FINS 67 



the first resembles that of the Herring and is supported by soft 

 rays, the second takes the form of a small flap, without any 

 supporting structures, composed entirely of fatty tissue and 

 covered with skin (Fig. 13c). This is known as the adipose 

 fin, and is found in all the members of the Salmon family, as 

 well as in many Characins, and in the majority of Cat-fishes. 

 In some of the latter the adipose fin is comparatively large 

 (Fig. 11), and in certain species may develop a few soft rays. 

 It is of interest to note that in the Mailed Cat-fishes {Loricariidae) 

 this fin is a triangular flap of skin, the front edge of which is 

 supported by a stout, movable spine (Figs. 32F; 42c), but in 

 some related naked forms {Cyclopium) from mountain streams 

 the spine has disappeared and the adipose fin has reacquired the 

 typical Cat-fish form. This pro\ddes one of the very rare cases 

 of reversible evolution, where an organ has become changed 

 and afterwards reverted to its original structure. 



The position of the dorsal fin or fins also exhibits a fair 

 amount of variation in diflferent fishes. In the Gar Pikes 

 (Lepidosteus) of North America (Fig. 48), and in the quite 

 unrelated Pike {Esox), both dorsal and anal fins are placed 

 well back towards the hinder end of the fish; in the Herring 

 [Clupea] and Carp {Cyprinus) the dorsal occupies a position 

 more or less in the middle of the back (Fig. 43) ; and in some 

 of the Cat-fishes the rayed fin is considerably nearer to the head 

 than to the tail (Fig. sSd). During growth the vertical fins 

 tend to undergo some change in form, those of young fishes 

 being generally higher than those of adults. The position 

 may also alter during the life of the individual fish, as in the 

 Herring [Clupea). In the larval stages the dorsal at first lies 

 close to the tail, but moves gradually forwards as growth 

 proceeds. In the grotesque Shrimp-fish (Centriscus) the arrange- 

 ment of the vertical fins is unique. The thin, bony cuirass 

 encasing the body ends behind in a long, stout spine, and the 

 two dorsal fins, crowded together at the hinder end of the fish, 

 are placed below the spine, the second actually pointing down- 

 wards. The tail has been deflected at an obtuse angle from the 

 trunk, and terminates in a small caudal fin, also pointing 

 downwards (Figs. 12; 42H). 



Among the spiny-rayed fishes, the rays at the anterior end 

 of the dorsal fin may be transformed into spines as in the Sea 

 Perch [Epinephelus) (Fig. 28e) or Fresh-water Sun-fish (Lepomis) 

 (Fig. 34e), or the spinous portion may be separated off as a 

 distinct fin, as in the Mackerel {Scomber) (Fig. 5A) or Cardinal- 



