U7 
44 _ LECTURE VIII. 
f ■( ' , ■' r* s T> ! /'O; j 
cess of nature which in the higher orders of ani- 
jpaals,-^ takes place; in, the internal c, a vijy 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 eacli^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 hollo w lungs, while 
in others, certain organs situated near the gills 
bear an appearance resembling, lungs, and these 
particularities of structme so much influenced the 
mind . of- Linnneus, that ,,J[ie placed most of the 
Fishes of, this pa.rticidar .tribe^ or what, are gener- 
ally termed the Cartilaginous Fishes, among the 
