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Show 1890.] OF THE EYE IN ARCTURUS. 367 shown to have affected all the component parts of the eye. The cornea was little (S. neara) or hardly at all (S. bromleyana) convex below ; the lens was granular, and could hardly have been transparent during life ; the rhabdom and retinules were not recognizable- at least in the form which they present in other (shallow-water) species. The amount of pigment present was comparatively small, or. as in S. bromleyana and S. gracilis, completely absent. I hope to show in the present paper a somewhat similar though less marked series of changes in the eyes of the deep-water Arcturi. Before the appearance of m y preliminary account of the genus Serolis (1), which contained a summary of observations upon the structure of the eye, but little had been done in investigating the histology of that organ in deep-sea Crustacea. Dr. P. P. C. Hoek, in his Report on the ' Challenger' Pycnogonida (6), mentioned that pigment is often absent from the eyes of deep-sea forms, and that the retina may be replaced by a mass of connective tissue, though the lens be present. The details given by Hoek are not very numerous. Since the publication of my Beport several other groups of deep-sea animals have been reported on. Mr. S. I. Smith (12) found that in the majority of species of Atlantic deep-sea Decapods the eyes have undergone certain structural changes ; these changes are partly in the alteration of the pigment, which becomes lighter coloured in the abyssal species, and partly in the reduction of the number of the visual elements. A considerable number of deep-sea Mollusca according to Pelseneer (8) have rudimentary eyes ; some are totally blind. Henderson found (7) with regard to the Anomura that degeneration was common in the eyes of abyssal forms; this degeneration was largely shown by the absence or reduction in quantity of the pigment. Here, however, there is no elaboration of detail and the points raised are not illustrated by figures. Animals that dwell in caves are, so far as absence of sunlight is concerned, subjected to the same conditions as are deep-sea animals. Packard (10), in investigating animals from the Kentucky caves, found various conditions of degeneration in the eyes, culminating in the total blindness of some species. The result, then, of all these investigations has been to show that the deep-sea fauna is chiefly made up of animals which are either blind or-if they have eyes-shoiv evident traces of degeneration in these eyes. I attempted to show, in considering the deep-sea Isopods, that the blind deep-sea genera were, at any rate for the most part, peculiar genera, and that those deep-sea Isopods with apparently well-developed eyes were closely allied to, if not identical with, forms living in shallow water. Thus it appeared reasonable to assume that the eyed forms were comparatively recent immigrants into deep water. This view has already, I find, been considered by Prof. Semper * to 1 'Animal Life,' Int. Scient. Series, p. 84. " We have become acquainted . . . with a wonderful deep-sea fauna, showing the same striking mixture of blind and seeing animals as the fauna of the caves. This case is all the more |