What defines the organisms in the phylum Mollusca?

Organisms in the phylum Mollusca are defined primarily by their soft bodies, which in many species are protected by a hard shell. This characteristic of having a soft body is fundamental to all mollusks, including snails, clams, and octopuses. The presence of a shell, which is made of calcium carbonate in many cases, provides structural support and protection to the soft-bodied organism, and is another defining feature of many mollusks, though some do lack shells altogether, as seen in octopuses.

The features that differentiate mollusks from organisms in other phyla include their unique body plan, which typically includes a muscular foot for movement, a visceral mass containing internal organs, and a mantle that secretes the shell if present. This combination of soft body structures and potential shell formation distinguishes them from other animal groups listed in the alternatives provided.