744 COMPARATIVE PHYSIOLOGY. 



shell of lime is " secreted " by the protoplasm. This taking 

 up of inorganic particles is not the only way in which the 

 tendency to form a protective covering is manifested in the 

 Protozoa. The Corticata are encased in a firm sheath which 

 shows many of the characters of true skeletal substances ; 

 while familiar organic compounds such as cellulose, gelatine, 

 and horny substances, are not unknown. Even in the 

 Protozoa, therefore, we see in germ the power, so charac- 

 teristic of higher animals, of producing by modifications of 

 their protoplasm, specific substances capable of affording 

 both support and protection. 



Skeletal tissues are usually characterised by the physical 

 property of being firm and often hard to the touch, while 

 generally retaining some elasticity, and the chemical one of 

 offering great resistance to ordinary chemical agencies. 

 They are naturally passive and inert, and, so far as the 

 internal skeleton is concerned, are formed in the connective 

 tissues, and not in relation to important organs, except in 

 pathological conditions. Lime salts are frequently associated 

 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 



