Fig. 1. Diagrammatic anatomy of Mercenaria mercenaria (adapted from Gainey et al., 2003). (A) Cross section of a clam. (B) Cross sections through portions of relaxed and contracted demibranchs cut in an anterior-posterior direction, i.e. in and out of the plane of the page with respect to the top figure. (C) Details of a water tube: with the musculature relaxed (left), and contracted (right). The water tube muscles are within the walls of the horizontal blood vessels, the vessels are not shown here. The longitudinal muscles lie between the water tube muscles (and the horizontal blood vessels) and the base of the gill filaments. bv, blood vessel; gf, gill filament; lm, longitudinal muscle; s, septum; wtm, water tube muscle.