254 THE EFFECT OF TEMPERATURE 



The direct dependence of pigmentation on light 

 seems to be proved by the generality of the reverse 

 phenomenon as observed in cave animals. Whenever 

 light is totally excluded, the pigmentation appears to 

 become diminished or abolished, whatever class of the 

 Animal Kingdom the individuals belong to. Thus 

 there have been found more or less unpigmented Coe- 

 lentera, Worms, Crustacea, Myriapoda, Arachnida, 

 Coleoptera, Fish, and other animals in the various sub- 

 terranean caves of Europe and North America.* 

 However, the abyssal fauna of the ocean, occurring at 

 depths such that (presumably) no light can penetrate, 

 includes numerous species which are just as much pig- 

 mented as those exposed to light. Thus Faxon f di- 

 vides deep sea Crustacea into two types; (1) those living 

 in the bottom mud, which are mostly pale in colour, and 

 often blind; (2) those which swim freely, have well-de- 

 veloped eyes, and are coloured bright red. He con- 

 siders that this red colour is due to the absence of light 

 at these profound depths, for S. Jourdain $ has shown 

 that two different species of Crustacea, which are brown 

 when exposed to light or partial darkness, become red 

 when placed in total darkness. MacCulloch and Cold- 

 stream have suggested a " theory of Abyssal Light " to 

 account for the existence at profound depths of these 

 Crustacea, and of the Fish, Mollusca, Crabs, and other 

 animals with well-developed eyes. This hypothesis 

 msists essentially in the idea that light diffused by 



* Packard, Memoirs of National Academy of Sciences, iv. p. 3, 

 1888. 



f Mem. Mus. Harvard, xviii. p. 251. 

 \ Comptes Rendus, Ixxxvii. p. 302, 1878. 



