the membrana fenestrata at the bases of the rhabdoms (night-position). When this happens the rays of light which strike the ommatidia at all sorts of angles, instead of being largely absorbed by the pigment, are refracted by the crystalline cones and distributed over the tops of the rhabdoms, passing freely from one ommatidium to another. In this way the eye acts on this occasion, not by mosaic vision, but on the principle of refraction, as in the Vertebrate eye. Of course the distinctness of vision is lost, but an immense economy in the use of light-rays is effected, and the creature can perceive objects and movements dimly in the dusk which by mosaic vision it could not see at all. The pigment is contained in living cells or chromatophores, and it is carried about by the active amoeboid movements of these cells with great rapidity. Now, besides the active adaptability to different degrees of light brought about in the individual by these means, we find Crustacea living under special conditions in which the eyes are permanently modified for seeing in the dusk, and this naturally occurs in many deep-sea forms. Doflein[118] has examined the eyes of a great number of deep-sea Brachyura dredged by the _Valdivia_ Expedition, and as the result of this investigation he states that the eyes of deep-sea Brachyura are never composed of so many ommatidia, nor are they so deeply pigmented as those of littoral or shallow water forms. At the same time an immense range of variation occurs among deep-sea forms which are apparently subjected to similar 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. _Cyclodorippe 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.