The euphausiid compound eye — a morphological re-investigation (Crustacea: Euphausiacea)

The ommatidia of the compund eye of the krill, Meganyctiphanes norvegica, are composed of the following cell types: corneagenous cells, main cone cells, accessory cone cells, distal pigment cells, and retinula cells. The corneagenous cells secrete the cuticular lenses, below which the crystalline cones are present. The bipartite crystalline cone is formed within the main cone cells, whereas the accessory cone cells invest the distal part of the cone and proceed proximally in the retina to the basement membrane. Six distal pigment cells are arranged around each ommatidium, though shared by adjacent ommatidia. The seven retinula cells form the rhabdom, and penetrate the basement membrane as axons. The basement membrane is formed by the conjoined extended prolongations from the accessory cone cells and sheets from the basal pigment cells.