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In the ommatidium of the apposition eye of Ocypode cursor eight retinula cells are found, where the eighth accessory cell has a characteristic shape and position distinct from the other seven retinula cells. For other decapods this cell has been assumed to be rudimentary. At least for Ocypode its contact with the rhabdome and the possession of an axon contradict this assumption. A functional specialization of this cell seems more probable.
In sections perpendicular to the optical axis a cross-shaped pigmentless structure appears at the distal end of the retinula. One bar of this cross (in any particular ommatidium the same) contains a spherical nucleus (Figs. 3, 4). All four bars seem to be in contact with the central rhabdome. The similar appearance of all four bars and the presence of only one nucleus in this region favour the assumption that all four bars belong to one and the same cell. The space between the bars is filled by the densely pigmented distal ends of the seven regular retinula cells: the dorsal space by cells 1 and 2, the anterior by 3 and 4, the ventral by 5 and 6, and the posterior by cell 7. The nuclei of these cells are elongated. The nucleus-containing bar tapers proximally into an axon (Fig. 6) which extends towards the basilar membrane peripheral to the rosette of the seven regular sense cells, close to the border of cells 6 and 7 (Fig. 7). Immediately adjacent to the basement membrane the retinula cells (in this case, their axons) diverge peripherally: the direction of No. 1 becomes dorso-posterior, No. 2 and No. 3 dorso-anterior, No. 4 and 5 ventro-anterior, No. 6–8 ventro-posterior (Fig. 9). In this asymmetric distribution the axons of the retinular cells pass through the basement membrane by openings which extend between the bases of neighbouring ommatidia in the next higher or lower row of ommatidia (Fig. 10). Thus a sequence of openings follows with alternately three and five cross sections of axons (Fig. 11).
