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 Amnio- 

 ccetes ; its topographical arrangement, its structure, its origin in muco- 

 cartilage. — The prosomatic skeleton of Amnioccetes ; the trabecular and 

 parachordals, their structure, their origin in white fibrous tissue. — The 

 mesosomatic skeleton of Linmlus compared with that of Ammoccetes ; 

 similarity of position, of structure, of origin in muco-cartilage. — The 

 prosomatic skeleton of Linmlus ; the entosternite or plastron compared with 

 the trabecular 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 epi- 

 thelial tube which formed the alimentary canal of the ancient type of 

 marine arthropods such as were dominant in the seas when the verte- 

 brates 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 verte- 

 brates from arthropods, the one skeleton being an endo-skeleton 

 composed of cartilage and bone, and the other an exo-skeleton com- 

 posed 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. 



