There are certain peculiarities in the structure of the eyes in some plankton Crustacea which suggest that the sense of sight is of special importance to their possessors, although we can hardly do more than guess at their special significance. Most Copepoda have only a single eye in the middle of the head, corresponding to the single eye of the nauplius larva, and of far simpler structure than the paired compound eyes of most other Crustacea. In many plankton species, however, this simple eye becomes much enlarged and complicated in various ways. The three parts of which it is normally made up may become separated from each other, and are sometimes increased in number to five, while lenses serving to concentrate the light are often developed by thickening of the overlying cuticle. The most elaborately constructed eyes are found in the family Corycæidæ. In _Copilia_ (Fig. 53) a pair of eyes of relatively enormous size are present. Each has in front a large biconvex lens set at the end of a conical tube which extends backwards to a smaller lens (like a telescope with object-glass and eyepiece), behind which, again, are the sensory cells, corresponding to the retina, enclosed in a tube of dark pigment, the whole apparatus being more than half the length of the body. These eyes, although paired, do not correspond to the paired compound eyes of other Crustacea, but have arisen by the separation and enlargement of two of the three divisions of the typical median Copepod eye. [Illustration: FIG. 53--_Copilia quadrata_ (FEMALE), A COPEPOD OF THE FAMILY CORYCÆIDÆ, SHOWING THE PAIR OF LARGE "TELESCOPIC" EYES. x 20. (After Giesbrecht.)] A peculiarity of the paired compound eyes found in plankton Crustacea of several different orders consists in the division of each eye into two parts, which differ in structure. In many Euphausiacea and Mysidacea, especially in those haunting the deeper strata (mesoplankton), this division of the eyes is well marked, a frontal or dorsal part having the separate elements of the eye (ommatidia) greatly lengthened and with reduced pigment, while the lateral part is of more normal structure. It seems probable, from the researches of Professor Chun, that the fronto-dorsal division is adapted for the perception of very faint light, while the lateral division will give a more accurate image of brightly illuminated objects. [Illustration: FIG. 54--_Phronima colletti_, MALE. FROM A SPECIMEN TAKEN IN DEEP WATER NEAR THE CANARY ISLANDS. × 12. (After Chun.)] In the pelagic Amphipoda, forming the suborder Hyperiidea, the eyes are of very large size, generally occupying almost the whole surface of the head, and giving the animals a very characteristic appearance, in contrast to the small-eyed, bottom-living Gammaridea. In the family Phronimidæ (Fig. 54) the eyes are each divided into two parts, differing in structure in the way just described.