vi BYES 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 crabs 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 schytet occurs 
in 7-128 metres, and has well-developed eyes; S. bronleyana, from 
730 to 3600 metres, has small and semi-degenerate eyes ; while S. 
antarctica i 730-2920 metres is completely blind. Lispognathus 
thompsoni 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, 7c. 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 omimatidia, eg. the 
Galatheid 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 
Aleock describes only one species, Parapylocheles scorpio, as 
having poorly pigmented eyes. An attempt to account for this 
was made by Milne Edwards and Bouvier,’ who pointed out that 
the truly deep-sea forms with well-developed eyes were always 
1 Ann. Sci. Nat. (Zool.) (7), xiii., 1892, p. 185. 
