The Decapods are an order of crustaceans within the class Malacostraca, including many familiar groups, such as crayfish, crabs, lobsters, prawns and shrimp. Most Decapods are scavengers. It is estimated that the order contains nearly 15,000 species in around 2,700 genera, with approximately 3,300 fossil species. Nearly half of these species are crabs, with the shrimps (c.3000 species) and Anomura (including hermit crabs, porcelain crabs, squat lobsters: c.2500 species), making up the bulk of the remainder.
As their name implies, all Decapods have ten legs; these are five pairs of thoracic appendages on the last five thoracic segments. The front three pairs function as mouthparts and are generally referred to as maxillipeds, the remainder being pereiopods. In many Decapods, however, one pair of legs has enlarged pincers; the claws are called chelae, so those legs may be called chelipeds. Further appendages are found on the abdomen, with each segment capable of carrying a pair of biramous pleopods, the last of which form part of the tail fan (together with the telson) and are called uropods.
Source: en.wikipedia.org/wiki/Decapoda - 25.05.2010
The perspective with this picture of a Decapod enhances the effect of the calipers, which are big, compared with the body.