of the skull and brain as more fully developed and segmented, so as to give a series of separate myotomes and perhaps separate nerves corresponding to the several furcal branches of the vagus; and we may very well suppose that the number of pharyngeal gill-slits was larger in the ancestral than in any living form, though it seems improbable that in any true Craniate did each gill-slit correspond to a distinct muscular segment.
An attempt to go further than this has been made by Dr Anton Dohm by the method of hypothesis and subsequent corroborative inquiry into facts of minute structure and embryological history. Making use of the principle of degeneration, Dohm started with the legitimate hypothesis that the branches of Vertebrata other than Craniata— viz., Cephalochorda, Urochorda, and (though at the time he commenced his work their structure was not fully understood) Hemichorda— were not to be regarded as permanent records of steps in the evolution of Craniata, but rather as greatly degenerate offshoots from the ancestors of that group, which could throw but little light on the character of their non - degenerate ancestors. A second fundamental assumption which led Dohm to his position was that the segmentation of the Craniates’ body-wall is a primitive and essential feature in their structure, and becomes more and more fully expressed instead of less developed the further we go back in their ancestry. Dohrn, in fact, assumes that what is called metameric segmentation is a phenomenon of structure which has occurred once only in the history of animal form, and that all segmented animals are genetically related and descended from a common segmented ancestor. Assuming this, he pointed to the existing Chaetopod Worms as most nearly representing at the present day the common ancestor of segmented animals. They have, as he pointed out, a high organization, little inferior to that of the lowest Craniata ; they possess a well - developed coelom, blood-vessels with red blood, a segmental series of nephridia (modified in some as gonaducts), segmental branchiae, and lateral locomotive organs; not a few develop cartilage as a skeletal support; and many show a concentration and fusion of segments to form a complex head, which resembles, so far, that of Craniata. The ventral in place of the dorsal position of the nerve-cord led Dohm to accept De Blainville’s conception that the dorsal and ventral surfaces are reversed in Vertebrata, as compared with Annelids, Crustaceans, and Insects, so that the Vertebrate is compared to an Insect walking with its ventral surface upward. This led further to the notion that the mouth of the Chaetopod or Annelid, which penetrates the nerve-cord, or rather passes between its two divaricated lateral constituents in those animals, has in Craniata disappeared, its place being taken by a new mouth derived from the modification of a pair of gill-slits. The remnant of the old mouth, which should, if the comparison instituted holds good, lie in Craniata somewhere on the dorsal surface of the cranial region, was sought by Dohm in some of the peculiar and hitherto unexplained median structures of the brain: at one time the fourth ventricle with its deficient roof was suggested as thus to be explained, whilst subsequently the curious median structures,—the pineal and pituitary bodies,—were called in as possibly thus significant.
Without pursuing further the elaboration of Dohrn’s views, it must be at once noted that, whilst the legitimacy of the assumption of degeneration must be admitted, the second assumption, viz., that metameric segmentation is a character bringing all forms showing it into a special genetic continuity, cannot be accepted.
The property of repeating units of structure, so as to build up a complex of many similar parts united to form one individual, is a very general one in organic forms, and is exhibited in various conditions by both animals and plants. Its simplest expression is found in cell-structure and the binary division of cells. It shows itself as affecting larger masses of structure in the arborescent colonies of Coelentera, in the radial or antimeric composition of Echinoderms and of Compound Ascidians, and in the linear or metameric segmentation of Worms, Arthropods, and Vertebrates. There is abundant evidence that this property is a general one, which may assert itself at any period in the history of a group of animals, and does not imply special unity of origin in forms which exhibit it. As pointed out in the article Hydrozoa, merogenesis —the name applicable to this phenomenon generally—may take an extreme and complete character, leading to the separation and independence of the units of structure produced; in that case it may be termed eumerogenesis. Or the process may be very partial, occurring only during a period of embryonic growth, and subsequently ceasing, so that later growth obscures or obliterates it altogether (dysmerogenesis). There is no ground for assuming that either one of these extremes is fundamental or original. Any mechanical or nutritional condition may lead to merogenesis in an organism in which the tissues have a certain reproductive capacity, or have not acquired final differentiation; and it will depend upon the balance of advantage, determined by natural selection, whether the segmentation (supposing the merogenesis to take the linear form) results in the separation of segment-buds, or in the formation of an annulate body, or leaves traces of its occurrence only in certain tissues and organs. The Cestoid Worms present within the range of a single group almost every grade of eumerogenesis and dysmerogenesis (Caryophyllaeus, Ligula, Taenia). In the otherwise amero-genetic Mollusca, Chiton and the pearly Nautilus show dysmerogenesis in certain organs, whilst the Planarian Worms frequently exhibit eumerogenesis in their bud-segmentation (to be compared with that of the Annelid Ctenodrilus described by Zeppelin, 9) and the elongated Nemertines only slight traces of dysmerogenesis.
