BOOK II. 



OF CARTILAGINOVS FISHES. 

 CHAPTER I. 



OF CARTILAGINOUS FISHES IN GENERAL. 



WE have seen that fishes of the cetaceous kind beai a strong re 

 nt.Tiblaiice to quadrupeds in their conformation ; those of the cartl- 

 aginous kinds are one remove separated from them : they form tiie 

 shade that completes the imperceptible gradations of Nature 



The first great distinction they exhibit is, in having cartilages of 

 gristles instead of bones. The cetaceous tribes have their bones en- 

 tirely resembling those of quadrupeds, thick, white, and filled with 

 marrow ; those of the spinous kind, on the contrary, have small slen- 

 der bones, with points resembling thorns, and generally solid through- 

 out. Fishes of the cartilaginous kinds have their bones always soft 

 and yielding; and age, that hardens the bones of other animals, ra- 

 ther contributes still more to soften theirs. The size of all fishes in- 

 creases with age; but from the pliancy of the bones in this tribe, they 

 seem to have no bound placed to their dimensions : and it is supposed 

 that they grow larger every day till they die. 



They have other differences, more obviously discernible. We have 

 observed, that the cetaceous tribes had lungs like quadrupeds, a hear 

 with its partition in the same manner, and apparatus for hearing; on 

 the other hand, we mentioned that the spinous kinds had no organs 

 of hearing, no lungs to breathe through, and no partition in the heart ; 

 but that their cold red blood was circulated by the means of the im- 

 pulse made upon their gills by the water. Cartilaginous fishes unite 

 both these systems in their conformation ; like the cetaceous tribes, 

 hey have organs of hearing, and lungs ; like the spinous kinds, they 

 have gills, and a heart without a partition. Thus possessed of a two- 

 fold power of breathing, sometimes by means of their lungs, some- 

 times by that of their gills, they seem to unite all the advantages of 

 which their situation is capable, and drawing from both elements 

 every aid to their necessities or their enjoyments. 



This double capacity of breathing in these animals, is one of the 

 most remarkable features in the history of Nature. The apertures by 

 which they breathe, are somewhere placed about the head ; either be- 

 neath, as in flat fish; on the sides, as in sharks; or on the top of th* 

 had, as in pipe-fish. To these apertures are the gills affixed, hui 



