ACTION OF THE FINS OF FISHES : FLYING FISH. 503 



of its centre of gravity. In other instances, the pectoral and 

 ventral fins move in such a manner as to assist the action of 

 the tail. In the Kays, the pectoral fins are developed to an 

 enormous extent (fig. 244) ; and being 

 directed horizontally, their action is vertical 

 like that of the wings of a bird. They are 

 furnished with a great number of joints, 

 by which they are rendered very flexible \ 

 and their surface may be thus increased 

 during the down-stroke of the fin, and 

 diminished during the -^p-stroke. If this 

 were not done, the action of the fins in 

 elevation would exactly counterbalance the 

 effect of their depression ; and no movement Flgt 244 *~ RAT ' 

 would be produced. The great power of the pectoral fins of 

 these Fishes seems connected with their want of an air- 

 bladder, which causes them to require a constant exercise of 

 force to keep them up in the water. Their propulsion forwards 

 is chiefly accomplished, as in other Fishes, by the action of the 

 tail. But sometimes the Rays change their position and swim 

 sideways, making horizontal strokes with the pectoral fins 

 (whose surface is then vertical), by which they are moved 

 through the water, and sustaining themselves by vertical 

 strokes of the tail, whose surface is then horizontal. 



667. The structure of the organs adapted for movement in 

 air bears great analogy to that of such expanded fins ; and 

 there are instances in which the same instruments may serve 

 both purposes. Thus there are Fishes which are able to quit 

 the water, and execute leaps of considerable length, supported 



Fig. 245. FLYING-FISH. 



upon their wing-like pectoral fins. These are known as 

 Fly ing- Fish (fig. 245) ; but it is not correct to speak of their 



