Mollusca — soft bodies and shells

This is the phylum of triploblastic animals with soft bodies. Snails, bivalves, octopuses, slugs, squid and cuttlefish belong to it.

Figure to be added

Figure 13.20 — several species of Mollusca: snail, cuttlefish and octopus.

They live in terrestrial, fresh water and sea water environments.
Multicellular and triploblastic, with soft bodies — which is why they are called molluscs.
The body has three main parts: the head, the muscular foot and the visceral mass. The body is not divided into segments.
They bear a body covering moistened by mucus.
Some bear an external or internal shell made of calcium carbonate (CaCO₃), secreted by the mantle.
The body shows bilateral symmetry.
They reproduce sexually and are generally unisexual.

Figure to be added

Figure 13.21 — a section through a mollusc: the head, the muscular foot and the visceral mass.

Segmentation is the thing to check

Annelids and molluscs are both triploblastic, both bilaterally symmetrical and both soft. The easiest thing separating them is segmentation: an annelid's body is divided into equal segments, a mollusc's is not.