150 CRUSTACEA CHAP. 



Crustacea of a roving habit, which were perhaps capable of 

 penetrating into better lit regions, and to whom well-developed 

 eyes might be useful, while the degenerate forms were sluggish. 

 This explanation cannot be held to account for the phenomenon, 

 as too many deep-sea forms with fairly normal eyes are known 

 which are never taken outside deep waters. Donein (loc. cit.~) 

 points out that in the Brachyura of the deep sea there is a 

 remarkable correlation between the degree of degeneration of 

 the eye and the size of the eggs the large-egged forms having 

 unpigmented and degenerate eyes, while the species with small 

 eggs have pigmented eyes. He supposes that the species with 

 large eggs undergo a direct development without pelagic free 

 swimming larvae, and that since they never reach the surface 

 their eyes never meet with the necessary stimulus of light for the 

 development of pigment ; whereas the small-egged species undergo 

 a pelagic larval existence when this stimulus is present and gives 

 the necessary initiative for the development of the pigment. 



Another factor enters into the question of eye-degeneration 

 in the Crustacea. The great majority of deep-sea animals, in- 

 cluding many deep-sea Crustacea, are phosphorescent, and it is 

 certain that although daylight never penetrates into the abysses 

 of the ocean, yet there is considerable illumination derived from 

 the phosphorescence of the inhabitants of these regions. 



Alcock * points out in this connexion that the Pagurids, which 

 are conspicuous in the great depths as animals with normally 

 developed eyes, carry about anemones with them, and these 

 organisms are very frequently phosphorescent to a high degree. 

 It may well be, therefore, that the Pagurids are enabled to use 

 their eyes in the normal manner owing to the phosphorescent 

 light which they carry about with them, and this use of phos- 

 phorescent light may apply to a number of deep-sea Crustacea 

 whose eyes are not at all or only partially degenerate. 



An extremely interesting case of the use of phosphorescent 

 light is given by Chun. 2 In a number of Euphausiids occurring 

 in deep waters each compound eye is divided into two parts a 

 frontal and ventro-lateral which differ from one another very 

 greatly in the nature and disposition of their oiumatidia. 



In the frontal portion (Fig. 104, A) the ommatidia are few in 



1 A Naturalist in Indian Seas, 1902. 

 2 "Atlantis," Bibliotheca Zoologica, Heft 19, 1896, \\ 193. 



