Bathyacmaea nipponica

Bathyacmaea nipponica is a species of very small (adults are typically about 6 mm in length), deep-sea limpet, a marine gastropod mollusk in the family Pectinodontidae.

For these reasons, along with a comparison of the development of the shell at the microscopic level, it has been argued that B. nipponica is not closely related to the Patelloidea or the Neolepetopsidae as one might expect based on simple morphological characteristics and similarity of appearance.

B. secunda does not have a series of arteries and veins— like all gastropods and indeed all molluscs, its blood is retained within various open body cavities or "sinuses" collectively referred to as a hemocoel.

Behind the nerve ring, the commissure of the pleural ganglia performs a characteristic "twist" common to many gastropods, the evolutionary result of torsion which placed the anus and the openings of the kidneys ("nephridial openings") near the head of the animal in order to accommodate the ancestral presence of a twisted shell (B. secunda does not have such a shell, but the streptoneurous condition of the pleural commissure remains).

The reproductive system of both males and females consists only of a single large disk-shaped gonad organ located above the animal's foot and extending around the lower posterior part of the visceral mass.