FISHES 269 



The whole body thereupon follows the direction assumed by the 

 head. 



(c) Since the back is the heaviest part of a fish's body, the animal, 

 while swimming, is always in a condition of unstable equilibrium. 

 (Hence, what is the position assumed by a dead fish ?) It must, 

 therefore, constantly balance itself (as man does unconsciously in 

 standing), in order to prevent itself from turning over on to its back. 

 This is effected by the action of the dorsal and anal fins (as well as by 

 the vertical caudal fin), all of which increase the vertical surface of the 

 body, and thus help to maintain it constantly in a vertical position in 

 the water (compare with a ship's keel). By virtue of their position, 

 these fins cut through the water with ease. During rapid progression 

 they are laid back. (Why ?) 



(d) The pectoral and ventral fins have varied uses. When swimming 

 slowly, the fish employs them as oars. (Why are they laid back in 

 rapid swimming?) 



Next, they serve for steering. Thus, if a fish at rest wishes to swim 

 over to the right, it has only to execute a few strokes with the left fin ; 

 on the other hand, if a fish swimming in a straightforward direction 

 wishes to turn to the right, it lays its left fins close up against the body, 

 and expands those of the right side. By this action (just as in a boat 

 steered by the oars) the speed of the right side of the animal's body is 

 impeded, and the body accordingly constrained to turn towards the 

 right. 



Again, the pectoral and ventral fins effect the ascent and descent of 

 the fish in the water. If a fish swimming in a horizontal position (let 

 line K Sch. repre- 

 sent the body of the K ^ Schi 



fish, K the position 

 of head, Sch. the 

 tail) desires to rise, 

 it places its pectoral 

 fins (B F) obliquely 

 backwards, so that 

 their anterior edge 

 comes to lie higher 



than their posterior edge. (Place your arm and hand in this position.) 

 As a result of the forward movement effected by the caudal fin, the water 

 tends to impede the pectoral fins. This is exactly as if a force (a) directed 

 parallel to the body of the fish were impinging on the under, or what is 

 now the anterior, surface of the fins. This force, acting obliquely on the 

 surface of the fin, is resolved (exactly as in avian flight, which see), 



18—2 



