Polyclads range from 3 millimetres (0.12 in) to 10 centimetres (3.9 in) in length with a flattened, roughly oval, body shape and, in many cases, a pair of short tentacles on the head.

They are distinguished from other related animals by the presence of a folded pharynx, an elongated intestine with numerous complex diverticula, and multiple ocelli.

In such associations, the worms may use the invertebrates as a source of food, or they may find protection from predators inside the structural framework of these hosts.

Acotyleans are major predators of sessile marine invertebrates such as all commercial bivalves species (including pearl and rock oysters), mussels, scallops and giant clams.

Cotyleans, on the other hand, with as many as 16 families, are prominent members of tropical coral reef communities and have bright, colorful bodies.