Crabs are related to lobsters and shrimps. However, they have evolved so that they can walk or run sideways, as well as burrow and swim. The body is covered by a chitinous shell called a carapace. A crabs abdomen has become fairly reduced and is tucked under the body. "This reduction is greatest among the true crabs, which lack the flaplike tail of the hermit crabs and their allies; instead, the abdomen serves as a brood pouch for the eggs. A crab's segmented body has several pairs of appendages, of which usually five pairs serve as walking legs and two as sensory antennae."(1) The pincers, called chelae, are located on the front legs ans are used for fighting, display, and feeding. Crabs can possess fairly complicated nervous systems and are considered to be more advanced. They can live in more extreme areas because of their ability to adapt well to the changing environment. Crabs can see fairly well with their compound eyes; their senses of smell and taste are also more advanced which helps them to find food and mates easier. "Reproductive and social behavior sometimes include complex mating rituals and communication techniques such as drumming or waving the pincers. Crabs tend to be aggressive toward one another, and the males often fight to gain access to the females."(2)

Some Pictures:

(*)Spider Crab
A Green Crab (this was taken off Sengacontacket Pond in Edgartown, MA)
Another Green Crab found on the coast of Woods Hole, Massachusetts. Green crabs are invaders to the United States' coasts from the Western Atlantic Ocean.
A Japanese Crab found on the coast of Woods Hole, Massachusetts. The Japanese Crab is a recent invader of the Eastern United States intertidal zone and now flourishes there. It is normally found in the Eastern Pacific and Indian Oceans.
A juvenile Rock Crab which was found on the coast of Woods Hole, Massachusetts. It is a native of the Eastern United States intertidal zone as a juvenile.
A picture of the Green Crab, Japanese, and Rock Crab getting cosy together :)
Deep-water decorator crab (Chorila longipes)
Armed hermit crab (Pagurus armatus)
(*) A hermit crab

(1)"Crab," Microsoft Encarta 96 Encyclopedia. 1993-1995 Microsoft Corporation. Funk & Wagnalls Corporation.
(2) Ibid.