The Origin of Vertebrates

by Walter Holbrook Gaskell

Such inverted retinas are found in the vertebrate lateral eyes and in the lateral eyes of the arachnids, but not of the crustaceans.

The evidence shows that all the invertebrate median eyes possess a simple upright retina, and in structure are remarkably like the right median or pineal eye of Ammocœtes; while the lateral eyes possess, as in the crustaceans, an upright compound retina, or, as in many of the arachnids, a simple inverted retina. The lateral eyes of the vertebrates alone possess a compound inverted retina.

This retina, however, is extraordinarily similar in its structure to the compound crustacean retina, and these similarities are more accentuated in the retina of the lateral eye of Petromyzon than that of the higher vertebrates.

The evidence afforded by the lateral eye of the vertebrate points unmistakably to the conclusion that the ancestor of the vertebrate possessed both crustacean and arachnid characters—belonged, therefore, to a group of animals which gave rise to both the crustacean and arachnid groups. This is precisely the position of the Palæostracan group, which is regarded as the ancestor of both the crustaceans and arachnids. In two respects the retina of the lateral eyes of vertebrates differs from that of all arthropods, for it possesses a special supporting structure, the Müllerian fibres, which do not exist in the latter, and it is developed in connection with a tube, the optic diverticulum, which is connected on each side with the main tube of the central nervous system. These two differences are in reality one and the same, for the Müllerian fibres are the altered lining cells of the optic diverticulum, and this tube has the same significance as the rest of the tube of the nervous system; it is something which has nothing to do with the nervous portion of the retina but has become closely amalgamated with it. The explanation is, word for word, the same as for the tubular nervous system, and shows that the ancestor of the vertebrate possessed two anterior diverticula of its alimentary canal which were in close relationship to the optic ganglion and nerve of the lateral eye on each side. It is again a striking coincidence to find that Artemia, which with Branchipus represents a group of living crustaceans most nearly allied to the trilobites, does possess two anterior diverticula of the gut which are in extraordinarily close relationship with the optic ganglia of the retina of the lateral eyes on each side.

The evidence of the optic apparatus of the vertebrate points most remarkably to the derivation of the Vertebrata from the Palæostraca.

CHAPTER III

THE EVIDENCE OF THE SKELETON

The bony and cartilaginous skeleton considered, not the notochord.—Nature of the earliest cartilaginous skeleton.—The mesosomatic skeleton of Ammocœtes; its topographical arrangement, its structure, its origin in muco-cartilage.—The prosomatic skeleton of Ammocœtes; the trabeculæ and parachordals, their structure, their origin in white fibrous tissue.—The mesosomatic skeleton of Limulus compared with that of Ammocœtes; similarity of position, of structure, of origin in muco-cartilage.—The prosomatic skeleton of Limulus; the entosternite or plastron compared with the trabeculæ of Ammocœtes; similarity of position, of structure, of origin in fibrous tissue.—Summary.

The explanation of the two optic diverticula given in the last chapter accounts in the same harmonious manner for every other part of the tube around which the central nervous system of the vertebrate has been grouped. The tube conforms in all respects to the simple epithelial tube which formed the alimentary canal of the ancient type of marine arthropods such as were dominant in the seas when the vertebrates first appeared. The whole evidence so far is so uniform and points so strongly in the direction of the origin of vertebrates from these ancient arthropods, as to make it an imperative duty to proceed further and to compare one by one the other parts of the central nervous system, together with their outgoing nerves in the two groups of animals.

Before proceeding to do this, it is advisable first to consider the question of the origin of the vertebrate skeletal tissues, for this is the second of the great difficulties in the way of deriving vertebrates from arthropods, the one skeleton being an endo-skeleton composed of cartilage and bone, and the other an exo-skeleton composed of chitin. Here is a problem of a totally different kind to that we have just been considering, but of so fundamental a character that it must, if possible, be solved before passing on to the consideration of the cranial nerves and the organs they supply.