116 THE VERTEBRATE SKELETON. 



the endoskeletal cartilaginous radialia are directly con- 

 tinuous with outgrowths from the dorsal and ventral arches 

 of the vertebrae, and form the main part of the fin. In 

 later types of Elasmobranchs the horny exoskeletal fin-rays 

 have comparatively greater prominence. In bony fish, as has 

 been already stated, the horny fibres are replaced by bony 

 rays of dermal origin, and at the same time complete reduc- 

 tion and disappearance of the cartilaginous radialia takes 

 place. 



THE CAUDAL FIN. 



The caudal region of the spinal column in fishes is of 

 special importance. It is distinctly marked off from the rest 

 of the spinal column by the fact that the ventral or haemal 

 arches meet one another and are commonly prolonged into 

 spines, while in the trunk region they do not meet but com- 

 monly diverge from one another. 



In some fish the terminal part of the caudal region of the 

 spinal column retains the same direction as the rest of the 

 spinal column. The blade of the caudal fin is then divided 

 into two nearly equal portions, and is said to be diphycercal. 

 This condition is generally regarded as the most primitive one ; 

 it occurs in the Ichthyotomi, Holocephali, all living Dipnoi, 

 Polypterus and some extinct Crossopterygii, and a few Selachii 

 and Teleostei. It occurs also in deep-sea fish belonging to 

 almost every group, and under these conditions obviously 

 cannot be regarded as primitive, but must be looked on as 

 a feature induced by the peculiar conditions of life. 



In the great majority of fish the terminal part of the 

 caudal region of the spinal column is bent dorsal wards, and 

 the part of the blade of the caudal fin which arises on the 

 dorsal surface is much smaller than is that arising on the 

 ventral surface. Such a fin is said to be heterocercal. 



Strictly speaking all fish whose tails are not diphycercal 

 have heterocercal tails, but the term is commonly applied 



