The shell and anatomy of Nipponomysella subtruncata is described. The bivalve is attached singly or in groups of up to nine on Siphonosoma cumanense, a burrowing intertidal sipunculan in south-west Japan. The species is a protandrous hermaphrodite. Specimens 1.4–2.5 mm long are males, which between 2.1 and 3 mm in length reverse sex and remain females. Reproduction peaks in summer and the annual number of clutches is small. Ripe oocytes, 84–88 μm diameter, are spawned into the suprabranchial cavity where they develop into 107-μm-long straight-hinged veligers. Following a planktotrophic period of unknown duration, the c. 360-μm-long spat normally settle upon and attach to the shells of larger, predominantly female, individuals. At a length of 1–1.6 mm they detach again and live separately thereafter. Sperm are transferred in spermatophores and stored within paired, mushroom-shaped receptacles located posteriorly in the female's suprabranchial cavity. The receptacles first appear in large males or in specimens in the process of reversing sex. Stored sperm probably survive long enough to fertilize more than one clutch. The anatomy of Nipponomysella is characteristic of the Montacutidae, and is of especial interest because of the unique structure of the sperm receptacles.