GAINEY, JR., L.F.* ; WALTON, J.; GREENBERG, M.J.: Neuromuscular anatomy of clam gills

The eulamellibranch gills of Mercenaria mercenaria contain three separate groups of muscles. Each gill filament has dorso-ventral muscle fibers running down its center. The interfilament tissue junctions, which connect adjacent gill filaments, contain longitudinal muscle fibers that run the length of the gill at right angles to the gill filament muscles. At the intersection of a gill filament and an interfilament tissue junction, there are muscle fibers connecting the two sets of muscles. The water tube epithelium contains a lattice-like network of muscle fibers running between the septa and the blood vessel, which lies midway between the septa of a water tube. Two distinct networks of serotonergic (5-hydroxytryptamine, 5HT) varicose fibers parallel the layout of the muscle fibers. Each gill filament contains a pair of serotonergic fibers that run parallel to the mucle fibers; these putative nerves lie between the epithelium and the muscle fibers within a filament. There is a separate nerve net associated with the longitudinal and water tube epithelium muscles. In each water tube, the nerve net consists of dorso-ventral nerve bundles running along the septa and blood vessel. These nerve bundles are connected by nerve fibers that run along the longitudinal muscles and the muscles of the water tube eipthelium. These anatomical observations confirm the results of previous pharmacological studies implying that 5HT is the excitatory neurotransmitter of the gill muscles.