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 Aeterocercal caudal fin an anal fin is added ventrally, 
so that the whole caudal fin thus formed is asymmetric with 
Fig. 310.—TAILs OF FISHES. 
Vertebral 
Column, 
Dorsal 
Part. 
Ventral Part. 
A, Protocercal. B, Heterocercal. C, Homocercal. 
a large dorsal portion into which the “tail” is continued. 
This is found in most sharks. The Zomocercal 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 Zeleostom: 
have this typeZof tail. 
