LIFE IN THE DEEP SEA. 591 



purposes, and may have other uses occasionally. There is no 

 reason why a constant emission of light should be more bene- 

 ficial than a constant emission of heat, such as takes place in the 

 case of our own bodies, and it is quite conceivable that animals 

 might exist to which obscure heat-rays might be visible, and to 

 which men and Mammals generally, would appear constantly 

 luminous. 



However, be the light beneficial to them or not, it seems 

 certain that the deep sea must be righted here and there by 

 greater or smaller patches of these luminous Alcyonarians, with 

 wide intervals, probably, of total darkness intervening ; very 

 possibly the animals with eyes congregate round these sources 

 of light. 



The nature of the light existing in the depths, has an 

 important bearing on the question of the colouring of deep-sea 

 animals. I examined the phosphorescent light emitted by three 

 species of deep-sea Alcyonarians with the spectroscope, and 

 found it to consist of red, yellow, and green rays only. Hence, 

 were the light in the deep sea derived from this source alone, 

 in the absence of blue and violet light, only red, yellow, and 

 green colours in animals could be effective ; no blue animals 

 were obtained in deep water, but blue animals are not common 

 elsewhere. 



It is remarkable that almost all the deep-sea shrimps and 

 Schizopods, which were obtained in very great abundance, are 

 of an intense bright scarlet colour, differing markedly in their 

 intensity of colouring from shallow-water forms, and having, 

 apparently for some purpose, developed an unusually large 

 quality of the same red pigment matter which colours small 

 surface Crustacea. 



Dr. Wallich refers at length hi his work, cited above* to the 

 absence of light in the deep sea, and explains the possibility of 

 persistence of colouring in deep sea animals, even though they 

 live in absolute darkness. Many deep-sea Holothurians are 

 coloured of a deep purple ; no doubt the colouring is useless in 

 their case, and is merely due to the persistence of a colouring 

 developed originally in shallow-water ancestors. 



* " The Atlantic Sea Bed," p. 108. 



