Krebs W, Lietz R
Cell Tissue Res. 1982;222(2):409-15. doi: 10.1007/BF00213221.
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.