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). of, anal fin; axl, axillary process; cr, caudal fin rays; 

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

 haemal 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 theTms~bf 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. 



