66 A HISTORY OF FISHES 



terus) or the Wels {Silurus), with a tiny flag-Hke fin in the 

 middle of the back (Fig. 28d), and one of the fancy varieties 

 of the Gold-fish {Carassius), in which it takes the form of a 

 huge sail-hke structure. In some fishes allied to the Carp, one 

 of the simple rays of the fin is stiff and spinous, and not infre- 

 quently saw-edged behind, but no true spinous fin is developed. 

 In the Cat-fishes {Siluroidea) a strong spine nearly always 

 precedes the remainder of the fin, and this has resulted from the 

 modification of one or more soft rays. It is often serrated on one 

 or both of its edges, or is provided with formidable barbs, 

 forming a powerful defensive weapon capable of inflicting a 

 nasty wound (Figs. 34K; 4 id). Sometimes this spine is attached 

 to the body by means of an elaborate ball-and-socket joint at 

 its base, enabhng the fish to keep it erect when alarmed. When 

 thus fixed, the spine cannot be depressed without breaking it, 

 but a rotary movement upwards and towards the body serves 

 to lower it again. 



A curious modification of the dorsal fin is found in the 

 Tarpon (Megalops), as well as in some of the members of the 

 Herring family (Clupeidae), the last ray being drawn out into 

 a long filament which is concave on its hinder edge and tapers 

 to a fine point (Fig. 13B). The purpose of this seems to be 

 connected with the leaping habits of this fish. According to 

 Mr. Mowbray, the Tarpon, preparatory to making a leap, 

 "lashes this whip around to one side of the body and clamps 

 it tight to its side." This adheres by suction to the body, and, 

 by keeping the fin rigidly to one side, aids the fish in deter- 

 mining the course of the jump, the turn naturally being made 

 towards the side to which the ray is pressed. 



Among those soft-rayed fishes with more than one dorsal fin, 

 mention may be made of the Gadoids, which belong to the 

 order of fishes known as Anacanthini (without spines) , a group 

 including such well-known food-fishes as the Cod, Pollack, 

 Whiting, Haddock, Hake, and Ling. In the first four of these 

 fishes there are three dorsal fins (Fig. 28b) ; in the others only 

 two. As their name implies, the fins of these fishes are sup- 

 ported entirely by soft rays. The little Rocklings {Motella), 

 members of the same order, have a series of free rays just in 

 front of the ordinary dorsal fin, and these may be continuously 

 and rapidly vibrated for long periods. The function of such 

 rays appears to be that of a sensory organ for locating and 

 detecting food. 



The Salmon {Salmo) also possesses two dorsal fins, but while 



