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 schytei_ 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_ in 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 degeneration 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 pigmented eyes composed of very numerous ommatidia, _e.g._ 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 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,[119] who pointed out that the truly deep-sea forms with well-developed eyes were always 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 (_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, including 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.