Mantle cavity

A pallium is a woollen vestment conferred on archbishops by the Pope, the mantle of a mollusc, or an anatomical term for the cerebral cortex.

The defining characteristic of all molluscs is the presence of the mantle. Snails have a mantle, and are thus a mollusc.

The siphon is part of the mantle of the mollusc, and the water flow is directed to (or from) the mantle cavity. A single siphon occurs in some gastropods. In those bivalves which have siphons, the siphons are paired.

A Periwinkle is a gastropod mollusc, an underwater snail. The gills of these animals are under the shell, under the mantle in the region where the shell meets the head.