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The anatomical organization of the two dorsal giant fiber systems of the earthworm Lumbricus terrestris is demonstrated in whole mounts and serial-section reconstructions based on backfillings of the ventral nerve cord with cobalt chloride. Both the medial and lateral fiber systems can be labeled selectively over more than ten body segments. They show a characteristic segmental pattern of collaterals with some modification in tail segments and of dorsal plasma protrusions in the unpaired medial giant fiber presumably representing openings in the myelin sheath. We found no multisegmental cobalt transport in other large neurons of the nerve cord. Cobalt passes through the segmentai septa between consecutive axonal elements of the metameric giant fibers and presumably also through commissural contacts between specific collaterals of the lateral giant fibers. Since these sites of contact are known to represent electrical synapses, cobalt coupling may, in L. terrestris, correlate with functional electrotonic coupling.

The distribution of serotonin-immunoreactive cell bodies and fibers in the ventral nerve cord of the earthworm has been investigated from whole-mount preparations and serial sections. Serotonin-immunoreactive neurons are organized in seven soma groups per ganglion; these are defined by cell number, soma shape, diameter and fiber projections. Positional variations of this pattern have been studied quantitatively. The number of labeled perikarya is constant in midbody ganglia, but increases markedly rostral to the posterior margin of the clitellum. Variability of position and cell number differs between the cell groups. Stained nerve fibers could only be partially traced; their distribution is described in relation to defined neuronal fiber bundles and segmental nerves. The distribution and morphology of serotonin-immunoreactive cells are compared with previous observations, based on pattern analysis of primary catecholamines and indolamines with the use of a formaldehyde-induced fluorescence technique. The possible role of serotonin-immunoreactive nervous elements in different nerve cord compartments is discussed with respect to physiological effects of serotonin in earthworms and other invertebrates.