746 COMPARATIVE PHYSIOLOGY. 



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 covering; 

 in some mammals it further furnishes powerful offen- 

 sive "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 our- 

 selves 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 



