The retina and retinal projection on the lamina ganglionaris of the crayfish Pacifastacus leniusculus (Dana).

The retinular cell morphology and ommatidia arrangement in the crayfish retina are studied. Each ommatidium contains eight receptor cells (R1-R8). Seven of them (R1-R7) contribute to a large spindle-shaped rhabdom with orthogonal layers of microvilli. Distal to the rhabdom of R1-R7, the 8th receptor cell forms a separate rhabdom with horizontal microvilli. The cell is four-lobed, is devoid of screening pigments, and forms a thin axon projecting past the first optic neuropile (lamina ganglionaris) and has a terminal in the second (medulla externa), indicating a separate function of the receptor. The axons from the eight receptors of one ommatidium project to different synaptic compartments (cartridges) in the lamina. A pattern is described where eight axons from three adjacent ommatidia join in one cartridge; and conversely, the axons from one ommatidium split and join to three cartridges. The seven axons from R1-R7 terminate in two levels of the lamina plexiform layer (epl 1 and epl 2), four in the distal and three in the proximal part. Among the monopolar ganglion cells, two types are found with lateral branches restricted to either of the two receptor terminal layers (M3 in epl 1 and M4 in epl 2) and axons terminating in the second optic neuropile. A correlation between the two orthogonal channels for e-vector discrimination and the two levels of terminals within the lamina is suggested. The retina is divided into dorsal and a ventral part with a mirror symmetry axis horizontally in the eye.