CLASS PISCES 



447 



portance in classification. The most primitive condition is 

 exhibited by very few if any living fishes, except in the embryo 

 or early larval stages. It is termed protocercal or diphycercal, 

 and is symmetrical both externally and in internal structure 

 (Fig. 377, A), The second type, or heterocercal tail, is not 

 symmetrical, and the vertebral column extends into the dorsal 

 lobe; this condition exists in the sturgeons (Fig. 382) and many 

 others. The stroke of the asymmetrical heterocercal tail forces 

 the anterior part of the body downward. This type is therefore 

 of advantage to and characteristic of those fishes that have a 



Fig. 377. — Two types of caudal fins. A, diphycercal (Polypterus). 

 B, homocercal. D, dermal fin supports ; N, notochord ; R, radials ; 

 R+N, neural spines. (From Dean; A, after Agassiz ; B, after Ryder.) 



ventrally situated mouth and feed on the bottom. The third 

 type, or homocercal tail, is externally symmetrical but internally 

 unsymmetrical (Fig. 377, B). The stroke of the homocercal tail 

 forces the fish straight forward. It is characteristic of fishes 

 with a terminal mouth and is the type possessed by most bony 

 fishes. 



Fins are normally used in locomotion through the water, but 

 may be modified for other purposes. For example, the pectoral 

 fins of the flying fishes (Fig. 394) are used somewhat like the 

 wings of an aeroplane to sustain the fish in the air during its 

 leap from the water; the pectoral fins of the African goby serve 

 the purpose of feet, enabling the fish to move about on the ground 



