Vertebrata

Vertebrata, a large branch of the animal kingdom, of which the characteristic members are mammals, birds, reptiles, batrachians, fish and cyclostomes, the craniate vertebrates of modern zoology. These include all the animals which possess "vertebrae," pieces of bone or cartilage jointed to form a "backbone" or spinal column (see Spinal Cord), although in some of the lower members of the group the segmentation of the spinal column is imperfect. That such animals formed a natural group was understood from the earliest times. Aristotle placed them together as "Enaima," or sanguineous animals, distinguishing them from the "Anaima," which he believed to be bloodless. Later it was discovered that the so-called bloodless animals contained uncoloured blood, and the vertebrates were distinguished as red-blooded, until G. L. C. F. D. Cuvier showed the existence of red blood in some other animals. C. Linnaeus made Mammalia, A y es, Amphibia and Pisces the first four classes of the animal kingdom, but suggested no corporate name for them. In 1788 A. J. G. K. Batsch united them into a great division, for which he proposed the name "Knochenthiere," bony animals. J. B. P. Lamarck carried the idea further, and first clearly recognized the importance of the vertebral column in classification; to him is due the division of the animal kingdom into Vertebrata, which included all the craniate vertebrates, and Invertebrata, which included all other animals. These names and the dichotomy they imply have persisted from their convenience, although zoological science has come to recognize that the groups are not morphologically equivalent and that the division is not logical. Cuvier showed that there were four groups in the animal kingdom, each corresponding to a definite type or plan of structure, and that craniate vertebrates composed only one of these groups, invertebrates including three. In the progress of zoology it has become clear that the coelomate animals fall into a very large number of distinct groups or types, and that the vertebrates are only one class amongst many morphologically distinct classes. It has been shown further that amongst the animals that Lamarck would have placed in the Invertebrata there are several which, although devoid of vertebrae or cranium, must be associated with vertebrates in any natural system. Closer investigation of the anatomy and embryology of the craniate vertebrates showed that the possession of a jointed vertebral column was not a fundamental characteristic of the group. In some creatures, such as sturgeons and lampreys, the position of the jointed vertebral column is occupied by an unjointed rod, the so-called notochord, whilst all the Vertebrata pass through an embryonic stage in which a similar elastic unjointed notochord exists as the precursor of the jointed column. It was further found that all the vertebrates of Lamarck displayed either in the embryonic condition alone, or both in embryonic and adult conditions, a set of passages leading from the anteriorlateral portion of the body into the cavity of the pharynx, and known as gill-slits, because in those creatures in which they become functional for aquatic respiration they lodge the gills or branchial tufts. Further, it was found that in all vertebrates the great central mass of the nervous system, known as the brain and spinal cord, is in reality a hollow tube with more or less thickened walls, developed as a strand of tissue along the dorsal surface of the embryo, which sinks downwards and inwards to form a hollow tube lying dorsal to the notochord.

In 1866 A. Kowalewsky, in a memoir that is one of the classics of vertebrate morphology, worked out the development of Amphioxus, then recognized as the simplest of the vertebrate group, and compared it with the development of an Ascidian, one of a group then termed Tunicate Mollusca, and showed that the latter creature, in its larval stage, possessed, like Amphioxus, a notochord, gill-slits and a hollow dorsally placed nerve-tube. In 1877 E. Ray Lankester published a classification of the animal kingdom in which he definitely associated all the Tunicates with the vertebrates, and subdivided Vertebrata as follows: 1ranch A., Urochorda, which contained the Tunicates and was characterized by the limitation of the notochord to the caudal region; Branch B., Cephalochorda, containing Amphioxus, in which the notochord extended from the extreme tip of the tail to that of the snout; Branch C., Craniata, containing the Cyclostomes, Pisces, Batrachia, Reptilia, A y es and Mammalia, in which the anterior extremity of the notochord ended in the base of a cranium. Later, F. M. Balfour adopted the system of Lankester, but proposed to replace the term Vertebrata, which was anatomically misleading, by the new term Chordata, as the latter term laid stress on the existence of the notochord as the fundamental character of the group. A. Kowalewsky had shown as early as 1866 that the marine worm Balanoglossus, described by Della Chiaje at the end of the 18th century, possessed a set of gillslits similar to those of Amphioxus and Tunicates. From 1884 to 1886 W. Bateson published a series of studies in which he suggested that there was present in Balanoglossus a representative of the notochord, and that a portion at least of its nervous system was a hollow, dorsally placed tube. On these grounds, coupled with the presence of gill-slits, he proposed to add yet a lower branch to the Chordata, to include Balanoglossus and to be termed Hemichorda, but neither Bateson nor zoologists who have written since have accepted the vertebrate affinities of Balanoglossus with complete confidence. Still more diffidently, S. F. Harmer and others have suggested that Cephalodiscus and Phoronis, still more lowly marine invertebrates, have claims to be associated with the Chordata.

