124 THE LIFE OF CRUSTACEA 



allies have, as Alcock says, " pallid, milky-yel 

 lack-lustre eyes which, though they may perhaps 

 serve to distinguish between light and darkness, can 

 never form a definite visual image." It is probable, 

 indeed, that these pale-coloured eyes are specially 

 adapted for vision in a dim light, for it has been 

 shown that in certain deep-sea Euphausiacea the 

 pigment-sheaths between the separate elements of 

 the compound eyes are greatly reduced, and ar 

 fixed in the position temporarily assumed by those 

 in the eyes of normal Crustacea when kept in the 

 dark. Be this as it may, there are many deep-sea 

 Crustacea which have well-developed and darkly- 

 pigmented eyes. Some of these are swimming forms, 

 which may at times migrate into the upper strata 

 of water to which some rays of light penetrate ; but 

 there are some cases of Crabs and other bottom- 

 living species that have well-developed eyes, although 

 they live at great depths. This would seem to sug- 

 gest that, although shut off from the light of day, 

 they are not condemned to grope in perpetual dark- 

 ness. Many deep-sea animals are known to be 

 phosphorescent, and it seems probable that the 

 large-eyed species may profit by the light emitted 

 by the glow-worms and fireflies of the abyss. Thus, 

 Alcock points out that the deep-sea Hermit Crab 

 Parapagurus pilosimanus (Plate XVI.), which lives in 

 partnership with a colony of sea-anemones which it 

 carries about with it, has large eyes, although it 



