Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting tail-like abdomen, usually hidden entirely under the thorax. They live in all the world's oceans, in freshwater, and on land, are generally covered with a thick exoskeleton, and have a single pair of pincers on each arm. They first appeared during the Jurassic period, around 200 million years ago.
Crab
Gecarcinus quadratus, a land crab from Central America
Crab (Pachygrapsus marmoratus) on Istrian coast, Adriatic Sea
Female crab Xantho poressa at spawning time in the Black Sea, carrying eggs under her abdomen
The Decapoda or decapods are an order of crustaceans within the class Malacostraca, and includes crabs, lobsters, crayfish, shrimp, and prawns. Most decapods are scavengers. The order is estimated to contain nearly 15,000 extant species in around 2,700 genera, with around 3,300 fossil species. Nearly half of these species are crabs, with the shrimp and Anomura including hermit crabs, porcelain crabs, squat lobsters making up the bulk of the remainder. The earliest fossils of the group date to the Devonian.
Decapoda
"Decapoda" from Ernst Haeckel's Kunstformen der Natur, 1904
Whiteleg shrimp, Litopenaeus vannamei (Dendrobranchiata: Penaeoidea)
Heterocarpus ensifer (Caridea: Pandaloidea)