4* LECTURE VIII. 



cess of nature which in the higher orders of ani- 

 mals takes place in the internal cavity of the 

 lungs, is brought about in Fishes externally by 

 means of the subdivided branchings of the gills. 

 These important organs the gills, are secured ex- 

 ternally by a strong bony flap on each side, called 

 the gill-cover, and which is generally edged with 

 a thin membrane, capable of extension or con- 

 traction by means of a certain number of elastic 

 arches with which it is internally furnished. The 

 form of the body varies greatly in the different 

 tribes : it must be almost unnecessary to add, that 

 the most common or general shape is that of an 

 oval, more or less contracted or sharpened at each 

 extremity, and slightly compressed on each side. 

 There is one tribe or order of Fishes in which the 

 gills differ in their structure from the rest, and in 

 some particular kinds have an appearance ap- 

 proaching to that of a kind of hollow lungs, while 

 in others, certain organs situated near the gills 

 bear an appearance resembling lungs, and these 

 particularities of structure so much influenced the 

 mind of Linnreus, that he placed most of the 

 Fishes of this particular tribe, or what are gener- 

 ally termed the Cartilaginous Fishes, among the 



