The presence of five pairs of thoracic legs (pereiopods) is the basis for .... decapod bodi

The presence of five pairs of thoracic legs (pereiopods) is the basis for the name decapod. The macrurous (shrimplike) species, which can be as small as 1 cm (0.5 inch), have elongated bodies with long abdomens, well-developed fan tails, and often long, slender legs. The brachyurous (crablike) types, which in the case of spider crabs can have spans of almost 4 m (12 feet) between their outstretched claws, have bodies that are flattened and laterally expanded, frequently with stout, short legs and reduced tail fans. Those decapods that have colonized terrestrial environments, such as some species of and , have evolved mechanisms to protect against desiccation and overheating while regulating the internal concentrations of their body fluids. Vascularization of the gill surfaces has made respiration possible on land for some species of decapods. Terrestrial decapods must usually return to the sea to spawn, while most freshwater decapods spend their entire life cycle in fresh wa! ter, commonly hatching their young as miniature adults. Decapods exist in a variety of relationships with other organisms. The two basic types of locomotion are swimming and crawling, though the macruran decapods are able to move swiftly backward by flexing their abdomens. In most decapods the eggs are carried cemented to the abdominal appendages until they are hatched. Decapods have three distinct body regions, each made up of segments, or somites: the head, thorax, and abdomen.