SKELETAL SUBSTANCES. 745 



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 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, 

 &c. 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. In the Crustacea it 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 nitrogen, and 

 give some, though not all, of the chemical reactions of 

 proteids. Collagen is found in the bones and cartilages of 

 Vertebrates ; it is characterised by yielding gelatin when 

 boiled with water. Unlike the substances previously men- 

 tioned, it is readily digested by pepsin, but is not affected 

 by tryptic ferments. In Vertebrates it is found as an intra- 

 cellular matrix, secreted by little patches of formative cells. 

 In Cephalopods in the head region there is a modified form 

 of collagen which is readily acted on by trypsin. Collagen 



