Fig. 1. Diagrammatic anatomy of Mercenaria mercenaria [adapted from Gainey and Greenberg (Gainey and Greenberg, 2003) and Gainey et al. (Gainey et al., 2003)]. (A) Clam on the half shell; anterior is towards the left. (B) Cross section of a clam. (C) Details of a water tube: with the musculature relaxed (left) and contracted (right); the water tubes run dorso-ventrally within the gill. The water tube muscles are within the walls of the horizontal blood vessels (the vessels are not shown here). (D) Details of a relaxed demibranch (as in C, left). This cross section is slightly out of the horizontal plane; thus, the filaments to the left of the dashed line are at the level of the interfilamentar tissue junctions that contain the longitudinal muscles whereas the filaments to the right of the line are at the level of the ostia and horizontal blood vessels. The walls of the horizontal blood vessels contain both the water tube muscles and a dense network of serotonergic nerves; neither of these is shown. Abbreviations: bv, blood vessel; fc, frontal cilia; gf, gill filament; hbv, horizontal blood vessel; lc, lateral cilia; lfc, latero-frontal cirri; lm, longitudinal muscle; o, ostium; s, septum; wt, water tube; wtm, water tube muscle; 5-HTn, serotonergic neuron.