Crustaceans (Subphylum Crustacea) are one of the primary groups of arthropods, alongside insects and arachnids. The group includes such familiar lifeforms as crabs, lobsters, crayfish, shrimp, krill and barnacles. Most crustaceans are free-living aquatic animals, but some are terrestrial (e.g. woodlice), some are parasitic (e.g. Rhizocephala, fish lice, tongue worms) and some are sessile (e.g. barnacles).