CARTILAGINOUS FISHES. 



627 



OF CARTILAGINOUS FISHES. 



CHAPTER CXLH. 



OF CARTILAGINOUS FISHES IN GENERAL. 



WE have seen that fishes of the cetaceous 

 kind bear a strong resemblance to quadru- 

 peds in their conformation ; those of the car- 

 tilaginous kinds are one remove separated 

 from them ; they form the shade that com- 

 pletes the imperceptible gradations of na- 

 ture. 



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 quadru- 

 peds, thick, white, and filled with marrow ; 

 those of the spinous kind, on the contrary, 

 have small slender bones, with points re- 

 sembling 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, 

 rather contributes still more to soften theirs. 

 The size of all fishes increases with age ; 

 but from the pliancy 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 obvi- 

 ously discernible. We have observed, that 

 the cetaceous tribes had lungs like quadru- 

 peds, a heart with its partition in the same 

 manner, and an apparatus for hearing; on 

 the other hand, we mentioned that the spin- 

 ous kinds had no organs of hearing, no lungs 

 to breath 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 con- 

 formation : like the cetaceous tribes, they 



MO. 53 Se 54. 



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 advan- 

 tages 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 breath, are some where placed 

 about the head ; either beneath, as in flat 

 fish; on the sides, as in sharks; or on 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 spin- 

 ous fishes ; from which, by this mark, they 

 may be easily distinguished, though other- 

 wise very much alike in appearance. From 

 these are bending cylindrical ducts, that run 

 to the lungs, and are supposed to convey the 

 air, that gives the organs their proper play. 

 The heart, however, has but one valve; so 

 that their blood wants that double circula- 

 tion which obtains in the cetaceous kinds ; 

 and the lungs seem to be rather as an inter- 

 nal assistant to the gills, than fitted for sup- 

 plying 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 

 hQurs after they are taken ; while the spinous 

 herring or mackarel expire a few minutes 

 4U 



