SKELETON. 



5 1 



CYCLOSTOMES have a persistent notochord, increasing in size with the 

 growth of the animal, and lacking constrictions since no centra are developed. 

 In the myxinoids there are neurapophyses and intercalaria deYeloped in the caudal 

 region; in the lampreys they occur in the trunk as well. 



FISHES. In the elasmobranchs the typical vertebrae are developed in cartilage, 

 with intercalaria in connection with the arches. Usually the centra undergo more 

 or less calcification (p. 43), the lime being either deposited in concentric rings 

 around the notochord (cyclospondylous vertebrae) or in radiating plates (astero- 

 spondylous) . In the trunk region each centrum often bears a pair of transverse 

 processes with short ribs at their extremities. In a few forms (skates, etc.) 

 embolomerism (p. 48) occurs in the tail, and in the holocephali the centra are 

 replaced by numerous rings of cartilage. In skates and hi Chimara there is a true 

 joint between the skull and the column, but in the sharks the anterior vertebrae are 

 fused together and to the skull. 



FIG. 47. 

 FIG. 47. Diagrammatic sections of elasmobranch vertebrae. 



C, asterospondylous. 

 FIG. 48. Cross-section of teleost vertebra; bone, black; cartilage, dotted. 



FIG. 48. 

 A, B, cyclospondylous; 



The ganoids vary greatly in vertebral characters, some of the Chondrostei 

 having only cartilage and some of the fossil forms lacked centra. On the other 

 hand, nearly the whole vertebra is ossified inAmia andLepidosteus, the latter having 

 opisthoccele vertebrae, a condition not reappearing until the amphibians, as all 

 other fishes in which centra are developed have amphiccelous vertebrae. 



As the name implies, ossification of vertebrae and other parts is common in 

 teleosts. The arches are almost always ossified, while the centra may be, or those 

 parts directly connected with the arches may remain cartilaginous while the rest 

 ossifies (fig. 48), so that the section presents a radiate figure as in the asterospondy- 

 lous sharks. Some teleosts have zygapophyses and a few genera have transverse 

 processes on some of the vertebrae. 



The dipnoans, so far as ossification of the vertebrae is concerned, are on a par 

 with the cartilaginous ganoids. There are no centra and the notochord grows 

 throughout life. 



AMPHIBIA, except the legless forms, have caudal, sacral, trunk, and a single 

 cervical vertebra, the sacrals being single except in a family of extinct anurans. 

 Zygapophyses and both kinds of transverse processes may be present. 



