Studies of the gill, digestive gland, and gonads of a large mytilid from the 13 degree N hydrothermal vent site have led to insights concerning the histology and functional anatomy of the gill, as well as the role of this organ in the nutrition of the bivalve. From observations of the digestive gland, it is concluded that this mytilid ingests bacteria, as well as other microscopic organisms of hydrothermal vent and euphotic surface water origin. Histology of the gonad reveals that reproduction in this species is similar to that of shallow-water coastal bivalves.