FINS 



59 



So far no mention has been made of the supporting skeleton 

 of the caudal or tail-fin. This is of a somewhat different nature 

 to that of the dorsal and anal, and involves a special modifica- 

 tion of the hinder end of the vertebral column. The tails of 

 adult fishes may assume one of three different forms, known 

 respectively as "diphycercal," "heterocercal," and "homo- 

 cereal," terms which may be freely translated as "twofold 

 tail," "unequal tail," and "equal tail" respectively. 



The diphycercal is undoubtedly the most primitive type, and 

 here the hinder end of the vertebral column is quite straight 



Fig. 25. STRUCTURE OF PECTORAL FINS. 



A. Cladoselachefyleri. (After Dean) ; B. Pleuracanthus decheni. (After Fritsch) ; 

 C. Australian Lung-fish {Epiceratodus forsteri) ; D. Cod (Gadtis callarias). 



and divides the fin into two equal lobes. Around the central 

 axis are arranged a series of rods which support the membrane 

 of the fin. Although characteristic of many of the early fishes, 

 it is doubtful whether any forms living to-day, however straight 

 and symmetrical their tails may appear to be, have a truly 

 primitive diphycercal tail. We find such a tail, however, in 

 almost all larval or embryonic fishes at a stage just before or 

 just after they leave the &gg, but merely as a transitory stage. 



The heterocercal tail, characteristic of adult Selachians and 

 some of the more primitive Bony Fishes, may best be studied in 

 such a form as the Dog-fish [Scyliorhinus) or Sturgeon {Acipenser). 

 Instead of being continued straight backwards, the hinder end 