It may be accepted definitely that Amphioxus and the Tunicates must be associated with the craniate vertebrates of Lamarck.

With regard to the terms Vertebrata and Chordata, usage still differs. Those who wish to make the names of the larger groups significant labels prefer the term Chordata, and on the whole seem to be prevailing, but there remain many zoologists who prefer the designation with historical associations, and regard it as immaterial if, in the advance of knowledge, the connotation may have been so changed that the term has become conventional rather than verbally significant.

The characters and affinities of the lower groups that have been included under Chordata are discussed in the articles Hemichorda, Balanoglossus, Phoronidea, Pterobranchia, Tunicata and Amphioxus, so that it is necessary here to deal only with the general characters of the Chordata or Vertebrata Craniata, and to consider the views that have been advanced with regard to the origin of vertebrates.

The Vertebrata Craniata share with the Cephalochordata the fundamental characters of the group Chordata. They are bilaterally symmetrical animals with a well-marked metameric segmentation of the muscles and muscle septa, with a gut opening by an anterior ventral mouth, with lateral gill-slits in the embryo or adult, and with a ventro-posterior anus; with a dorsal tubular central nervous system, under which lies in the embryo or adult an unsegmented notochord of endodermal origin; with the body prolonged posteriorly to the anus to form a metamerically segmented tail containing notochord, nervous system and muscles; with a spacious coelomic cavity and separate blood-vascular system. They differ from the Cephalochordata in the extreme cephalization of the anterior segments of the body, including the formation of an enlarged brain with paired sense organs, the nose, eyes and auditory apparatus, and the formation of a cranium, and in the structure of the skeleton, heart, liver and organs of excretion and reproduction.

