Apical region of the crayfish retinula.
The base of the crystalline tract, the distal part of the eighth retinular cell and its rhabdomer constitute a structural unit in the apical region of the retinula of Astacus fluviatilis and A. leptodactylus, shielded from the blood by a special covering cell.
D000818:Animals; D003400:Astacoidea / Q000648:ultrastructure*; D005123:Eye / Q000648:ultrastructure; D008854:Microscopy, Electron