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. Piatt, 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 thaey 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 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 oculo-motor 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 primitiver 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 mid- and 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 nephrostomes 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 ventro-posterior 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
