PRODUCTS OF METABOLISM. 743 



pigment known as haemocyanin is dissolved. This con- 

 sists of a proteid substance united to copper, and is the only 

 albuminoid present in the plasma. It is very widely spread 

 among Gasteropods, Crustaceans, &c., but is not universal. 

 Its absence in some Crabs, which have apparently no com- 

 pensating metal containing pigment, perhaps indicates that 

 too much stress should not be laid upon its. respiratory 

 significance. Lipochrome pigments are very frequently 

 present in the blood of Crustaceans and Cephalopods ; their 

 use is unknown. 



If we compare the condition seen in Cephalopods with 

 that found in Vertebrates, we find that in the latter it is the 

 red blood corpuscles which are the oxygen carriers, while in 

 the former the plasma alone subserves respiration. Even in 

 Vertebrates, however, the waste carbonic acid is carried in 

 the plasma in combination with its soda, so that the plasma is 

 not entirely unconcerned with respiration. In both Verte- 

 brates and Cephalopods the plasma has a nutritive function. 



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 utilized for different purposes. Thus many 

 pigments which are widely distributed seem to be practi- 

 cally 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 struc- 

 tures. In many cases the organism merely takes up inor- 

 ganic particles from the surrounding medium, and with these 

 fashions a shell for itself, as we find in some of the Fora- 

 minifera. In most of the Foraminifera, however, a true 



