Crab

(invertebrate zoology) The common name for a number of crustaceans in the order Decapoda having five pairs of walking legs, with the first pair modified as chelipeds. The common name for members of the Merostoma.
(navigation) To drift sideways or to leeward, as a ship.

The name applied to arthropods in sections Anomura and Brachyura of the reptantian suborder of the order Decapoda, class Crustacea. The term crab is also sometimes used for two species of sucking lice (order Anoplura) which prey upon humans. Phthirus pubis is the crab louse which inhabits the pubic region.

Section Anomura includes over 1400 species in 12 different families. Commonly called hermit, king, sand, or mole crabs, the anomurans have lobsterlike abdomens which bend beneath the cephalothorax in crablike manner, setting them apart in a somewhat ill-defined group.

Section Brachyura encompasses the so-called true crabs, with reduced abdomens folded snugly beneath the cephalothorax. More than 4500 species making up 28 families have been grouped into subsections. In addition to subsections Dromiacea, Gymnopleura, and Oxystomata, the Hapalocarcinidea has one family of the same name containing three genera of small crabs with elongated abdomens. Subsection Brachygnatha is the largest group of brachyurans and includes the most typical crabs. It has 20 families encompassing over 3700 species which occupy a variety of habitats, undergo different types of development, and exhibit contrasting patterns of behavior. See also Crustacea; Decapoda (Crustacea).