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Kingdom and Phylum
Crabs, along with all other animals, belong to kingdom Animalia, which excludes bacteria, algae, protists and plants. Members of Animalia are all heterotrophs (they get their energy from other organisms) and multicellular. Kingdom Animalia is divided into phyla; crabs belong to phylum Arthropoda, along with insects, trilobites, chelicerates and other crustaceans. Members of Arthropoda have exoskeletons that they can molt as they grow, segmented bodies and jointed appendages on those segments. Crabs are grouped into a subphylum within Arthropoda called Crustacea; this subphylum contains aquatic arthropods.
Class
Class Malacostraca contains crabs as well as lobsters, shrimp and more unfamiliar crustaceans. Malacostracans have a head, thorax and abdomen that each contain a specific number of appendages. Like all crustaceans, Malacostracans have heads made up of five fused segments; their thorax has eight segments and their abdomens six. Malacostracans usually have a carapace or shell covering their bodies.
Order
Crabs belong to Decapodia, an order that also includes lobsters, crayfish and shrimps. Decapods are marked by their five pairs of legs, which are simple or pincer-like legs. The infraorder Brachyura contains true crabs, which, unlike lobsters, crayfish and shrimp, have short, flat, wide abdomens. Members of Brachyura also always have chelipeds (claw-like front legs) and reduced antennae.
Families
Crabs make up several families within the order Decapodia. Rock crabs belong to Cancridae, sumo crabs to Dorippidae, and deep sea crabs to Geryonidae. Carrier crabs make up the family Homolidae, while king crabs belong to Lithodidae and spider crabs to Majidae. Parapaguridae consists of hermit crabs, while the family Portunidae is made up of the only crabs that can swim. These families of crabs are further divided into genera and finally species; there are approximately 4,500 known crab species.