If we deny Dohrn’s assumption with reference to segmentation, we are no longer led in the direction of the Annelids (Chaetopods) in our search for the ancestry of the Craniate Vertebrata.
The fact that the notochord is the forerunner of the segmented vertebral column, and is itself never segmental, instead of being a difficulty, acquires directive significance. The fact that the nerve tube is dorsal, and not ventral, no longer requires the large assumption that animals have reversed their habitual carriage, but suggests that the Craniates’ ancestor had a dorsal median nerve, which has increased in size and importance so as to become the nerve-tube of existing forms.
The explanation of the curious structure of the brain will have to be found otherwise than in the assumption of a perforating pharynx,—an assumption which the recent discovery of the true nature of the pineal body has rendered untenable in the latest form advocated by its ingenious author, whose speculations, nevertheless, deserve the fullest recognition as having stimulated inquiry and guided observation.
Balfour (10 ) in 1878 refused to adopt Dohrn’s views, and considered it probable that the dorsal position of the nerve-cord in Vertebrata could be accounted for, without any assumption of a substitution of a pair of gill-slits for the original mouth, by assuming that primitively the nerve-cord consisted of two lateral cords, as seen at the present time in the Nemertine Worms, and that these cords have coalesced dorsally in Vertebrata, just as it is clearly demonstrable that two originally lateral cords have coalesced ventrally to form the Annelid’s ventral nerve-chain.
The comparison of the Vertebrates’ nervous system with that of the Nemertines had already been made by the present writer, as cited by Hubrecht (11 ) in connexion with the latter’s discovery of a complete sub-epidermal nerve-tunic in those worms. Hubrecht has more recently on two occasions (12 and 13 ) developed an interesting and important comparison of Nemertine and Vertebrate structure. He has in the first place suggested that the notochord of Vertebrata is nothing more than a modified survival of the proboscidean sheath of the Nemertines, whilst the oral invagination of the epidermis, in connexion with the hypophysis cerebri of the Vertebrate, may be a last remnant of the proboscis itself. More conclusively he has drawn attention to the median dorsal nerve of Nemertines as corresponding to the Vertebrate cerebro-spinal nervecord, whilst the great lateral nerve-cords of Nemertines, and the lateral ganglia in which they expand anteriorly, are compared to the lateral ganglia of the cephalic region of Craniate Vertebrata and the nerve of the lateral line (see fig. 6). The comparison is strengthened by the existence of a metameric series of transverse nerves in the Nemertine, which correspond in respect of their metamerism and their connexion with a dorsal median trunk, with the spinal nerves of Craniata. Hubrecht is careful to insist that he does not regard the Nemertines as representing the direct ancestry of Vertebrata ; but he points out that from the primitive condition of an elongate animal, with a plexiform nerve-tunic, it is readily conceivable that a form was developed in which the nervous tissue was concentrated in three cords,—a median dorsal and two lateral,— and from such a form we can derive the Craniates’ condition by excessive development of the median tract and relatively small development of the lateral cords, whilst the Nemertines’ condition would be attained by the converse process. The tubular condition of the cerebro-spinal nerve-cord of Vertebrata, it may here be remarked, is now very generally regarded as being in its origin a purely developmental feature. It was primitively separated from the epidermis by delamination and in-sinking, and the mode of formation by invagination of a canal has been substituted in accordance with a general embryological law of growth, which is that bulky structures originating beneath a surface from the cells forming that surface take up their position in embryonic growth by invagination of the parent surface. The tubular form, having thus started, seems to have been utilized during one phase of Vertebrate evolution for the respiration of the nervous tissue, by the introduction through an anterior unclosed pore of a current of water, which escaped by the neuranal canal (as in larval Amphioxus).
