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. {119}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 Ammocoetes; its topographical arrangement, its structure, its origin in muco-cartilage.--The prosomatic skeleton of Ammocoetes; the trabeculæ and parachordals, their structure, their origin in white fibrous tissue.--The mesosomatic skeleton of Limulus compared with that of Ammocoetes; similarity of position, of structure, of origin in muco-cartilage.--The prosomatic skeleton of Limulus; the entosternite or plastron compared with the trabeculæ of Ammocoetes; 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.