1 6 BRITISH FRESH-WATER FISHES 



the extremity of the tail, the dorsal fin upon the back, and 

 the anal fin upon the ventral line behind the vent. In the 

 eel the vertical fin still remains continuous, running from a 

 point on the dorsal line distant from the snout about twice 

 and a half the length of the head, round the point of the tail 

 and along the ventral line as far as the vent, which is three 

 times and a half the length of the head from the snout. 

 Locomotion is effected by a serpentine, lateral action of the 

 whole spinal column, aided by an undulatory movement 

 along the whole length of the fin. 



The next stage is well shown in the flat-fishes, which have 

 the caudal fin well separated from the continuous vertical fin, 

 thereby divided into two fins, dorsal and anal. These spread 

 fringe-like along both margins of the unsymmetrical body. 

 In the flounder, the dorsal fin extends from the narrowest 

 part of the tail to a point immediately over the left eye, and 

 the anal fin is pushed so far forward as to displace the ventral 

 fins, causing them to hold a position on the throat in advance 

 of the pectorals. The vent also appears close below the gill- 

 opening, in advance of the stomach and intestinal tract. 



Besides acting as a keel, or steadier, the dorsal fin is 

 constantly employed by most free-swimming fish as a supple- 

 mentary rudder. In the Acanthopterygii, or Spiny-finned 

 Fishes, it is usually too rigid for this purpose on account of 

 the spinous rays ; but some idea of its use may be obtained 

 by watching a trout in an aquarium when it is swimming to 

 and fro. The limits of the tank compel the creature to turn 

 repeatedly ; every time it does so, the dorsal fin may be 

 noticed to be strongly bent in the direction the fish wants to 

 go. It will be observed also that even when the trout is at 

 rest, apparently asleep, with its body motionless, every one 

 of the fins (except the second dorsal or adipose fin) are con- 

 tinually in motion as balancers. 



Before proceeding farther in consideration of the functions 

 of fins, it will be convenient to refer to the general design of 



