THE SKELETAL SYSTEM 



203 



THE AXIAL SKELETON 

 Evolution of the Vertebral Coltunn. Nothing like a vertebral column 

 appears in any invertebrate, so that the earlier portions of its history are 



Fig. 156. — Diagram of the skeletogenous tissue in the caudal region of a vertebrate. 

 bv, blood-vessels; d, corium; epmu, epaxial muscles; lis, horizontal septum; hymy, 

 hypaxial muscles; 7nsd, msv, dorsal and ventral median septa; tnys, myosepta; n, spinal 

 cord; nc, notochord. (From Kingsley's "Comparative Anatomy of Vertebrates.") 



/NEURAL PROCESS 



B. CESTRACION, A SHARK. 



Fig. 157. — A, The skeleton of a cyclostome, Petromyzon; B, The skeleton of an 

 elasmobranch, Cestracion. Elasmobranchs were the first animals to invent paired 

 appendages and the skeletal elements to support them. Marked differences in the axial 

 and branchial skeletons of cyclostomes and elasmobranchs also appear. (Redrawn 

 after Dean.) 



unknown; though, if amphioxus gives the clue, it was once no more than a 

 medial dorsal fold of the alimentary canal. Its first certain beginnings are 



