Crustaceans are invertebrate animals in the phylum Arthropoda, subphylum Crustacea, and include the barnacle, crab, crayfish, lobster, shrimp, water flea, and pill bug. Although largely aquatic, a few crustaceans are found in terrestrial habitats. Of the nearly 40,000 aquatic species of crustaceans, only about 10% occur in freshwater habitats. Crustaceans have evolved a variety of specialized body forms and behaviors to cope with both aquatic and terrestrial environments, however, all crustaceans share several characteristics: a hard exoskeleton; jointed, paired appendages; and three body regions (head, thorax, and abdomen). The head and thorax regions are sometimes combined into a cephalothorax.