Decapoda

The Atlantic Ghost Crab, Ocypode quadrata (Decapoda: Pleocyemata: Reptantia: Eureptantia: Meiura: Brachyura: Eubrachyura: Thoracotremata: Ocypodidae) on Martinique, Antilles Photo by Patrick Verdier, via Wikipedia.

The most successful of all the malacostraca, the Decapoda ("ten-footed") appeared during the Devonian but remained insignificant until the start of the Mesozoic, when they underwent a tremendous evolutionary radiation, perhaps in response to the extinction of the Palaeozoic shrimp-like malacostractans at the end of the Permian.

The group includes all the most familiar animals we ordinarily think of as crustaceans - prawns, shrimp, crayfish, lobsters, and crabs, There are thought to be around 15,000 extant species in some 2,700 genera. Nearly half of these species are crabs, with the prawns or shrimp (Dendrobranchiata) and a group called the Anomura (including hermit crabs, porcelain crabs, squat lobsters, and others), making up the bulk of the remainder. While some decapods are tiny, others such as ordinary lobsters, the cocconut crab, and the Japanese spider crabs are easily the largest living arthropods, although still exceeded in size by Paleozoic eurypterids and arthropleurids. Approximately 3,300 fossil decapod species are know, although this obviously would only be a tiny fraction of all those that ever lived. - modified from Wikipedia, MAK120530