THE SKELETAL SYSTEM 



147 



of the trunk, the difference correlated with a difference in the relation of 

 the coelom to the vertebrae. In the trunk region, where the body-cavity 



Fig. 134. — Diagram of the skeletogenous tissue in the caudal region of a vertebrate. 

 bv, blood-vessels; d, corium; epmu, epaxial muscles; hs, horizontal septum; hytny, 

 hypaxial muscles; msd, msv, dorsal and ventral median septa; mys, myosepta; n, spinal 

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



. CARnL>CE3 



/NEURAL PROCESS 



B CESTRACION, A SHARK. 



Fig. 135. — 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.) 



lies, the haemal arch of each vertebra is incomplete, while in the caudal 

 region each arch is complete with a median spinous process. The noto- 

 chord persists intervertebrally and the centra are biconcave. The 



