vi EVES OF DEEP-SEA CRUSTACEA 149 



conditions of darkness, a variation stretching from almost normal 

 eyes to their complete degeneration and the fusion of the eye- 

 stalks with the carapace ; and this variation is very difficult to 

 account for. A very frequent condition for crahs living at about 

 100 fathoms, and even more, is for either the irido-pigment or the 

 retino-pigment to be absent, for the number of ommatidia to be 

 reduced, and for the corneal lenses to be greatly arched. There 

 can be little doubt that these crabs use their eyes, not for mosaic 

 vision, but to obtain the superposition-image characteristic of the 

 Vertebrate eye. In deeper waters, where no daylight penetrates 

 at all, this type of eye is also met with, and also further stages 

 in degeneration where all pigment is absent, and the ommatidia 

 show further signs of reduction and degeneration, e.g. Cyclo- 

 dorippe dromioides. In a few forms, e.g. Cymonomus granulatus 

 among Brachyura, and numerous Macrura, the ommatidia may 

 entirely disappear, and the eye-stalks may become fused with the 

 carapace or converted into tactile organs. 



Progressive stages in degeneration, correlated with the depth 

 in which the animals are found, are afforded by closely related 

 species, or even by individuals of apparently the same species. 

 Thus in the large Serolidae of Antarctic seas, Serolis scliytei occuis 

 in 7-128 metres, and has well-developed eyes ; S. Ironleyana, from 

 730 to 3600 metres, has small and semi -degenerate eyes ; while S. 

 <i ntnrctica in 730-2920 metres is completely blind. Zispognathus 

 tliompsoni is a deep-water spider-crab, and the individuals taken 

 at various depths are said to exhibit progressive stages in degenera- 

 tion according to the depth from which they come. 



At the same time many anomalies occur which are difficult 

 to explain. In the middle depths, i.e. at about 100 fathoms, 

 side by side with species which have semi-degenerate or, at any 

 rate, poorly pigmented eyes, occur species with intensely pig- 

 mented eyes composed of very numerous ommatidia, e.g. the 

 (Jalatheid Munidopsis and several shrimps, while in the true 

 abysses many of the species have quite normal pigmented eyes. 

 This is especially the case with the deep-sea Pagurids, of which 

 Alcock describes only one species, Parapylocheles scorpio, as 

 having poorly pigmented eyes. An attempt to account for this 

 was made by Milne Edwards and Bouvier, 1 who pointed out that 

 the truly deep-sea forms with well-developed eyes were always 



1 Ann. Sci. Xdt. (Zool.) (7). xiii., 1892, p. 185. 



