Neuronal elements in snail tentacles as revealed by horseradish peroxidase backfilling

The snail tentacle ganglion is a prominent structure that innervates an epithelial pad sensitive to wind and odors. Its neural composition, and that of the sensory pad, was studied in the terrestrial snail Achatina fulica by applying horseradish peroxidase to the distal end of the cut tentactle nerve. Five types of neurons were labelled by the procedure: receptors, located in the subepithelial region; three kinds of interneurons, located in the ganglion and its digitlike extensions; and a bipolar neuron, located in the dermo-muscular wall of the tentacle. In contrast to earlier descriptions based on silver stains, the present results demonstrate the presence of neurons of large size (soma diameters up to 46 microns). Also, contrary to earlier interpretations, the results indicate that all five identified cell types send axons directly to the CNS.