FISHES. 



229 



In the primitive condition the vertebral axis continues 

 straight to the tip of the tail, the caudal fin being developed 

 symmetrically on either side (diphycercal fin, Fig. 234, A'). 

 This is found in but few forms. In elasmobranchs and many 

 ganoids the heterocercal type occurs (Fig. 234, B^. Here the 



Fig. 234. Different forms of tails of fishes, from Hertwig after Zittel. A, 

 dipfiycercal {Polyplerus) ; B, heterocercal {^Acipenser^; C, homocercal (Aviia); 

 D, homocercal l^Salmo). 



axis of the body is bent abruptly upwards ; and besides the 

 caudal of the diphycercal condition a secondary lobe develops 

 on the lower side, giving the tail an unsymmetrical appearance. 

 The homocercal condition (Fig. 234, Cand U) is derived from 

 this last by the greater development of the ventral lobe, so that 

 the fin appears symmetrical externally, although, as will be seen 

 from the figures, the skeletal parts are of the heterocercal type. 



