34 COLOUR IN NATURE chap. 



again in further detail when we come to consider 

 the colours of plants. 



So far as is yet certainly known, chlorophyll is 

 the only pigment by means of which organisms can 

 avail themselves of the carbon of the atmosphere, 

 though it is quite possible that some other pigments 

 may yet be shown to possess this property. The 

 suggestion has been made in the case of the bright 

 pigments of coral polypes, as well as of various 

 other pigments, notably of the purple pigment of 

 Beggiatoa roscopersicina (see Chap. III.). 



Pigments fulfilling the function of haemoglobin 

 are perhaps fairly numerous, but there is some doubt 

 connected with many of these so-called respiratory 

 pigments. Of those certainly respiratory, haemo- 

 cyanin is by far the most important. 



The question as to whether any other pigments 

 besides those of the nature of chlorophyll and the 

 respiratory pigments are of direct physiological 

 importance, is a somewhat difficult one. According 

 to many, some pigments, such as the colouring 

 matter of the Red Algae and the dark pigments of 

 many animals, are of great importance in protecting 

 delicate tissues from the injurious effects of certain 

 of the rays of white light. 



Again, the pigment described by Dr. MacMunn as 

 enterochlorophyll, which is so common in connection 

 with the alimentary tract in many Invertebrates, is 

 supposed by some to be of importance in digestion 

 or assimilation. There are other suggestions of the 

 same kind, but there is as yet little certainty, the 

 functions of the large majority of pigments being 

 quite unknown. 



