FINS : THEIR USES. 59 



like flap of membrane (fig. 15, B., p. 178). la 

 many fishes only one dorsal fin is present, as in 

 the bream of our fresh waters. This single fin 

 sometimes attains hnge proportions. In an ex- 

 tinct fish of the Eocene age of the world's history, 

 named Semiophorus, the single dorsal fin was 

 longer than the whole body ! It is of enormous 

 size in one of the sword-fishes, Histiophorus (fig. 

 13, p. 135). The dorsal, like the ventral fins, as 

 we shall see presently, are sometimes modified to 

 form a sucker, as in the sucking-fish, Echeineis. 

 In this, the spiny rays of the dorsal fin are 

 composed of two halves, bent the one towards the 

 right, and the other towards the left, and forming 

 a support to a double series of transverse lamellas ; 

 the whole sucker is of an oval shape and surrounded 

 by a membranous fringe. By means of this 

 disc, writes Dr Giinther, the sucking-fishes "are 

 enabled to attach themselves to any flat surface, 

 a series of vacuums being created by the erection 

 of the usually recumbent lamellae. The adhesion 

 is so strong that the fish can only be dislodged 

 with difficulty, unless it is pushed forward by a 

 sliding motion. The c suckers ' attach themselves 

 to sharks, turtles, ships, or any other object 

 which serves their purpose. . . . Being bad 

 swimmers they allow themselves to be carried 

 about by other animals or vessels, endowed with 

 a greater power of locomotion." In the " fishing- 

 frog " (Lophius) the spines or rays of the dorsal fin 

 are separate, one from another, and bear flag-like 

 membranous appendages resembling short fronds 

 of sea-weed. By their means the fish comes to 

 harmonise so completely with its surroundings 