Evidence points to the origin of the Cephalochordata and the Craniata from a common ancestor in which metameric segmentation of the mesoblast and the nervous system was complete and regular. This condition has been retained by Amphioxus, but in the Craniata has been much modified. The lateral mesoblastic plates with their contained coelom are unsegmented in craniates, although traces of the primitive segmentation are visible in the development of Cyclostomes. The dorsal mesoblastic somites with the segmental musculature derived from them retain the segmental condition in Amphioxus and in the trunk region of craniates, but in the head region of the latter there has taken place a fusion or cephalization more pronounced in the higher forms, where the head is distinct from the trunk, than in lower forms where the head passes gradually into the trunk. The exact number of somites which have been cephalized is difficult to estimate, and certainly varies in different cases, but it appears to be certain that three, immediately anterior to the otic region, have been transformed into the optic muscles. Those behind the otic region (metaotic somites) vary from nine to eleven, and in Cyclostomes give rise to segmental muscles in series with those of the trunk. In true fish and higher Craniates the anterior one or two of these metaotic somites practically disappear, whilst of the remainder none form complete segmental muscles, but various portions of them give rise to muscles associated with the branchial apparatus (epibranchial and hypobranchial), the dorsal portions fading away. In other words, the metameric series continued from the trunk to the anterior end of the body in the ancestral form, retained by the Cephalochorda, and of which traces remain in the development of the Craniata, has been modified in the adult Craniata by the suppression of certain portions and the specialization of other portions to form an unsegmented structure. The process of cephalization, with, however, less complete destruction of the segmental arrangement, has also affected the anterior nerves of Craniata and brought about the distinction between cranial and spinal nerves which is a feature of the Craniates. The ancestral form must be supposed to have given off from its central nervous system lateral nerves segmentally arranged in pairs. Each member of each pair possessed two roots, a dorsal and a ventral root, possibly remaining separate, as in the Cephalochordata and the cranial nerves of Craniata, possibly joining to form a common trunk, as in the spinal nerves of Craniata. The ventral roots consisted of motor fibres passing straight outwards to innervate the segmental muscles derived from the dorsal somites; the dorsal roots took a longer course, arching outwards and round the body to supply the visceral muscles, the mucous membranes, the skin and the sense organs connected with these. It appears, moreover, that the ventral roots remained in strict association with the muscular somites to which they corresponded, and wandered beyond their own segmental areas only with these muscles, whereas the ramifications of the dorsal fibres had a wider range and were less closely bound to segmental regions. Such a primitive condition has been retained by Amphioxus, but in the case of Craniata only by the spinal nerves. Almost every great anatomist has contributed to working out the history of the cranial nerves, and it would be a hopeless task to make a just allocation of credit for the various steps which have led to our present knowledge, but the names of C. Gegenbaur, F. M. Balfour, A. M. Marshall, J. W. van Wijhe, N. K. Koltzoff, Miss J. B. Platt, J. Beard, H. V. Neal and E. S. Goodrich are conspicuous. The Craniates are characterized by the presence of ten pairs of cranial nerves, numbered usually I. to X., from before backwards, with a course and distribution fundamentally identical throughout the group from the lowest fish to man, whilst in the higher forms an additional eleventh and twelfth pair have been assumed from the trunk or neck. Pairs I_ and II. are the nerves of special sense of smell and sight, and in all probability are morphologically .distinct from true segmental pairs. Pairs III. to X. represent various portions of primitive segmental pairs, modified in association with the cephalization of the anterior region of the body. III., IV. and VI. innervate the muscles of the eyeball, and represent the ventral roots of the three prootic somites; the dorsal root of the anterior of these three passes to the anterior portion of the head as the so-called nervus ophthalmicus profundus. The V. of human anatomy, the trigeminal, is formed almost entirely from the dorsal root of the nerve of the second prootic somite, whilst the VII. or facialis of human anatomy similarly represents the greater part of the dorsal root of the third prootic somite, whilst the remaining and lesser portion of that root forms the VIII. or auditory nerve of human anatomy. The IX. or glossopharyngeal represents the dorsal root of the first metaotic somite, the ventral root of which persists in Cyclostomes but disappears together with the somite in higher Craniates. The X. or vagus of human anatomy represents the dorsal root of the second metaotic somite. The backward extension of the vagus to supply the regions corresponding to the posterior gill-slits and internal viscera has been interpreted variously. The explanation at first sight most probable, and that has been advocated by Gegenbaur and many other anatomists, is that the dorsal roots corresponding to a number of somites have fused to form a single system. The ventral roots of the somites in question have a varying fate, being fully represented in the Cyclostomes by nerves to musculature developed from these somites, whilst in the higher forms thay have in great part disappeared. Evidence seems to point to a similar disappearance of the dorsal roots of the branchial somites posterior to the first supplied by the vagus; but as remnants of them have been traced in the development of the various Craniates, it seems as if the vagus were not in reality a compound nerve, but the extension of the nerve arising from a single dorsal segmental root. Notwithstanding some dubiety in detail, the main proposition remains clear: the cranial nerves of Craniates have arisen, in the course of a process of cephalization, from a primitive set of segmental nerves in series with those of the trunk, by a suppression of certain portions and 4 an expansion and specialization of other portions. The work of a large number of anatomists has shown that the fundamental morphological characters of the cranium and brain, organs in which the Craniates are most clearly marked off from Cephalochordates, are fundamentally alike throughout the group. The original crude theory of L. Oken and the poet Goethe, that the skull was composed of expanded and fused vertebrae, was disproved by T. H. Huxley and Gegenbaur. There can be little doubt, however, that the region behind the infundibulum, consisting of part of the optic capsules, the anterior extremity of the notochord, the parachordals (for details as to these see article Skeleton) and the corresponding lateral and dorsal elements with their suspended visceral arches represent at least three cephalic somites, and that the process of cephalization has played an important part in the formation of the cranium as it has in the case of the nerves and muscles of the head. The region of the cranium anterior to this is probably a forward growth of the primitive head, produced in association with the development of the organs of smell and sight, and thus is different in kind from the posterior region. But as Amphioxus is obviously degenerate in the region of the head, no source of information exists as to the exact mode in which the development of the head of the ancestral vertebrate took place.