There is a wide gap between any form presenting an approach to a Nemertine Worm and the most simple Craniate Vertebrate which can be imagined still provided with the organization characteristic of all Craniata. To pass from such a Worm-like animal to a Craniate, we have to account for and introduce, amongst other new developments, (1) a greatly increased metamerism, showing itself in the segmentation of the muscles of the body-wall and in the repetition of the nephridia; (2) the characteristic sense organs; (3) the lateral and median longitudinal folds or continuous fins; (4) the cartilaginous rods and bars of the skeleton; (5) the gill-slits, even if we admit the notochord to be represented by the proboscidean sheath.
It remains to inquire whether the structure of the other Vertebrata throws light on this long hypothetical passage from the simple Worm phase to the elaborate Craniate, or suggests any other ancestry.
The Cephalochorda. Cephalochorda are Vertebrata in which there is no anterior dilatation of the nerve-tube to form a brain (see fig. 6) and no specialized skeletal brain-case. The notochord extends from one extremity of the elongate body to the other as a tapering unconstricted rod, passing anteriorly some distance in front of the nerve-cord. The longitudinal muscles of the body-wall are divided by transverse fibrous septa into a series of segments (sixty-two in Amphioxus lanceolatus), the more anterior of which are in front of the mouth and not in any way fused to form a head or cranial structure. Dense connective tissue (differing but little from cartilage) forms an unsegmented sheath to the notochord and an unbroken neural canal above it, in which the nerve-cord lies. The same tissue forms a series of metamerically repeated fin rays, which support the base of a median fin extending along the entire dorsal surface. The fin is continued ventrally from the caudal extremity as far forward as the anus, but without fin rays. Two lateral up-growths of the body-wall (the epipleura) extend one on either side from the head as far back as the anus. Each of these is divided into three regions,—(1) an anterior, which forms the praeoral hood; (2) a median, which forms the wall of the great branchial chamber, the two folds meeting one another and coalescing in the ventral mid-line, excepting where they leave a posterior median aperture, the atriopore; and (3) the post-atrioporal praeanal ventral fin (extending between atriopore and anus), which is formed by the complete coalescence of the two folds behind the atriopore.
The originally double character of this part of the ventral fin is indicated by the double series of metameric fin rays which support it. It is probable that these “epipleural” folds of Amphioxus correspond to the opercular folds and lateral fins of Craniata. No cartilaginous fin rays are developed in the atrio-pleural (opercular) region of the epipleura; but a longitudinal unsegmented bar of cartilaginous consistency strengthens its side and bounds a lymphholding canal (x in fig. 8).
The gill-slits in Amphioxus are very numerous (one hundred or more), and have no numerical relation to the metameres of the muscular body-wall, though the first few which appear in the embryo correspond at the time to successive myomeres, —a relation which they subsequently lose. The sides of the gill-slits are supported by chitinous (?) bars, and each slit is divided into two equal portions by a longitudinal tongue or bar, which grows out from the dorsal margin of the slit soon after its first formation. The number of gill-slits increases continually throughout the life of Amphioxus by the formation of new ones at the posterior border of the pharynx, whilst the myomeres do not increase in number after early embryonic life.
The nerves given off from the dorsal nerve-cord of Amphioxus are of two kinds, —dorsal and ventral. The dorsal nerves correspond in number and position to the myomeres, right and left, except in the most anterior region of the body, where two larger pairs of dorsal nerves are given off from near the extremity of the nervecord, and pass forward, supplying the region which lies in front of the termination of the musculature. The ventral nerves are minute, and are given off numerously, right and left, from the nerve-tube throughout its length. The dorsal and ventral nerves of a single myomere appear to correspond, respectively, to the dorsal and ventral roots of a spinal nerve of a Craniate.
There is a single olfactory pit in Amphioxus, which rests upon the left side of the anterior termination of the nerve-cord (see fig. 7, 4). Within the cavity of the nerve-cord at the same point a patch of brown pigment is present (eye-spot). There are no representatives of the lateral eyes of Craniata and no otocysts.
There is no representative of the Craniates' swim-bladder in Amphioxus. A single wide diverticulum of the alimentary canal represents the liver of Craniata ; the pancreas is unrepresented.
