Morphology of the compound eyes of Nebalia herbstii Leach, 1814 (Leptostraca, Nebaliidae).

The Nebalia compound eyes are scotopic apposition eyes that provide a roundabout view. Each ommatidium is equipped with a large, layered retinula, a flat cornea, a eucone crystalline cone made of four cone cells, and eight retinula cells arranged in three cell pairs that exhibit mirror symmetry (R1/R6, R2/R5 and R3/R4) as well as two single cells (R7 and R8). Cone cell threads run between retinula cells R1 and R2, R3 and R4, R5 and R6, R8 and R1. A plane of symmetry divides the compound eyes into anterior and posterior parts containing ommatidia arranged in mirror symmetry and having different R-cell chirality. The findings are discussed with respect to the structural features of other crustacean eyes and basic features of the arthropod compound eye.