This is usually a circle or horseshoe shape around the mouth that is covered with cilia. The cilia are used to move water through the lophophore and that's how they catch plankton. On Brachiopods the covering could be mistaken for a shell but technically its not produced in from a mantle like the shell of Mollusks . Your best to look at them in smaller classes. Univalves Have only one part to their shell and include animals like snails and abalone.
Bivalves Have two parts to their shell and include such species as oysters and scallops.
They include animals like chitons which because of the multi-plate structure are very good at holding on to curved rocks. In general you can say that arthropods have an exoskeleton or a hard external shell. These guys have 4 pairs of walking legs plus claws. These crabs have 3 pairs of walking legs plus claws. Although they have an exoskeleton it is softer and thus they tend to use empty mollusk shells for protection.