The vascular system is singularly incomplete: large trunks exist, but few branches and no heart, whilst the blood itself is colourless, and communicates (as in Craniata by the lymphatic “hearts”) with the coelomic fluid at various points. A contractile ventral trunk runs along the lower face of the slit pharynx, and sends vessels right and left up the successive bars; these vessels unite above, as in Craniata, to form a double " dorsal aorta,” which posteriorly becomes a single vessel. A portal system of veins can be traced in connexion with the hepatic caecum.
No system of lymphatic vessels, nor lymphatic “glands,” nor a spleen exist; but the coelom, and certain other spaces in the connective tissue, contain coagulable lymph, and correspond to the lymph spaces of Craniata.
There is no series of nephridia, nor a renal organ formed by the coalescence of nephridia, nor are gonaducts present. The “brown funnels,” a pair of funnel-shaped tubes discovered by Lankester (14 ), place the coelomic space of the opercular (epipleural) downgrowths of the body-wall in communication with the space which these folds enclose. They appear to be identical with the “collarpores” of Balanoglossus, and it is doubtful whether they represent nephridia.
In the larval Amphioxus there is developed from the left anterior coelomic pouch a glandular tube and a sense-organ, which are represented in the adult by the structures marked/ in fig. 8, B. This tube is probably the same thing as the subneural gland of Ascidians and as the proboscidean gland and pore of Balanoglossus. Quite distinct from the foregoing is a nephridial tube lying on the left side behind the mouth of the larval Amphioxus. All are probably of the nature of nephridia. In the adult Amphioxus the nephridial tube is in an atrophied condition, though large and active in a late larval stage, when the olfactory pit opens into the neural canal. Hatschek (15 ) describes this condition in “ausgebildete" but not in adult examples.
The gonads are distinct ovaries and testes; they are developed in distinct male and female individuals in corresponding positions, viz., in that part of the coelom which is carried downwards in the descending right and left outgrowth (epipleura, opercula) of the body-wall, which encloses the atrial or branchial chamber. The gonads are twenty-six pairs in number, corresponding to the 10-36 myomeres. They are devoid of ducts, and discharge their products by dehiscence into the atrial chamber, whence they pass to the exterior, either by the atriopore or by entering the pharynx through its slits, when they are ejected by the mouth.
In many respects Amphioxus, the only representative of Cephalo-chorda, bears evidence of being derived from a more highly organized ancestry. Its mode of life (burrowing in the sand in shallow water, whilst its general build is that of a swimming animal) and the nature of its food (diatoms, &c., carried into the pharynx by ciliary currents) in themselves suggest such a history. The vascular system is elaborate in plan yet incomplete in detail, suggesting an atrophy of its finer branches, which is consistent with the small size of Amphioxus and the general principle that a complex vascular system can only be developed in an animal which has attained to a certain bulk. The absence of well-developed sense organs and of “cephalization” in an animal which has attained to such elaboration of structure as is shown by the pharynx and atrial chamber, and which has such well-developed muscles to the body-wall, is an inconsistency best explicable by degeneration; so, too, the existence of the elaborate series of fin rays, which are out of proportion to the mechanical requirements of so small a form.
Degenerate though Amphioxus must be, the ancestor from which it started on its retrogressive course was probably a long way behind any living Craniate. There is no reason to suppose that this ancestor had a cranium, or that the muscular segments and segmental nerves in its cephalic region were fused and welded. Amphioxus has probably lost, as compared with that ancestor, lateral eyes and otocysts, nephridia, and, above all, size. The epipleural folds which now form oral hood, branchial opercula, and coalesced ventral fin were probably originally less developed lateral ridges, protecting the gill-slits anteriorly and posteriorly, serving by their undulations to assist in locomotion, whilst the median fin and its rays were large and functional.
One of the most curious features in the structure of Amphioxus is its asymmetry. The anus is on the animal’s right side, the nasal pit upon its left; the myomeres on the two sides of the notochord do not coincide; and the right and left dorsal spinal nerves do not arise vis-à-vis to one another. There is no conclusive reason for regarding this as an ancestral feature, although the early larval form is as curiously asymmetrical as the adult. Amphioxus habitually rests upon the sand, lying upon one side of the body, and it is possible that the distortion is related to this habit, as in the case of the Pleuronectid Fishes.
