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 Ammo- 
cetes; its topographical arrangement, its structure, its origin in muco- 
cartilage.—The prosomatic skeleton of Ammoccetes; the trabecule and 
parachordals, their structure, their origin in white fibrous tissue.—The 
mesosomatic skeleton of Limulus compared with that of Ammoceetes ; 
similarity of position, of structure, of origin in muco-cartilage.— The 
prosomatic skeleton of Limulus; the entosternite or plastron compared with 
the trabecule of Ammoccetes; similarity of position, of structure, of origin 
in fibrous tissue.—Summazy. 
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 orgaus they supply. 
