YOU can always recognize a limpet by the shape of his shell. It is oval at the base and pointed at the top, like a miniature tent. Its shape has also been compared to that of the broad-brimmed hats often worn by Chinese coolies.

When you touch a moving snail, he draws in his foot. When you touch a limpet, he protects himself by drawing his tent-like shell down tight over his body. It is difficult to pry him off the rock or other object to which he is attached.

Snails, limpets, clams and mussels belong to a large group of animals called mollusks, which includes most of the animals commonly called shellfish, except crustaceans.

Snails and limpets are the gastropods, or "stomach-footed ones" of this group. Clams and mussels are the "hatchet-footed ones." When you examine the shells, you can see why snails and limpets are also referred to as univalves and clams and mussels as bivalves.

Fresh-water limpets are usually classified with the air-breathing snails, for, like snails, the limpets have a head, one pair of tentacles, one lung, one foot, and a shell. However, the limpet's shell is never coiled like that of other snails, and he does not wander about as they do.

Fresh-water limpets are very small. Those you are most likely to find are about onequarter inch in length.

Like other water snails, the limpet builds his shell in one piece, and makes it larger by adding new layers around the bottom. The shells are usually dark gray, and difficult to see among the plants and other animals in your collection, particularly if the limpet is attached to a rock of the same color.