The fine structure of the compound eyes of mysids (Crustacea: Mysidacea)

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Abstract
The ultrastructure of the compound eyes of five species of mysids (Crustacea: Mysidacea), Praunus flexuosus, Siriella norvegica, Mysidopsis gibbosa, Neomysis integer and Erythrops serrata, is described. The ommatidia are constructed on a common plan, but there are considerable differences in detail. Common features include the arrangement of the cornea, crystalline cone and the basement membrane. The number of retinular cells differ: in Neomysis and Erythrops there are seven, whereas in the other species there are eight, the eighth cell forming a distal rhabdom, which consequently is lacking in the ommatidia of Neomysis and Erythrops. Another difference is the epirhabdom, which is lacking in Erythrops, but present in the other species. The epirhabdom is an extracellular structure, probably serving as a dioptric element. The pigment arrangement is similar in the first four species. The pigment shield consists of the distal pigment, distal reflecting pigment, proximal pigment (in the retinular cells) and the proximal reflecting pigment. The distal and proximal pigments are dark screening pigments. In addition to these, there are basal red pigment cells, which are mainly located below the basement membrane. In Erythrops there are three kinds of pigment cells: distal pigment cells, distal reflecting pigment cells and basal red pigment cells. Besides the basal red pigment cells, the distal pigment cells contain red pigment granules.