Abstract
The morphology of the ventricle of the fresh‐water lamellibranch, Elliptio complanatus, was investigated. Contrary to the condition reported previously in Tritogonia verrucosa, the two atria in Elliptio communicate with the ventricular lumen through separate openings, each guarded by an atrio‐ventricular valve. Fixation of ventricle for electron microscopy with 2.5% buffered glutaraldehyde did not appear to shrink the tissue, in spite of the low blood osmolarity to which the muscle is adapted. Ventricle tissue is composed of smooth muscle fibers, containing a central nucleus, glycogen, mitochondria, paramyosin, dense bodies and “attachment plaques,” much like the ventricle of the salt‐water clam, Venus (Mercenaria) mercenaria.