Types and arrangements of neurons in the crayfish optic lamina.

The neural arrangements in the optic lamina of the crayfish Pacifastacus leniusculus Dana have been studied by light microscopy by means of silver impregnation techniques. The lamina is composed of columnar synaptic compartments (cartridges). Each cartridge is composed of seven receptor terminals distributed in two layers and second-order monopolar neurons connecting the lamina with the second synaptic region, the medulla externa. The neurons found in the lamina consist of five classes: monopolar neurons, centrifugal small-field neurons, tangential neurons, multipolar cells (possibly of a glial nature) and photoreceptor axons (fig. 13). Among the monopolar cells, five types are classified (M1-M5) according to their lamina arborizations. Two types are stratified (M3 and M5) corresponding to the photoreceptor terminal strata. On this basis, the lamina plexiform layer is subdivided into two layers (epl1 and epl2). The remaining monopolar neurons have lateral processes in both layers, two of them within one cartridge (M1 and M2) and one over several cartridges (M5). There is one type of small-field centrifugal neuron (C1) and two types of tangential medulla to lamina neurons (Tan1 and Tan2), both having processes covering a large number of cartridges. Multipolar cells with cell bodies distal (MP1) or proximal (MP2) to the plexiform layer send processes to several cartridges. The receptor axons consit of three types. One has terminals in epl1 or epl2, the second has its terminal in epl1 and a thin process to epl2, and the third (corresponding to the 8th retinular cell) bypasses the lamina and has a terminal in the medulla externa. A brief comparison is made with the neural arrangements in the lamina of the Norway lobster Nephrops norvegicus L.