Neuroanatomy of the visual afferents in the horseshoe crab (Limulus polyphemus).

The central nervous system of Limulus consists of a circumesophageal ring of fused ganglia and a paired ventral nerve cord. The anterior portion, the protocerebrum, receives sensory inputs including visual information. Three optic nerves, one each from the lateral eye, median ocellus, and ventral eye enter each side of the protocerebrum. The central connections of each optic nerve were determined by staining cut nerve trunks with cobalt chloride. The lateral optic nerve innervates the lamina, medulla, optic tract, ventral central body, and ocellar ganglion. The branching patterns of single axons, probably those of eccentric cells in the lateral eye retina, were observed. Single, large-diameter axons in the lateral optic nerve ramify at seven loci including sites in each of the structures innervated by the lateral optic nerve as a whole. The median optic nerve innervates the ocellar ganglion, central body, optic tract, and medulla. Three types of branching patterns were observed for single, large-diameter fibers in the median optic nerve. One type bypasses the ocellar ganglion and innervates the central body. A second type passes through the ocellar ganglion and optic tract without branching and innervates the posterior medulla. A third type innervates the ocellar ganglion, ventral central body, optic tract, and medulla. The ventral optic nerve is composed of large-diameter axons of ventral photoreceptors. Each axon enters the ganglion cell layer of the medulla and branches over a planar area less than 150 micrograms in diameter. We also observed that axons from mechanoreceptors on the anterior carapace innervate the posterior neuropil of the medulla, and that about 5% of the neurons in the medullar ganglion cell layer send axons to the ipsilateral circumesophageal connective.