However we may estimate Amphioxus, we are not led by it, though its muscular metamerism is so well marked, a single step in the direction of the Annelids, neither are we led directly, it is true, in the direction of Nemertina in connexion with those points, as to relationship of notochord with proboscis sheath and nerve-cord with median dorsal nerve, insisted on by Hubrecht. But it will be seen below that, by the agreement of Amphioxus with Balanoglossus in the structure of the perforations of the pharynx, in the possession of collar pores, and in the praeoral glandular body, we do arrive at an important connexion with Nemertine-like forms.
The Urochorda. Urochorda are Vertebrata which, with the exception of the group larvalia (Appendicularia, Fritillaria, Oikopleura), have receded very far indeed from the characteristic Vertebrate structure, showing neither notochord nor nerve-cord, and gill-slits only of the most highly modified and aberrant form; some, however (certain Ascidians), pass through a larval condition in which these structures are present in the normal form. It is necessary for the purposes of the present article to confine our attention to Larvalia and to the larval forms which retain ancestral characters. (For a description of the whole group, see the article Tunicata. ) In Urochorda thus signalized the notochord never reaches forward into the anterior part of the body, but is confined to the tail (hence Urochorda). The longitudinal muscles of the region traversed by the notochord show traces of metameric segmentation, which are probably survivals of a more complete development of myomeres in ancestral forms (16). There is no trace in Larvalia of fin rays or other skeletal structure. Corresponding to the opercular folds and epipleura of Craniata and Cephalochorda are ridges of the body-wall, which protect the pharyngeal gill-slits, and may give rise, as in Cephalochorda, to an enclosed atrial chamber with atriopore. The gill-slits in these larval forms are few in number (one or two pairs), but in many of the aberrant Urochorda (by far the majority of the group) they become excessively numerous and complicated in structure, and are supported by a chitinous (?) framework, as in Cephalochorda. It has been suggested that the fenestrated structure of the pharyngeal wall in Tunicata does not represent a series of gill-slits, but a single pair of slits subdivided. This suggestion is worthy of further consideration.
The cerebro-spinal nerve-cord is tubular and presents itself as a dilated cerebral vesicle in front of the notochord, and as a narrower part running along the whole length of the notochord.
Sense organs are present—a single eye with pigment and lens, a single otocyst, and an olfactory pit (Larvalia). The mouth is dorsal in position in the Ascidian tadpole, but subterminal in Larvalia. The pharynx is wide, and is followed by a narrow oesophagus, stomach, and intestine, which does not open ventrally but turns upwards to the anus. The Larvalia have a rudimentary heart and no vascular system,—a fact connected with their diminu tive size. For the same reason no vascular system develops in the Ascidian tadpole until it has ceased to be locomotive and has entered upon its later development; but in the larger adult Urochorda a contractile heart and a well-developed vascular system are present.
No undeniable nephridia are present in Larvalia nor in the larval Ascidian, and no structure comparable to the collar pores of Balanoglossus or the atrio-coelomic funnels of Amphioxus is known in them.
The subneural gland, however, a glandular tube opening anteriorly near the mouth of the pharynx, appears to be identical with the praeoral larval gland of Amphioxus and the proboscis pore and gland of Balanoglossus. It is probably to be regarded as a nephridium, and has been compared by Julin and Van Beneden to the pituitary body of Craniata, with which it corresponds in position and development.
The gonads of Larvalia are developed in irregular masses on the walls of the coelom, ovaries and testes in the same individual.
As above indicated, there is a small section of Urochorda which retain in adult life the tadpole-like form and the essential Vertebrate organs which are exhibited by the larvae only of other Urochorda, and by a few only of these. This necessitates a primary division of the branch into two grades. [table]
Urochorda are so extremely aberrant, and show so little more than a transient developmental indication of the essential Vertebrate organs, that we cannot hope to get much positive information from them on the subject of Vertebrate ancestry. Only the minute Appendiculariae (Larvalia) retain the Vertebrate structure through life, and they are obviously, on account of their minute size, extremely degenerate. It is possible to make hypotheses as to the greater or less elaboration of the ancestors of Urochorda, and to maintain even that their ancestry had reached as high a condition as that shown by Craniata ; on the other hand, it does not seem likely that their point of divergence from the main ancestral line leading to Craniata was lower than, or even so low as, that at which Amphioxus branched off. The differentiation of trunk and tail by the limitation of the notochord anteriorly is a nearer approach to Craniate structure than that shown by Amphioxus, whilst the definite development of a brain of considerable relative size places Urochorda nearer to the Craniates than is Amphioxus. The metameric myomeres so strongly developed in this last are not absent in Urochorda, as is often maintained, but exist in a rudimentary form, indicating that they had once a fuller development.
