STRUCTURE OF FISHES. 
101 
the other organs of sense are formed upon a very inferior 
type. They swallow it without much division in the mouth; 
but it seems to undergo rapid digestion. The blood of some 
Fish, whose muscular activity is peculiarly great, is rich in 
red corpuscles, and of a temperature not much lower than 
that of Mammals; but, generally speaking, it contains much 
less solid matter than that of the warm-blooded Yertebrata, 
and its temperature follows that of the surrounding medium. 
90. Although Fishes breathe by gills instead of by lungs, 
these gills are connected with the mouth, so that the water 
which passes over them is received into it, in the same man¬ 
ner as the air is in the higher Yertebrata. This is a character 
which distinguishes the position of the gills of fishes from 
that of the corresponding organs of any of the inferior tribes. 
They are lodged in a cavity on each side of the throat; and this 
cavity opens outwardly, either by one large valve-like aperture 
on either side, or by several; through these apertures the 
streams of water which have been taken in by the mouth, 
and forced over the gills by the action of its muscles, make 
their exit. 
91. All Fishes are oviparous; and the number of eggs which 
they produce is generally prodigious. It is very seldom that 
after the eggs have been deposited and fertilized, the parents 
take any further concern in regard to them; though there 
are a few instances in which a kind of nest is made, and 
others in which the egg is retained and hatched within the 
body, so that the young comes forth alive. This last is the 
case with the Sharks and Kays, which, notwithstanding that 
their skeleton is cartilaginous, are higher than Fishes generally 
in several other parts of their organization. 
92. All the animals which are destitute of a vertebral 
column are called Invertehrata; and this division into the 
Yertebrated and Invertebrated groups was formerly regarded 
as the first step in the classification of the animal kingdom. 
But it was pointed out by Cuvier, that in the Invertebrated 
division are comprehended three groups, of which the mem¬ 
bers differ as much from one another as they do from Yerte¬ 
brated animals; and that each of these ought, therefore, to 
rank with the first, as a primary division. This is evident 
to those who are but slightly acquainted with the structure 
of the animals already named (§ 69) as characteristic speci- 
