STBUCTUEE 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 Vertebrata, 

 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 Vertebrata. 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. 



9 1 . 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 Eays, 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 Invertebrata ; and this division into the 

 Vertebrated 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 Verte- 

 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- 



