The Subphylum Crustacea belongs to the Phylum Arthropoda. It is a diverse group that includes crabs, lobsters, shrimps, ostracods, barnacles and woodlice. The Crustacea is the most diverse group of arthropods after the insects, including over 800 families, and many subgroups.

The most common body plan is a tripartite arrangement, consisting of a head, thorax, and abdomen. Unlike the insects, where the head and thorax are separated by a flexible joint, in the Crustacea they are almost always rigidly fused together. Sometimes the head and thorax are covered by a carapace, producing a single functional unit known as a cephalothorax. Some species have a prominent anterior projection from this cephalothorax called the rostrum.

The Crustacea are characterized by these features:

Below is a generalised diagram of a crustacean, showing the basic body plan.

Redrawn from Pechenik 1996.