Their shells are made of calcium carbonate. The shells are very decorative on the inside for many marine species, and the outside for many snails. Most mollusks have at least two stages of life and their second stage (or adult phase) is often marked by bearing a shell.

Do all mollusks have shells?

Mollusks usually have a shell (although some do not). Mollusks also have an extension of the body wall called the mantle. This portion of the animal's anatomy is responsible for secreting the shell. The mantle encloses the mantle cavity which contains the Ctenidia (gills), anus and excretory pores.

What mollusks have no shells?