There are about 52,000 described Crustaceans, but it is thought that many more await description. The body plan varies considerably and in some the head and some thoracic segments have become fused into a cephalothorax; and in others the carapace covers most of the body.
The cuticle is mainly composed of calcareous material, with some chitin and protein; and this no doubt restricts their colonization of water with a low pH.
There are many leg modifications, e.g. walking legs, paddles, food collection, and claspers for mating. Although some are terrestrial, e.g. land crabs, all except the woodlice must return to the water to breed.
In the smaller aquatic crustaceans there is a tendency for gaseous exchange to take place over the entire body.
They are separated into nine Classes (see the menu above).
Humans eat more Crustacea than any other invertebrate Phylum. It is often the remains of these meals, the middens, that indicate sites of past human habitation.
Chromatophores are pigment containing cells, and can be quite large. However the pigment can be concentrated in a tiny area so small as to be invisible, or dispersed throughout the cell making it visible. The cells usually have irregularly shaped sides so that they fit together like a jig-saw making a number of colour combinations possible, that allow the animal to change colour rapidly. It is thought that colour change is one form of communication.