16 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 Acanthopterygti, 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 
a 