it is still less possible to lay down anything definite as to how far the structure of the brain of Craniates conforms with a theory of origin by a process of cephalization of metameric segments. The minute expansion at the anterior end of the nerve tube of Amphioxus cannot be called a brain, whilst the brain of all the Craniates is identical in morphological type and so complex that it must have behind it a long history of development. The embryonic Craniate brain appears as three dilatations of the neural tube, respectively the posterior or hind-brain, continuous with the spinal cord, the mid-brain and the fore-brain. From the hind-brain there arises the medulla oblongata or myelencephalon behind, and the metencephalon in front, the dorsal wall of which gives rise to the cerebellum. The hind-brain is closely similar in structure to the spinal cord, and gives rise to all the segmental cranial nerves except the patheticus and motor oculi. The sides of the mid-brain thicken and give rise to the optic lobes; its floor forms the crura cerebri, whilst the oculomotor and patheticus nerves take origin from it. The fore-brain divides into a posterior thalamencephalon and an anterior telencephalon. Thickenings of the floor of the thalamencephalon give rise to the optic thalami; the paired optic lobes grow out from its sides; the pineal body, which primitively was a pair of dorsal eyes, grows from the roof and the infundibulum from the floor. The telencephalon in front grows out secondarily to an extent progressively increasing in the higher groups and forms the corpora striata, the cerebral lobes and the rhinencephalon. The most plausible interpretation is that the midand hind-brains represent a cephalized continuation of the spinal cord, probably originally metamerically segmented, whilst the fore brain has been developed primitively in association with the organs of smell and hearing, and secondarily in connexion with the increasing elaboration of the higher functions of the brain and the development of the association centres of which the cerebrum is the seat.

The details of the structure and development of the sense-organs, gill-slits and visceral organs of Craniates are sufficiently discussed in the articles dealing with the separate classes of the group. It is necessary to refer, however, to new light thrown on the structure and morphology of the renal excretory organs due chiefly to the investigations of Goodrich. The excretory organs of the vast majority of invertebrate coelomate animals are essentially what are known as nephridia. Nephridia in their simplest form are excretory tubules growing from the exterior inwards, and removing from the surrounding tissues or blood vessels waste matter which they discharge to the exterior. In many cases these tubules acquire secondary openings to the coelom, termed nephrdstomes and serving to remove waste matter from that space. Finally, in metamerically segmented invertebrates the nephridia frequently appear in segmentally disposed pairs. Gegenbaur, C. Semper, B. Hatschek, and many other anatomists have compared the kidneys of Craniates with nephridia, supposing the segmental tubules with their coelomic apertures to represent nephridia, which, instead of discharging directly to the exterior by pores in the segments in which they are situated, have come to discharge at each side into a longitudinal common duct with a posterior aperture. The excretory system of Amphioxus undoubtedly consists of true nephridia, morphologically identical with those of the invertebrate coelomates. The latter, however, may also possess a different set of organs, also frequently appearing as segmentally arranged tubules. These are the genital funnels which develop outwards from the coelom, and serve for the discharge of the genital products. It is with the latter that the segmental tubules of the Craniata are to be compared, and the possession of a different type of excretory organ is one of the most vital distinctions between the Craniata and the Cephalochordata.

