Fig. 53—Copilia quadrata (Female), a Copepod of the Family Corycæidæ, showing the Pair of Large "Telescopic" Eyes. x 20. (After Giesbrecht.)

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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.

Fig. 54—Phronima colletti, Male. From a Specimen taken in Deep Water near the Canary Islands. × 12. (After Chun.)

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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.