Encyclopedia Mollusks M aybe you think that snails, clams, mussels, squid, and octopods are very different. Yet, they are all in the same category of animals known as mollusks and are structurally similar. Some are very rare and are only found in very deep-water. Mollusks have three body regions. Mollusks usually, but not always, have a shell, which is secreted by a body wall called the mantle.
Many mollusks have a tongue of sorts, called a radula.
The radula is rough like sandpaper. Mollusks have well developed body organs that are used in the respiratory, circulatory and nervous systems. The stomach-foots include snails, limpets and abalones, which have shells. Slugs and nudibranchs are also stomach-foots, but do not have shells. A few stomach-foots are found on land.