A subset of the arthropods, a large group of invertebrates. Crustaceans are characterised by having a hard exoskeleton that must be moulted as the animal grows. Crustaceans are normally divided into six subsets, the most well-known of which includes crabs, lobsters, woodlice and shrimp. This leads people to assume that crustaceans have lots of appendages ("legs"). While this is true for most crustaceans, some have very few or none.
Crayfish and brine shrimp are crustaceans. Some, like crabs, are adapted to live on land and in water.