The Hemichorda. Hemichorda comprise the single genus Balanoglossus— formerly classified by Gegenbaur as Enteropneusta, an independent phylum of the animal kingdom. They are Vertebrata of worm-like form, elongate and somewhat flattened from above downwards. In front of the mouth is a long cylindrical proboscis, and behind it a collar, the free margin of which is turned backwards, and corresponds to the opercular epipleural folds of Cephalochorda and Craniata. This agreement is supported by the existence of a pair of collar pores opening into the coelom of the collar, as the “brown funnels” of Amphioxus open into the epipleural coelom of that animal. A proboscis pore, opening on the left side into the praeoral coelom of the proboscis (paired in B. kupfferi), is exactly representative of the similarly placed pore which in the young Amphioxus (according to Hatschek, 15 ) leads into the tubular organ derived from the left coelomic chamber of the praeoral lobe of that animal. The whole surface of the body is ciliated, as in Nemertines and Echinoderms, and as in no other Vertebrates. Following the collar is a perforated region of the body,—gill-slits opening from the outer surface into the pharynx. In the young form there is for a time, as in Ap-pendiculariae and the Ascidian tadpole, only one pair of gill-slits, but they subsequently increase in number as the animal grows in length. They resemble in form and structure those of Amphioxus. The notochord (h in fig. 10) arises at the anterior end of the hypoblast in the young, and grows forward, forming a support for the base of the proboscis. It is limited to this very small region. The cerebro-spinal nerve-cord originates by a delamination of a solid cord of epiblast in the mid-dorsal line of the middle third of the body; then by invagination of its two ends it extends as a tube both anteriorly and posteriorly. A general network of nervefibres (and cells ?) exists beneath the epidermis all over the body. The blood-system is peculiar, consisting of an anterior heart and a dorsal and ventral vessel; these are united by a plexus of subcutaneous vessels. The musculature of the body-wall is not broken into successive myomeres; but, on the other hand, the gonads (ovaries or testes) are sac-like, and, as in Amphioxus, are repeated in a series throughout a great length of the body. In the pharyngeal region the gonad sacs agree in number with the gill-slits. There are no nephridia (unless proboscis pore and collar pores are to be so regarded); but the connective-tissue cells of the body-cavity are active as excreting agents, as in Echinoderms and in Urochorda, and a large glandular organ in the proboscis attached to the end of the notochord appears to have to do with this function. Not the least remarkable fact about Hemichorda is the nature of their larvae. No other Vertebrata present larval forms which indicate the nature of the early ancestral history in what we may call prae-chordal times; however interesting the Ascidian larva, or the young Amphioxus, and the embryo dog-fish, they do not take us out of the Vertebrate area. Some species of Balanoglossus (? B. minutus), however, pass through a banded ciliate larval condition, which was known as Tornaria, and was considered to be an Echinoderm larva allied to Bipinnaria, before its relation to Balanoglossus was discovered. It is not possible to view the Tornaria larva of Balanoglossus as otherwise than identical with Echinoderm larvae, and it results that Balanoglossus and the Echinoderms have remote genetic affinities of a special kind.
No classification of Hemichorda is possible beyond an enumeration of the species :— [table]
It seems that in Balanoglossus we at last find a form which, though no doubt specialized for its burrowing sand-life, and possibly to some extent degenerate, yet has not to any large extent fallen from an ancestral eminence. The ciliated epidermis, the long Worm-like form, and the complete absence of segmentation of the body-muscles lead us to forms like the Nemertines. The great proboscis of Balanoglossus may well be compared to the invaginable organ similarly placed in the Nemertines. The collar is the first commencement of a structure destined to assume great importance in Cephalo-chorda and Craniata, and perhaps protective of a single gillslit in Balanoglossus before the number of those apertures had been extended. Borrowing, as we may, the nephridia from the Nemertines, and the lateral in addition to the dorsal nerve, we find that Balanoglossus gives the most hopeful hypothetical solution of the pedigree of Vertebrates. Space has not permitted us to go so fully into pros and eons as the speculative nature of the subject requires; but we give the final conclusion to which our consideration of the structure of the four great branches of the Vertebrata leads in the form of the accompanying genealogical tree.
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