Fig. 42—A Deep-sea Lobster (Nephropsis stewartii), from the Bay of Bengal. Reduced. (After Alcock and Anderson.)
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Fig. 43—Munidopsis regia, a Deep-sea Galatheid from the Bay of Bengal. Reduced. (After Alcock and Anderson.)
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In animals that live in perpetual darkness we should expect to find, in accordance with the principle of adaptation which runs through the whole of organic nature, that the eyes are wanting or imperfectly developed. In a great many deep-sea animals this is indeed the case. The deep-sea Lobsters of the genus Nephropsis ([Fig. 42]), which are very closely allied to the Norway Lobster (Nephrops) of shallow water, have very short and slender eye-stalks hidden under the rostrum, and showing at the tip only the merest traces of what was once an eye. In the lobster-like Eryonidea (see [Fig. 46], p. 133), the reduced eye-stalks are firmly fixed in notches in the front edge of the carapace. Some of the deep-sea Crabs and Prawns seem also to be totally blind. In a great many cases degeneration has not quite gone so far, and the eyes are present, although much reduced and modified. Thus, the very numerous deep-sea species of Galatheidæ, belonging to the genus Munidopsis ([Fig. 43]) and its allies have, as Alcock says, "pallid, milky-yellow, 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 are 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 suggest that, although shut off from the light of day, they are not condemned to grope in perpetual darkness. 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 descends to depths of at least 2,000 fathoms; and he suggests that the Crab may be able to see its way by the light emitted by the zoophytes.
PLATE XVI