Anatomy of and patterns of water currents through the mantle cavities of pleurotomariid gastropods.

The paired pallial organs of pleurotomariids (slit shells) are generally interpreted as representing the primitive gastropod condition. Relatively few individual pleurotomariids have been observed alive; most anatomical descriptions have been based on retracted, preserved material. In this study, live animals were observed in still and moving water, videotaped, photographed, and relaxed prior to anatomical description. The pleurotomariid Perotrochus maureri formed a siphon with its mantle at the posterior end of the slit in the shell. As expected, water was expelled from the mantle cavity through this siphon. Dye released along the aperture, the area traditionally considered to be the incurrent window, was never observed to enter the mantle cavity. Instead, water entered through the slit just anterior to the siphon. A similar pattern was observed in the deep-slit pleurotomariid Entemnotrochus adansonianus. Thus the "anal" slit provided the sites for the incurrent as well as the excurrent windows. This surprising flow pattern is inconsistent with the traditional scenario of early gastropod evolution.