PRODUCTS OF METABOLISM. 749 



Products of Metabolism. 



In the course of those processes of breaking down and 

 building up of protoplasm, which constitute what is called 

 the metabolism of the animal, we constantly find that 

 certain by-products are formed. These may be simply 

 waste matters, capable of subserving no useful purpose in 

 the animal economy, or they may have important functions. 

 As we ascend in the scale, we find that these by-products are 

 more and more utilised for different purposes. Thus many 

 pigments which are widely distributed seem to be practic- 

 ally functionless, but in particular cases they come to be of 

 importance in producing protective coloration, and so on. 

 Among the products of metabolism we will discuss here only 

 two groups — the skeletal tissues and the colouring matters. 



The skeletal tissues of animals. — Even in the very 

 simplest forms of life we find that the soft protoplasm is 

 frequently provided with protective structures. In many 

 cases the organism merely takes up inorganic particles from 

 the surrounding medium, and with these fashions a shell for 

 itself, as we find in some of the Foraminifera. In most of 

 the Foraminifera, however, a true 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 

 tective 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 characteristic 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 



