FINS 



75 



term " gephyrocercal " (bridge tail) has been applied, is found 

 only in the Sun-fishes and the Pearl-fishes (Fierasfer), and 

 represents a very specialised condition. 



So much for the median or unpaired fins. The paired fins, 

 corresponding respectively to the arms and legs of the land 

 vertebrates, are absent in the Lampreys and Hag-fishes (Cyclo- 

 stomes), but, with few exceptions, one or both pairs are developed 

 in other fishes. 



The pectoral fins vary very little in position, being situated 

 just behind the gill-opening or openings, and placed near the 

 lower edge of the body in some fishes and higher up on the 

 sides in others. The pectorals of the Sharks are considerably 

 larger than those of the generahty of Bony Fishes, being used 



Fig. 33. — ^SHAPES OF CAUDAL FIN. 



A. Lunate or crescentic ; b. forked ; c. emarginate ; d. truncate ; e. rounded ; 

 F. pointed ; G. double emarginate. 



almost entirely for steering purposes (Fig. 34b). A Shark seems 

 to be quite incapable of making a sudden stop, and never uses 

 the pectorals as brakes, being compelled to swerve to one side 

 of an obstacle which lies in its path. The enormous, flattened, 

 lobe-like pectoral fins of the Rays and their relations {Hypo- 

 tremata), joined to the sides of the head and body and forming 

 the principal organs of locomotion, have been already described 

 (Fig. 34a). They may also be used for steering, especially in 

 those forms in which the tail has degenerated to a mere fila- 

 m.ent. As in the case of the median fins, the paired fins of the 

 Selachians are completely covered by skin and muscles, no 

 trace of the rays being visible externally. 



In the Bony Fishes these fins are nearly always relatively 

 small, paddle-shaped organs, and only that part of the fin 



