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, is 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 

 flying 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 or the body by alternate contractile and 

 expansive movements, as the medusae. 



In these different displays of voluntary motion, the mus- 



