7SO COMPARATIVE PHYSIOLOGY. 



with some of the common skeletal substances, but this is by 

 no means universal even for the same substance. Thus the 

 collagen of the bones of Vertebrates is associated with 

 abundant lime salts, while that of the cartilages contains an 

 inconsiderable quantity. Again, chitin in the Crustacea is 

 strongly impregnated with lime, while in Insecta lime salts 

 are practically absent. Within the limits of the Cephalo- 

 poda, the conchiolin of the "shell" may be associated with 

 lime in one genus and quite devoid of it in another. 

 Within the Mollusca, indeed, we find every stage in shell 

 development represented, from the papery " shell " of 

 Aplysia to the enormous edifices seen in some of the 

 tropical forms. It seems difficult in these cases to avoid 

 the conclusion that the disproportionate bulk is due to 

 necessities of growth, and has no relation to the needs of 

 the animal. 



The following is a brief account of some of the more 

 important skeletal substances : — 



Tunicin. — Tunicin, or animal cellulose, is a carbohydrate very 

 similar to, if not identical with, the cellulose of plants. It occurs in 

 the test of Tunicates as a cuticular product of the epidermal cells, 

 and is said to have been also found in some cases in the body of the 

 animal. Dr. Ambronn asserts that he has found a body giving similar 

 chemical reactions in connection with the chitin of Arthropods, and 

 also in some Molluscs. 



Chitin and Conchiolin.— Chitin and Conchiolin (or Conchin) 

 may serve as examples of skeletal substances containing nitrogen, but 

 giving only one of the proteid reactions. Several other well-known 

 substances are included in this group, such as spongin, byssus-substance, 

 etc. All are characterised by their great resistance to chemical 

 agents. 



Chitin is characteristic of Arthropods, but also occurs in the shell of 

 Lingula and in "cuttle-bone." It yields, on decomposition, reducing 

 substances of the nature of sugar, and is a derivative of a carbohydrate. 

 It is a product of ectodermal cells, and is the only organic skeletal 

 substance in Arthropods. It is believed to be formed by the union of a 

 substance of the ammonia group with a carbohydrate. In the Crustacea 

 chitin is usually associated with lime salts and with various pigments. 



Conchiolin is found in Bivalves, Gasteropods, and some Cephalopods. 

 It strongly resists the action of mineral acids, and, like chitin, is 

 unaffected by ferments. It varies greatly in composition, even within 

 the limits of a species, and is probably a mixture of nearly related 

 substances. The substance which forms the horny axis in Gorgonida 

 and Antipatharia is closely allied to conchiolin. 



Collagen and Keratin.— Collagen and Keratin are well-known 

 examples of skeletal substances which contain sulphur as well as 