Origin of the Vertebrata

The recorded fossil history carries us backwards with comparative ease from the highest mammals to the lowest members of the Craniates. Remains of the latter, abundant in the palaeozoic rocks, were undoubtedly true Craniates, allied with the Cyclostomes and the lower fishes, but showing no more than superficial and dubious resemblances to the members of any other group. We have to rely upon general inferences which lead to much ingenious argument and little certain result. The Craniates can be traced back to fishes not unlike the modern shark or dogfish with little dubiety. The Cyclostomes, although true Craniates, present an obviously simpler type of structure: the head is less cephalized and therefore less distinct from the trunk; lower jaw, true teeth and dermal armature are absent, whilst there are other simplifications in the structural type. Very general assent could be obtained for the proposition that one stage in the ancestry of the Vertebrates must have been not unlike a simplified Cyclostome, a bilaterally symmetrical coelomic animal, elongated and fish-like in shape, but without paired limbs, with a smooth, soft skin, a ventral mouth without teeth or lower jaw and probably surrounded by labial palps, with lateral gill-slits and a ventroposterior anus; with an unsegmented notochord and a dorsal tubular nerve cord. The brain, however, must have been expanded, and there must have been paired organs of smell, two lateral eyes and probably two dorsal eyes, and a large paired auditory apparatus. The mesoblastic system of muscles and fibrous skeleton was highly and regularly segmented, but in the anterior region cephalization had proceeded to a considerable extent. The resemblances between such a creature and Amphioxus are so close that they cannot be dismissed. Amphioxus no doubt is specialized in many respects, and probably degenerate in others, just as, if we go to the other pole of the Craniates, we know that although the Anthropoid Apes are the nearest living representatives of the ancestor of man, they are specialized in many respects and almost certainly degenerate in other respects. If we carry those processes of progressive change by which the Cyclostome type has passed into the low fish type, and the low fish type into the higher Craniate type, backwards towards Amphioxus we reach the conception of an ancestral creature essentially a Cephalochordate, differing no doubt from Amphioxus in various details, as one member of a group differs from another, but specially marked by the possession of better developed cranial sense organs and by the presence of a coelomostomic instead of a nephridial excretory system. Paired sense organs of an elaborate character have arisen in many groups, and there seems to be no special difficulty in supposing that those characteristic of Craniates have arisen independently in that group, Amphioxus, although in that respect partly degenerate, being degenerate from a stage in which the cephalic sense organs were extremely simple. The different type of excretory system presents even less theoretical difficulty, as both types of segmental funnel exist amongst Invertebrates and may even be present in the same animal. If we follow the process of progressive change still further back, we reach a stage in which cephalization had practically disappeared, and where even metameric segmentation was in a much less advanced condition. The tadpoles of Ascidians, and still more remotely Balanoglossus, although still less than Amphioxus to be regarded as actual ancestral vertebrate types, give images of some of the many phases in which the ancestral type may have been exhibited. It is needless to say that the creatures exhibiting such a stage in the ancestry of the Vertebrates would have formed simply one in the vast series of marine coelomate types which the anatomy of the Invertebrates shows us to have existed. Its distinguishing features would have been the presence of gill-slits, of the skeletal rod, known as the notochord, and of the dorsal tubular nervous system. We cannot make even profitable guesses as to the exact conditions under which these features, or the corresponding features of other coelomate types, arose in the kaleidoscopic differentiation of form, but consideration of the general morphology of the nervous system enables us to see the Chordate ancestor in its true perspective amongst other coelomic groups. In the Coelentera the nervous system appears as a diffused layer of cells and fibres, underlying, and in close connexion with, the epidermis. This diffused layer may thicken in special regions, forming rings round apertures, radial bands, and so forth, whilst in the intervening areas it disappears. In the different groups of Coelomates specialized bands and strands have formed in this way from a primitive diffuse system, giving rise to the nervous patterns distinctive of the various groups, whilst a second process, that of inward migration from the epidermis, produces further changes. In the Turbellaria there have been formed two ventrolateral cords with variously placed anastamoses; in the Trematodes, two ventral, two lateral and two dorsal cords with variously placed anastomoses, and in the Cestodes two lateral and in some cases one dorsal cord. In the Nemertea the primitive continuous subepidermal sheath is retained with two lateral and sometimes one dorsal thickening. In the Nematodes there are one dorsal, one ventral and at each side two lateral thickenings, sometimes separated cords, sometimes mere sub-epidermal bands, whilst the traces of a circum-oesophageal ring may be regarded as another specialization of the primitive complete sheath. In Balanoglossus there is a continuous sheath with a dorsal and ventral band, the latter in certain regions showing traces of a tubular structure. In Annelids and Arthropods there are two ventral bands tending to unite in the median ventral line, and a circum-oesophageal collar. In the Chordates there is a continuous dorsal band, which secondarily migrates inwards and becomes tubular. In almost any of these types, as the individual becomes more integrated, there is a tendency for the nervous matter of the specialized areas to become still further massed; and in bilaterally symmetrical animals with forward progression and the beginning of cephalization a specially important mass forms something comparable with a brain in special relation with the sense organs of the primitive head. If the problem of vertebrate origin be considered from the wide `point of view of comparative anatomy, it becomes no more difficult nor remarkable than the differentiation of any other type amongst simple, marine, unsegmented, or little segmented, wormlike creatures. It is obvious, however, that such a theory of origin cannot expect confirmation from the geological record, as it supposes a differentiation of the main chordate characters in a stage too simple to leave fossil remains.

