HISTORY OF FISHES. 



BOOK II 



OF CARTILAGINOUS FISHES. 



CHAP. I. 



OF CARTILAGINOUS FISHES IN GENERAL. 



WE have seen that fishes of the cetaceous 

 kind bear a strong resemblance to quadrupeds 

 in their conformation ; those of the cartilagi- 

 nous kinds are one remove separated from 

 them ; they form the shade that completes the 

 imperceptible gradations of nature. 



The first great distinction they exhibit is, 

 in having cartilages or gristles instead of 

 bones. The cetaceous tribes have their bones 

 entirely resembling those of quadrupeds, thick, 

 white, and filled with marrow; those of the 

 spinous kind, on the contrary, have small 

 slender bones, with points resembling thorns, 

 and generally solid throughout. Fishes of 

 the cartilaginous kinds have their bones al- 

 ways soft and yielding ;*and age, that hardens 

 the bones of other animals, rather contributes 

 still more to soften theirs. The size of all 

 fishes increases with age ; but from the plian- 

 cy of the bones in this tribe, they seem to 

 have no bounds 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 ce- 

 taceous tribes had lungs like quadrupeds, a 

 heart with its partition in the same manner, 

 and an 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 impulse made upon their gills 

 by the water. Cartilaginous fishes unite both 

 these systems in their conformation : like the 

 cetaceous tribes, they have organs of hearing, 

 and lungs ; like the spinous kinds, they have 

 gills, and a heart without a partition. Thus 



possessed of a twofold power of breathing, 

 sometimes by means of their lungs, sometimes 

 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 fea- 

 tures in the history of Nature. The apertures 

 by which they breathe, are somewhere placed 

 about the head ; either beneath, as in flat 

 fish ; on the sides, as in sharks; or the top of 

 the head, as in pipe-fish. To these apertures 

 are the gills affixed, but without any bone to 

 open and shut them, as in spinous fishes ; from 

 which, by this mark, they may be easily dis. 

 tinguished, though otherwise very much alike 

 in appearance. From these are bending cy- 

 lindrical ducts, that run to the lungs, and are 

 supposed to convey the air, that gives the or- 

 gans their proper play. The heart, however, 

 has but one valve ; so that their blood wants 

 that double circulation which obtains in the 

 cetaceous kinds ; and the lungs seem to be 

 rather as an internal assistant to the gills 

 than fitted for supplying the same offices as in 

 quadrupeds, for they want the pulmonary vein 

 and artery. 



From this structure, however, the animal 

 is enabled to live a longer time out of water 

 than those whose gills are more simple. The 

 cartilaginous shark, or ray, live some hours 

 after they are taken ; while the spinous her- 

 ring or mackarel expire a few minutes after 

 they are brought on shore. From hence this 

 tribe seems possessed of powers that other 

 fishes are wholly deprived of; they can re- 

 main continually under water, without ever 

 taking breath ; while they can venture their 

 heads above the deep, and continue for hours 

 out of their native element. 



We observed, in a former chapter, that 



