The Origin of Vertebrates

by Walter Holbrook Gaskell

Throughout this book my attention has been especially directed to both Limulus and the scorpion group in endeavouring to picture to myself the ancestor of the earliest vertebrates, because the Eurypteridæ possessed such marked scorpion-like characteristics; so that in considering the origin of a special sense-organ, such as the vertebrate auditory organ near the junction of the prosoma and mesosoma, it seems to me that the presence of such marked special sense-organs as the flabellum on the one hand and the pecten on the other, must both be taken into account, even although the former is an adjunct to a prosomatic appendage, while the latter represents, according to present ideas, the whole of a mesosomatic appendage.

From the point of view that the VIIIth nerve represents a segment immediately posterior to that of the VIIth, it is evident that an organ in the situation of the pecten, immediately posterior to the operculum, i.e. according to my view, posterior to the segment originally represented by the VIIth nerve, is more correctly situated than an organ like the flabellum, which belongs to a segment anterior to the operculum.

On the other hand, from the point of view of the relationship between the scorpions and the king-crabs, it is a possibly debatable question whether the pecten really belongs to a segment posterior to the operculum. The position of any nerve in a series depends upon its position of origin in the central nervous system, rather than upon the position of its peripheral organ. Now, Patten gives two figures of the brain of the scorpion built up from serial sections. In both he shows that the main portion of the pectinal nerve arises from a swelling, to which he gives the name ganglion nodosum. This swelling arises on each side in close connection with the origin of the most posterior prosomatic appendage-nerve, according to his drawings, and posteriorly to such origin he figures a small nerve which he says supplies the distal parts of the sexual organs. This nerve is the only nerve which can be called the opercular nerve, and apparently arises posteriorly to the main part of the pectinal nerve. If this is so, it would indicate that the pectens arose from sense-organs which were originally, like the flabella, pre-opercular in position, but have shifted to a post-opercular position.

The Origin of the Parachordals and Auditory Cartilaginous Capsule.

In addition to what I have already said, there is another reason why a special sense-organ such as the pecten is suggestive of the origin of the vertebrate auditory organ, in that such a suggestion gives a clue to the possible origin of the parachordals and auditory cartilaginous capsules.

In the lower vertebrates the auditory organ is characterized by being surrounded with a cartilaginous capsule which springs from a special part of the axial cartilaginous skeleton on each side, known as the pair of parachordals. The latter, in Ammocœtes, form a pair of cartilaginous bars, which unite the trabecular bars with the branchial cartilaginous basket-work. They are recognized throughout the Vertebrata as distinct from the trabecular bars, thus forming a separate paired cartilaginous element between the trabeculæ and the branchial cartilaginous system, which of itself indicates a position for the auditory capsule between the prosomatic trabeculæ and the mesosomatic branchial cartilaginous system.

The auditory capsule and parachordals when formed are made of the same kind of cartilage as the trabeculæ, i.e. of hard cartilage, and are therefore formed from a gelatin-containing tissue, and not from muco-cartilage. Judging from the origin already ascribed to the trabeculæ, viz. their formation from the great prosomatic entochondrite or plastron, this would indicate that a second entochondrite existed in the ancestor of the vertebrate in the region of the junction of the prosoma and mesosoma, which was especially connected with the sense-organ to which the auditory organ owes its origin. This pair of entochondrites becoming cartilaginous would give origin to the parachordals, and subsequently to the auditory capsules, their position being such that the nerve to the operculum would be surrounded at its origin by the growth of cartilage.

On this line of argument it is very significant to find that the scorpions do possess a second pair of entochondrites, viz. the supra-pectinal entochondrites, situated between the nerve-cord and the pectens, so that if the ancestor of the Cephalaspid was sufficiently scorpion-like to have possessed a second pair of entochondrites and at the same time a pair of special sense-organs of the nature either of the pectens or flabella, then the origin of the auditory apparatus would present no difficulty.

It is also easy to see that the formation of the parachordals from entochondrites homologous with the supra-pectinal entochondrites, would give a reason why the VIIth or opercular nerve is involved with the VIIIth in the formation of the auditory capsule, especially if the special sense-organ which gave origin to the auditory organ was originally a pre-opercular sense-organ such as the flabellum, which subsequently took up a post-opercular position like that of the pecten.

The Evidence of Ammocœtes.