Reference must be made, however, to definite theories of the origin of Vertebrates which have been successively urged by anatomists. A. Dohrn, if not the inventor, was the most ingenious advocate of the Annelid theory. He recognized the fundamental importance of segmentation in vertebrate structure and sought for a highly segmented ancestor. Partly influenced by Ray Lankester's studies on degeneration, he held that the apparently simplest living members of a group may give misleading clues with respect to the ancestral line, and he devoted much brilliant anatomical and embryological work to develop the thesis that Amphioxus and the Tunicates were degenerate offshoots from a higher vertebrate stock. He took a Chaetopod worm as the closest living representative of the stock of all segmented animals, and in particular of the Vertebrates, laying stress on the segmentation, the large coelom, the segmental excretory tubules, the vascular system with red blood, the segmentally disposed branchiae, the lateral organs of locomotion, and the tendency to form a distinct head. The chief difficulty was the nervous system, and this he explained by accepting an idea propounded many years before by De Blainville, that the dorsal surface of Vertebrates was homologous with the ventral surface of Annelids and Arthropods. He assumed that the ancestral type was a marine creature in which reversal of surface was of little physiological moment. He supposed that a new mouth had been formed, probably by a coalescence of a pair of gill-slits on what was to be the ventral surface of the vertebrate, and that the old invertebrate mouth with the downward turn of the anterior end of the alimentary canal, between the diverging ends of the ventral nerve cords, was to be sought for in the roof of the vertebrate brain, possibly the pineal body. Dohrn's theory has failed to find acceptance for many reasons, of which the chief are the difficulty as to reversal of surfaces, the knowledge that segmentation occurs independently in many groups of animals and in different organs, greater knowledge of the vascular, excretory and nervous systems, and in particular the discovery that the pineal body was a degenerate eye. F. M. Balfour from the first refused to accept Dohrn's theory and suggested that the dorsal position of the nerve cord in Vertebrates could be accounted for by supposing that the primitive condition was a lateral cord at each side such as were then known to occur in Nemertines, and that these cords had fused dorsally in Vertebrates,. ventrally in Annelids. A. A. W. Hubrecht soon afterwards discovered the existence of a continuous nerve sheath in Nemertines, and he and Ray Lankester suggested a Nemertine origin for Vertebrates, and homologized the notochord with the proboscis sheath, Ray Lankester, in particular, pointing out that the tubular condition of the vertebrate nervous system was secondary, that it consisted essentially of a dorsal band, which sank inwards, and that the canal might have been at first an epidermal water canal. These authors were emphatic in laying stress on the view that no actual Nemertine could be supposed to represent the vertebrate ancestor, but that the Nemertines were to be taken merely as showing the kind of material out, of which the vertebrate structure, and in particular the vertebrate nervous system, might have arisen. The view adopted in this article as given above, is in reality an extension of the Hubrecht-Lankester theory.

