Neuronal connectivity patterns in the compound eyes of Artemia salina and Daphnia magna (Crustacea: Branchiopoda).

The neuronal types and patterns in the visual system of the species Artemia salina and Daphina magna have been studied with the Golgi method and electron microscopy. The lamina contains five classes of neurons: photoreceptor axons, monopolar, centrifugal, tangential and amacrine neurons. The terminals of the receptor axons are distributed in two (A. salina) or three (D. magna) layers. The dilated terminals have an extensive and wide array of fine branches. One axon from each ommatidium bypasses the lamina and terminates in the medula in A. salina. A. salina has four types of monopolar neurons, two of which are stratified, whereas in D. magna only two types are found, one of which is bistratified. Tangential T-neurons connect the lamina with the protocerebrum. D. magna has in addition one tangential T-neuron connecting both the lamina and the medulla with the protocerebrum. In both species monopolar-type centrifugal neurons connect the medulla and the lamina, whereas that of A. salina has a wide laminar distribution. Both species also have amacrine cells in the lamina. The medulla contains, besides those shared with the lamina, transmedullary neurons (two types in A. salina), amacrine cells and neurons originating in the protocerebrum. "Cartridge"-type synaptic compartments are lacking in the investigated species, although a periodic arrangement is discernible in the distal portion of the lamina of A. salina. The receptors from three types of specialized contacts in Artemia, one of which involves a dyad. D. magna has only one-to-one synapses. Neurosecretory fibres are absent in A. salina.