74 COMPARATIVE PHYSIOLOGY. 



as respiratory, or as waste products resulting from the 

 decomposition of such, is very small ; in the great majority 

 of cases we can say nothing as to function. In some cases, 

 however, we can point to the physical or chemical conditions 

 which favour the development of pigments. Thus in some 

 animals the pigments indicate the normal reaction of the 

 tissues. For example, those sea anemones which contain 

 peptic ferments are red, those which contain tryptic, yellow 

 or brown. Again, light and absence of oxygen are neces- 

 sary for the development of certain of the black pigments ; 

 the black pigment in a frog's skin disappears in an 

 atmosphere of pure oxygen. It is a fact of common observa- 

 tion that portions of animal's bodies which are shaded from 

 the light tend to be pale in colour. Another interesting 

 point about colouring matters is that they are not always 

 produced by the animal in which they occur. Thus green 

 oysters owe their colour to an insoluble pigment taken from 

 the diatoms of the food, and deposited in the tissues ; 

 the colour of " red mullet " is perhaps due to the crustaceans 

 of the food. It has been suggested that the uniformity of 

 tint observed in many animals living in the same environ- 

 ment, as, for example, in the Sargasso Sea, may be due to 

 a similar cause. 



Most of the pigments fall into chemical groups ; of these 

 the best defined and perhaps most widely spread is the 

 Lipochrome group. The Lipochromes are characterised (i) 

 by their colour, which varies from yellow through orange to 

 red ; (2) by giving in the dry state a blue coloration with 

 strong H 2 SO 4 ; (3) by their ready decomposition when 

 exposed to light, when they lose their colour and yield 

 cholesterin ; (4) by the fact that they consist only of 

 carbon, hydrogen, and oxygen. They occur in both plants 

 and animals, especially in the "liver" in Arthropods and 

 Molluscs, in the skin and body fluids of Crustacea, in the 

 skin of many Vertebrates, &c. According to Krukenberg, 

 the lipochromes probably often take origin from fatty bodies; 

 if this is correct, their association with the " liver " in 

 Invertebrates is interesting. 



The above is a very brief account of some of the chief 

 known facts of animal coloration. It may serve to show 

 that much must be discovered as to the primary physiological 



