The Origin of Vertebrates

by Walter Holbrook Gaskell

Starting with the working hypothesis that the central nervous system of the vertebrate has arisen from the central nervous system of the arthropod, but has involved and enclosed the alimentary canal of the latter in the process, so that there has been no reversal of surfaces in the derivation of the one form from the other, we have been enabled to compare closely all the organs of the head-region in the two groups of animals, and in no single case have we been compelled to make any startling or improbable assumptions. The simple following out of this clue has led in every case in the most natural manner to the interpretation of all the organs in the head-region of the vertebrate from the corresponding organs of the arthropod.

That it is possible to bring together all the striking resemblances between organs in the two classes of animals, such as I have done in preceding chapters, has been ascribed to a perverted ingenuity on my part—a suggestion which is flattering to my imaginative powers, but has no foundation of fact. There has been absolutely no ingenuity on my part; all I have done is to compare organs and their nerve-supply, as they actually exist in the two groups of animals, on the supposition that there has been no turning over on to the back, no reversal of dorsal and ventral surfaces. The comparison is there for all to read; it is all so simple, so self-evident that, given the one clue, the only ingenuity required is on the part of those who fail to see it.

The great distinction that has arisen between the two head-regions is the disappearance of appendages as such, never, however, of important organs on those appendages. If the olfactory organs of the one group were originally situated on antennules, the olfactory organs still remain, although the antennules as such have disappeared. The coxal excretory organs at the base of the endognaths remain and become the pituitary body. A special sense-organ, such as the flabellum of Limulus or the pecten of scorpion, remains and gives rise to the auditory organ. A special glandular organ, the uterus in the base of the operculum, remains, and gives rise to the thyroid gland. The branchiæ and sense-organs on the mesosomatic appendages remain, and even the very muscles to a large extent. As will be seen later, the excretory organs at the base of the metasomatic appendages remain. It is merely the appendage as such which vanishes either by dwindling away, or by so great an alteration as no longer to be recognizable as an appendage.

This dwindling process was already in full swing before the vertebrate stage; it is only a continuation of a previous tendency, as is seen in the dwindling of the prosomatic appendages in the Merostomata and the inclusion of the branchiæ within the body of the scorpion. Already among the Palæostraca, swimming had largely taken the place of crawling. The whole gradual transformation from the arthropod to the vertebrate is associated with a transformation from a crawling to a swimming animal—with the concomitant loss of locomotor appendages as such, and the alteration of the shape of the animal into the lithe fish-like form. The consideration of the manner in which this latter change was brought about, takes us out of the cranial into the spinal region.

If we take Limulus as the only living type of the Palæostraca, we are struck with the fact that the animal consists to all intents and purposes of prosomatic and mesosomatic regions only; the metasoma consisting of the segments posterior to the mesosoma is very insignificant, so that the large mass of the animal consists of what has become the head-region in the vertebrate; the spinal region, which has become in the higher vertebrates by far the largest region of the body, can hardly be said to exist in such an animal as Limulus. As to the Eurypterids and others, similar remarks may be made, though not to the same extent, for in them a distinct metasoma does exist.

In this book I have considered up to the present the cranial region as a system of segments, and shown how such segments are comparable, one by one, with the corresponding segments in the prosoma and mesosoma of the presumed arthropod ancestor.

In the spinal region such direct comparison is not possible, as is evident on the face of it; for even among vertebrates themselves the spinal segments are not comparable one by one, so great is the variation, so unsettled is the number of segments in this region. This meristic variation, as Bateson calls it, is the great distinctive character of the spinal region, which distinguishes it from the cranial region with its fixed number of nerves, and its substantive rather than meristic variation. At the borderland, between the two regions, we see how the one type merges into the other; how difficult it is to fix the segmental position of the spino-occipital nerves; how much more variable in number are the segments supplied by the vagus nerves than those anterior to them.

This meristic variation is a sign of instability, of want of fixedness in the type, and is evidence, as already pointed out, that the spinal region is newer than the cranial. This instability in the number of spinal segments does not necessarily imply a variability in the number of segments of the metasoma of the invertebrate ancestor; it may simply be an expression of adaptability in the vertebrate phylum itself, according to the requirements necessitated by the conversion of a crawling into a swimming animal, and the subsequent conversion of the swimming into a terrestrial or flying animal.

However many may have been the original number of segments belonging to the spinal region, one thing is certain—the segmental character of this region is remarkably clearly shown, not only by the presence of the segmental spinal nerves, but also by the marked segmentation of the mesoblastic structures. The question, therefore, that requires elucidation above all others is the origin of the spinal mesoblastic segments, i.e. of the cœlomic cavities of the trunk-region, and the structures derived from their walls.

Proceeding on the same lines as in the case of the cranial segments, it is necessary in the first instance to inquire of the vertebrate itself as to the scope of the problem in this region. In addition to the variability in the number of segments so characteristic of the spinal region, the complete absence in each spinal segment of a lateral root affords another marked difference between the two regions. Here, except, of course, at the junction of the spinal and cranial regions, each segmental nerve arises from two roots only, dorsal and ventral, and these roots are separately sensory and motor, and not mixed in function as was the lateral root of each cranial segment. Now, these lateral roots were originally the nerves supplying the prosomatic and mesosomatic appendages with motor as well as sensory fibres. The absence, therefore, of lateral roots in the spinal region implies that in the vertebrate none of the musculature belonging to the metasomatic appendages has remained. Consequently, as far as muscles are concerned, the clue to the origin of the spinal segments must be sought for in the segmentation of the body-muscles.