FUNCTIONAL-MORPHOLOGY OF HETERODONAX-BIMACULATUS (LINNE, 1758) (BIVALVIA, PSAMMOBIIDAE)

Heterodonax bimaculatus (Linne, 1758) apparently is restricted to the coasts of temperate and tropical America. In southern Brazil, this spe cies occurs infaunally in shallow bay areas on coarse sand substrata. A comparison is made between H. bimaculatus and other infaunal Psammob iidae wherein the most significant adaptions concern the organs of the mantle cavity. The major structural features and ciliary currents are described. Special attention is paid to the ctenidia, labial palps an d stomach, and its functional morphology is similar to Gari and Asaphi s. The stomach of Heterodonax bimaculatus could not be defined as Type V as exists in all studied Psammobiidae. The major typhlosole does no t send a flare into the right caecum but describes a loop very close t o the mouth of it. Of all known species of Psammobiidae, only H. bimac ulatus has a stomach of Type IV. The species can be regarded as being near the ancestral condition in the process of evolution in the Tellin acea and so is at the base of the Psammobiidae lineage.