254 CARTILAGINOUS FISHES. 



than gristle or cartilage, and of course composed 

 almost entirely of gelatine, without any admixture of 

 those salts of lime to which bones and shells owe their 

 stiffness. But when we come to examine the matter, 

 we find this structure a remarkable instance of con- 

 trivance and economy. The absence of lime in the 

 skeleton gives wonderful pliancy to the bodies of these 

 fishes ; while the granulated, tuberculated, or spinous 

 surface, is a coat of mail to them against enemies, and 

 not unfrequently a powerful weapon of aggression. 

 Their external bony substances differ a good deal 

 from bone. They contain a portion of carbonate of 

 lime, as well as phosphate, and thus hold an inter- 

 mediate rank between bones and shells ; and place 

 those fishes to which they belong as an intermediate 

 link between the osseous fishes and the shelled mollusca ; 

 the latter of which have not even the cartilaginous 

 rudiments of a skeleton, and have their covering chiefly 

 of carbonate of lime. We find a similar gradation in 

 those marine animals that are covered with crusts, such 

 as the crab and the lobster. Their internal bones 

 are cartilaginous, and the external crusts are com- 

 posed of carbonate and phosphate of lime. On 

 land, we find the same gradation in many of the 

 reptiles. Their bones are cartilaginous, while the 

 indurated matter is accumulated in the external scales 

 and crusts. Generally, however, there is a difference 

 in the composition of those appendages, which shows 

 that each is fitted to the element in which it is to live. 

 The scales and crusts of land animals are horny, or 

 composed almost entirely of gelatine, which, though it 

 can bear the action of the air, and a considerable change 



