Some, like whelks, have a sharp edge of the shell. Some have a poisonous radular tooth, e.g., cone shells. Some are suspension feeders like Crepidula . This is the largest of the three subclasses and almost all are marine. The operculum that covers the aperture is composed of protein and calcium carbonate.
Water travels across the osphradium.
Larval stages include the trochophore and the veliger. In this group there is reduction or loss of shell and operculum. Not surprisingly therefore they have chemical defences. They undergo detorsion in development. The mantle cavity and the usual gills are reduced or lost.