True crabs are decapod crustaceansand belong to a group called the Brachyura. They have a very short projecting "tail" and their small abdomens are completely hidden under the thorax. There are almost 7,000 speciesof true crabs with over 800 species freshwater.
Other animals, such as hermit crabs, king crabs, porcelain crabs, horseshoe crabs and crab lice do not belong in the Brachyura - the true crabs.
All crabs have one pair of pincers (chelipeds) and four pairs of walking legs. They are the first pair of legs on a crab and are used for holding and carrying food, digging, cracking open shells and warning off would be attackers. The carapace protects the internal organs of the head, thorax and gills. The eyes are on the ends of short stalks and the mouthparts are a series of pairs of short legs, specialised to manipulate and chew food. The abdomen is small and tightly held under the body. The sexes are separate and the size of the abdomen distinguishes them.
For much more detailed information refer to the "World of Crabs" on this website.