AbstractThe neural plexus in theAplysiagill is comprised of monopolar, bipolar, and multipolar neurons. Histochemical procedures reveal acetycholinesterase (AChE), serotonin (5HT), and possibly dopamine present in plexus neurons. The three cell morphologies in the plexus are found in specific regions of the gill. Monopolar cells singly and in clusters lie along branches of the branchial and ctenidial nerves. The largest cluster, the gill ganglion, is located on the branchial nerve; ganglion neurons show a marked similarity to neurons in the central nervous system. AChE is present in monopolar neurons; its presence was demonstrated using acetylthiocholine as a substrate with inhibitors specific for esterases other than AChE. Monoamines are also contained in monopolar cells; yellow and green fluorescence, resulting from exposure to paraformaldehyde vapor, indicated that 5HT and catecholamines, respectively, are present in the cells. Bipolar neurons, containing AChE, are located beneath the ciliated epithelium in the pinnule. Multipolar, monopolar, and bipolar cells are found in the gill musculature. Single neurons appear to innervate at least two muscle bundles. Evidence is also presented suggesting that cells containing different neurotransmitters can terminate in the same muscle. Acetylcholine, 5HT, and dopamine, when infused in the gill, elicited discernable movements for each agent. The plexus, with its apparent hierarchy of neurons, has a number of properties ascribed to the central nervous system. The hierarchy is discussed in relation to the physiology of plexus neurons and to the behavior manifested by the gill.