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Each such complex of cells underlying the lenticle of a compound eye is called an “ommatidium”; the entire mass of cells underlying a monomeniscous eye is an “ommataeum.”

Each ommatidium is an elongated cone with its broad extremity abutting against the corneal lenticle.

C, Ocular unit or ommatidium of compound Eye of Scutigera. len, corneal lenticle; c.c, crystalline cone; 1, pigmented cells of outermost tier; 2, 3, retinular cells of middle and innermost tiers; rbd, rhabdomeres; n. opt, optic nerve; pg, pigment cells.