I50 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. Doflein (oc. 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 1s 
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. 
Aleock ' 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.” 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 ommatidia. 
In the frontal portion (Fig. 104, A) the ommatidia are few in 
1 4 Naturalist in Indian Seas, 1902. 
2 “Atlantis,” Bibliotheca Zoologica, Heft 19, 1896, p. 193. 
