VERTEBRAL COLUMN 



41 



vertebral column, and in these Fishes the anterior vertebra are 

 fused into a single mass. 



In the caudal region of Amia the centra are mostly double, an archless 

 pleuro- or post-centrum alternating with an inter- or pre-centrnm. A some- 

 what similar condition is found in the Jurassic Eurycormus and other fossil 

 Ganoids. 



As a rule Elasmobraiichs and Ganoids possess a greater number of 

 vertebrae (in Alopecias vulpes there are 365) than Teleosts, in which we 

 seldom meet with more than 70 : the Eel, however, possesses more than 

 200. 



The caudal region of the vertebral column deserves particular 

 attention in Fishes, and the condition of this region in Amphioxus, 

 Cyclostomi and Dipnoi, may be taken as a starting-point. In 

 these, the notochord extends straight backwards to the hinder end 

 of the body and is surrounded quite symmetrically by the tail- 

 fin, which is therefore spoken of as protocercal or diphycerccd 

 (Fig. 30). This condition is also met with in many Fishes of the 



FIG. 30. TAIL OF Protopterns. 



Devonian strata as well as in young stages of Teleostei. In the 

 latter, however, the ventral half of the tail-fin with its sup- 

 porting skeleton (haemal arches and fin-rays) is, as a result of un- 

 equal growth, more strongly developed than the dorsal, and the end 

 of the vertebral column becomes bent upwards, thus giving rise to 

 a heterocercal tail. This form of tail may be recognised externally, 

 as in many Elasmobranchs, Ganoids, and numerous fossil Fishes ; or 

 may be masked by a more or less symmetrical tail-fin, as in Lepi- 

 dosteus (Fig. 31), Amia, and more particularly in most Teleosts L 

 (e.g. Salmo, Fig. 32), in which the heterocercal character is only 

 visible internally. The posterior end of the vertebral column 

 is then frequently represented by a rod-like urostyle, and in 

 Teleosts one or more wedge-shaped hypural bones (enlarged haemal 

 arches) generally occur directly beneath it (Fig. 32). 



1 The term homocercal is sometimes used to describe the masked lieterocerca 

 condition of the tail in Teleostei. 



