The fine structure of the retinula of the compound eye of Astacus fluviatilis.

Abstract

In the eye of Astacus fluviatilis one retinula is made up by eight retinular cells. Its length is approximately 190 μm, its diameter about 40 μm at its distal, and 30 μm at its proximal pole. The nuclear regions of the retinular cells are situated distally and partly surround the end of the crystalline tract. Proximally the retinula is embedded in a tapetal layer of about 50 μm thickness made up by cells which contain gaseous vacuoles. The fused and banded rhabdome is slender and spindle-shaped and extends in the central axis of the retinula from the end of the crystalline tract down to the basilar membrane. The rhabdome consists of microvilli of the seven pigmented main retinular cells and is enveloped by their cell bodies throughout its length. An axon originates from the seven main retinular cells about 50 μm distally of the basilar membrane. It is continuously connected with the rhabdome-forming part of the cell by a thin cytoplasmic sheet. The eighth retinular cell devoid of pigment granules is in connection with the distal end of the rhabdome and sends forth a small axon taking its course with the axons of the other retinular cells.