The structure and function of the stomachs of nine bivalves are examined. These animals belong to the Gastrotetartika, Gastrotriteia, and Gastropempta, the three orders of the Polysyringia, a new sub-class of the Bivalvia suggested by Purchon (1960a). It is found that in all three orders there are common ciliary tracts and grooves. For example, tracts are always found in association with the duct openings, and a distinction between the right duct tract and the posterior sorting area is drawn. Another tract which is a constant feature of these stomachs, but which has not been noted before, is the antero-dorsal tract. A generalized polysyringian stomach is constructed. All the stomachs examined function similarly, there being set up in the lumina circulating currents of the gastric fluid. In general, large particles and masses of particles are recirculated until they are broken down, and small heavy particles are rejected by the sorting areas, of which three main types are described. Light particles in suspension in the stomach fluid are carried by the circulating current to the vicinity of the opening of the ducts of the digestive diverticula.

The form of the primitive polysyringian stomach and the evolution of sorting areas are postulated. Possible steps in the evolution of the gastropemptan stomach through the gastrotetartikan form are suggested. The function of the appendix is that of a temporary store for large, heavy particles which escape the pallial sortingjnechanisms. It is noted that this organ occurs in all three orders of the Polysyringia.