Bilateral Symmetry with segmentation reduced. Body covered with a thin membrane called the mantle . This structure may secrete the shell.
Gas exchange occurs through the gills or the lining of the mantle cavity.
Nitrogenous wastes are excreted through nephridia . Open circulatory system. Does not contain many blood vessels. Reproduction sexual with the development of the trochophore larvae. Its body cavity is completely lined with mesoderm.