In the eyes of higher animals the retina is more than a mosaic of independent photoreceptor elements. Even in a primitive eye such as the compound eye of the horse-shoe crab, Limulus, neural interconnections exist and interaction takes place among the receptor units (the ommatidia).
The structure and function of the ommatidia in Limulus has been described by Hartline, Wagner and MacNichol (1952) and recent studies of its microstructure by electron microscopy have been made by Miller (1957). Each ommatidium is a functional unit. In response to illumination of its facet it generates trains of nerve impulses in the major optic nerve fiber that arises from one of the cells within it. A network of nerve fibers lies back of the ommatidia, interconnecting them. This plexus has been figured in a recent paper (Hartline, Wagner and Ratliff, 1956), and has been shown to mediate the interacting influences.
In the Limulus
HARTLINE HK. Inhibitory Interaction in the Limulus Eye—Abstract. AMA Arch Ophthalmol. 1958;60(4):734-738. doi:10.1001/archopht.1958.00940080754017