;88 COMPARATIVE PHYSIOLOGY. 



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 mentioned, it is readily digested by pepsin, 

 but is not affected by tryptic ferments. In Vertebrates it is found as an 

 intracellular 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 is said to have been 

 found in Sipunculus^ in Holothurians, and in Brachiopods. 



The dead epidermal cells of many Vertebrates form a cuticle of 

 keratin over the living cells below. The process is said to be one of 

 dehydration ; but it is not a simple drying up, as it occurs quite as 

 markedly in aquatic animals. In the hairs and nails of mammals, the 

 feathers of birds, the scales of fishes, keratin forms a protective cover- 

 ing ; in some mammals it further furnishes powerful offensive " horns." 

 Keratin is also found in the egg-shells of Birds, Reptiles, and 

 Selachians ; in the first group it is associated with lime salts. It also 

 occurs in the sheath of nerve-fibres, which is explicable enough when 

 we remember that in development the nerves arise from the ectoderm. 

 Keratin has also been found among worms. It is extremely resistant 

 to the action of ferments. 



The colouring matters of animals. Colour in animals 

 is either due directly to pigments, or, as in the case of 

 structural colours, is simply a light effect. To the latter 

 division belong the often brilliant colours of some annelids, 

 and the gorgeous metallic tints of the plumage of some 

 birds. In this section we confine ourselves to the 

 pigments. 



Physiologically, we may classify pigments in various ways : 

 there are the respiratory pigments, of which haemoglobin is 

 perhaps the best example ; the waste products, such as the 

 pigments of some butterflies' wings (which are allied to uric 

 acid), and probably the pigments of bile ; finally, there are 

 numerous pigments of whose primary physiological meaning 

 we can say nothing, but which may be secondarily of use 

 in producing protective, warning, or sexual colouring. Such 

 are the pigments of the skin in Crustacea, caterpillars, 

 Amphibians, and so on. 



The most important respiratory pigments are haemo- 

 globin, haemocyanin, and haemerythrin ; some others have 

 been named by different authors, but their respiratory 

 significance seems uncertain. 



Hemoglobin occurs in all the Craniate Vertebrates, and also not 

 infrequently among the different Invertebrate classes, usually in isolated 

 members of groups. It consists of a pigment, hsematin, united to a 



