176 COMPARISONS OF STRUCTaKE IN ANIMALS. 



cai-tilage, generally tougli and firm, but some- 

 times extremely delicate ; and, in these fishes, 

 we find a considerable modification of the ver- 

 tebras composing the spinal column. As exam- 

 ples of this group, we may mention the shark, 

 the sturgeon, the skate, the ray, the lamprey, 

 the myxine, and the lancelet. 



In the shark and stm-geon, the bodies of the 

 vertebra are circular in form ; and, in many 

 cases, there is a central communication between 

 one cup, or double conical cavity, and another, 

 throughout the -whole length of the spine. In 

 the skate, remarkable for the enormous deve- 

 lopment of the pectoral fins, the bones of the 

 shoulder, in a cartilaginous state, are attached 

 to two large lateral processes at the upper part 

 of the spine, while the cartilaginous framework 

 that supports the ventral fins is simple and 

 unconnected with the vertebral column. The 

 anterior portion of the spine in this fish is not 

 divided into distinct pieces, while, in the pos- 

 terior part, "the number of vertebral arches 

 is twice as great as that of the separate bodies 

 of the vertebrse." In the lamprey, the bodies 

 of the vertebras are annular, and perforated in 

 the centre ; hence a continuous canal, filled 

 with gelatine, runs through the spinal cokunn. 



