PISCES. 



435 



sand-eel, or it may be broken up into numerous dorsal fins, 

 a caudal and numerous anal fins. The caudal fin may be 

 one of three kinds. The simplest, found in larval fishes, is 

 the protocercal. In this the " tail " or prolongation of the 

 body lies symmetrically in the centre of a symmetrical fin. 

 In the heterocercal caudal fin an anal fin is added ventrally, 

 so that the whole caudal fin thus formed is asymmetric with 



Fig. 310. — Tails of Fishes. 



Ventral Part. 



Ventral Part. 

 A, Protocercal. B, Heterocercal. C, Homocercal. 



a large dorsal portion into w^hich the ''tail" is continued. 

 This is found in most sharks. The ho7nocercal caudal fin 

 is itself symmetrical, the ventral portion being of the same 

 size as the dorsal, but the ^'tail" is bent up into the dorsal 

 half, showing that this type has a secondarily acquired 

 symmetry through the heterocercal stage. Most Teleostomi 

 have this type^of tail. 



