Electron microscopic studies of planaria. III. Some observations on the fine structure of planarian nervous tissue

Abstract

Abstract The small size of the nerve fibers of the planarian nervous system render the details of its neuroanatomy invisible to optical microscopy. Electron microscopic studies on the nervous system of Dugesia dorotocephala are described showing the neuropil of the brain, the origin of the ventral nerve cords from the brain neuropil, synaptic junctions in the neuropil, and “giant” fibers in the longitudinal conduction system. Two major kinds of synapses can be discerned as well as the pre‐ and post‐synaptic sides of each. There appear to be three major kinds of cells comprising the brain; neurons, neurosecretory cells, and neuroaccessory cells. The neurons are unmyelinated, contain neurotubules, and appear to synapse only in the centrally located neuropil.