138 PHILOSOPHY OF ZOOLOGY. 
of their fins ; these are termed flying-fish. The power of 
flight, among the invertebral animals, is confined to in- 
sects with six feet. Here the wings are distinct from the 
feet, and vary in number from two to four. They are 
membranaceous in their structure, and are, in general, cover- 
ed with hairs or scales. Spiders are able to move in the 
air by means of their threads. 
In the action of flying, the tail, especially in birds, 1s of 
great use in regulating their rise and fall, and even their la- 
teral movements. 
5. Swimming—is the same kind of action in water, as 
flymg is in air. The organs which are employed for this 
purpose, resemble the oars of a boat in their mode of action, 
and in general possess a considerable extent of surface and 
freedom of motion. The former condition enables them 
to strike the surrounding fluid with an oar of sufficient 
breadth to give progressive motion to the body; and the 
latter permits the same organ to be brought back to its for- 
mer position for giving a second stroke, but in a different 
direction, and without offering so great resistance. The 
centre of gravity is so placed, that the body, when in ac- 
tion, shall rest upon the oars or swimmers, or be brought 
by certain means to be of the same specific gravity with the 
water. 
The animals which are furnished with oars or swimmers, 
and are capable of performing this action, are chiefly con- 
fined to the vertebral, and to those with articulated limbs 
among the invertebral animals. 
Swimming, however, is not confined to those animals which 
are furnished with oars or swimmers. Many animals move 
with ease in the water, by means of repeated undula- 
tions of the body, as serpents, eels, and leeches ; or by 
varying the form of the body by alternate contractile and 
expansive movements, as the meduse. 
In these different displays of voluntary motion, the mus- 
