PISCES 



105 



or homocercal caudal fin (Fig. 56, E), is the result of a foreshortening 

 of the terminal portion of the caudal axis and results in the typically 

 "fish-tailed" type of fin. In a teleost fish such as the salmon the 

 young fish has first a diphycercal, then a heterocercal, and finally a 

 homocercal type of fin. Various modifications of these three main 

 types are found and will be commented upon in appropriate places. 



Fig. 56. — ^Types of Caudal Fins. A, Diphycercal, with equal dorsal and ven- 

 tral lobes; B, Heterocercal (Selachii); C, Modified diphycercal (some teleosts); 

 D, Heterocercal (Chondrostei); E, Homocercal (teleosts); F, Abbreviated het- 

 erocercal (some Holostei). af, anal fin; axl, axillary process; cr, caudal fin rays; 

 def, dorsal lobe of caudal fin; df, dorsal fin; ef, epicaudal lobe of caudal fin; ha, 

 hsemal arches; hf, hypocaudal lobe of caudal fin; na, neural arches; nt, notochord; 

 r, dermal fin rays. (From Lankester's "Treatise on Zoology," Vol. IX. [A & C. 

 Black].) 



The most primitive type of paired fin is believed to be that seen in 

 Cladoselache (Fig. 65). The "lobe-fins" of the Crossopterygii are 

 next in primitiveness, while the fins of other groups are specialized 

 types derived from these two primitive types. As will appear later 

 the "lobe-fin" is the most nearly hand-like in architecture and is 

 believed to have given rise to the hand-type of paired appendage seen 

 in primitive land vertebrates. 



