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 Eays, 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 he 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 
would he 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 Kays 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 
Flying-Fish (fig. 245); but it is not correct to speak of their 1 
Fig. 244.—Ray. 