The theory of vertebrate origin that has been most elaborately expounded is W. H. Gaskell's hypothesis that they are descended from Arthropods. Gaskell accepts Dohrn's view of the importance of segmentation and of the degeneracy of Amphioxus and Tunicates, but rejects the conception of a reversal of surfaces. He takes the larval stage of a Cyclostome as the most generalized living representative of the essential vertebrate type, and selects Limulus, the kingcrab, in a very general way, as the closest living representative of such an Arthropod type as might have been the vertebrate ancestor. The starting-point of Gaskell's theory is the conception of the vertebrate nervous system as a band of nervous tissue which immediately underlies and gradually grows up round a distinct epidermal tube, the tube which forms the vesicles of the brain and the central canal of the spinal cord. Ray Lankester had already applied this to the Nemertine theory, but Gaskell urges that it affords an immediate comparison with Arthropod structures. The ventral mouth of Limulus leads vertically upwards through a ring of nervous tissue, the circumoesophageal commissure, into an expanded stomach,. and from this the digestive tube runs back to the anus immediately dorsal to the ventral nerve chain. For Gaskell the infundibulum is the Arthropod oesophagus, the ventricles of the brain are the stomach, and the spinal canal leading back to fuse 'with the anus at the neurenteric canal is the Arthropod digestive tract. In the Vertebrate a new digestive tract has been formed, probably from a structure corresponding to the branchial chamber of Arthropods. The lateral halves of the ventral nervous system of the Arthropod, where they diverge on either side of the oesophagus, represent the crura cerebri of Vertebrates, whilst the supra-oesophageal ganglia represent the fore-brain. Gaskell has instituted an elaborate comparison, extending to very minute details of structure, and finds remarkable analogies between the organs of Arthropods and structures in the Vertebrates. From the palaeontological side, he points out that at the time when the earliest known Craniates were abundant, large Arthropods, essentially like Limulus, were also abundant. He thinks it probable that Vertebrates arose from a dominant invertebrate group, and points to many resemblances in detail between the Silurian Arthropods Palaeostraca and the Craniate Ostracoderms of the same horizon.

BIBLIOGRAPHY.-F. M. Balfour, Monograph on the Development of Elasmobranch' Fishes (1878); W. Bateson, "Balanoglossus," in the Quarterly Journal of Microscopical Science (1884, 1885, 1886); J. Beard, "The System of Branchial Sense-Organs and their Associated Ganglia in Ichthyopsida," in the Quart. Journ. Micr. Sci. (1885); A. Dohrn, "Studien zur Urgeschichte des Wirbelthierkorpers," in the Mitth. Zool. Sta. (Naples, 1882, 1884, 1885, 1886, 1904); W. H. Gaskell, The Origin of Vertebrates (1908); C. Gegenbaur, "Die. Metameric des Kopfes," in the Morph. Jahrb. (1888); Grundziige der Vergleichenden Anatomie (various editions from 1870); E. S. Goodrich,. volume on "Vertebrata Craniata," in Lankester's Treatise on Zoology (1909) (a notable review of the subject, to which the writer of this article is specially indebted); B. Hatschek, "Die Metameric des Amphioxus and des Ammocoetes," in Vertr. Anat. Ges. (Wien, 1892), and Anat. Anz. (1893); A. A. W. Hubrecht, "On the Ancestral Form of the Chordata," in the Quart. Journ. Micr. Science (1883); "The Relation of the Nemertea to the Vertebrata" (ibid. 1887); A. Kowalevsky, "Le Developpement de l'Amphioxus lanceolatus," in Arch. Sci. Phys. Nat. (1866); "Entwickelungsgeschichte der einfachen Ascidien," in St Petersb. Acad. Sci. (1867), and summarized in the Quart. Journ. Micr. Sci. (1870); N. K. Koltzof, "Metameric des Kopfes von Petromyzon Planeri," in Anat. Anz. (1899); "Entwickel. d. Kopfes von Petromyzon Planeri," in Bull. Soc. Imp. Nat. Moscow (1902); E. Ray Lankester, "Notes on Embryology and Classification," in Quart. Journ. Micr. Science (1877); article "Vertebrata," in Ency. Brit. (9th ed.); A. M. Marshall, "The Segmental Value of the Cranial Nerves," in the Journ. Anat. and Phys. (1882); H. V. Neal, "Segmentation of Nervous System in Squalus acanthias," in Bull. Mus. Comp. Zool. (Harvard, 1898); J. W. van Wijhe, "Ueber das Visceralskelet, u. die Nerven des Kopfes der Ganoiden u. von Ceratodus," in Niederl. Arch. f. Zool. (1879, 1882); various authors (A. Dendy, H. Gadow, J. S. Gardiner, W. H. Gaskell, E. S. Goodrich, E. W. MacBride, E. Ray Lankester, P. Chalmers Mitchell, A. Smith Woodward), "Discussion on the Origin of Vertebrates," in the Proc. of the Linnaean Society (London, 1910). (P. C. M.)