Crayfish, also known as crawdads or crawfish, belong to the phylum Arthropoda and the subphylum Crustacea. Like other members of their phylum, crayfish have external skeletons made of chitin, a circulatory system that lacks dedicated blood vessels and a simplified nervous system similar to that found in worms.
Crustaceans are distinguished from other members of their phylum by their adaptations for life in water. Other arthropods, such as insects, have book lungs or tracheae; but crustaceans, including crayfish, have gills. Crayfish resemble small lobsters, and both crayfish and lobsters belong to Decapoda, a group within Arthropoda that consists of animals with five legs and 20 body segments.