mollusk, also spelled mollusc, any soft-bodied invertebrate of the phylum Mollusca, usually wholly or partly enclosed in a calcium carbonate shell secreted by a soft mantle covering the body.

Which mollusc does not have shell?

Slug is a terrestrial gastropod mollusc without shell. Octopus is another example, which is an aquatic cephalopodan mollusc.

Why do molluscs have shell?

As the mollusk ages, the shell becomes thicker and sturdier. As such, older mollusks usually have strong shells. The primary purpose of the shell is protection. It protects the animal against predators and guards the internal organs.