Molluscs show an amazing degree of diversity, yet all have certain features in common. All have soft bodies and have a strong muscular foot, which is used for movement or grasping. They also have gills, a mouth and an anus. A feature unique to molluscs is a file-like, rasping tool called a radula. This structure allows them to scrape algae and other food off rocks and even to drill through the shell of prey or catch fish.
The diversity of molluscs impressively demonstrates how a basic body plan can evolve into a variety of different forms adapted to specific environments. For example, the hard shell in a land-dwelling snail is relatively large and serves to protect the animal. In the fast-swimming squid, however, the shell is reduced to a small internal pen-shaped structure.