They also possess a calcified exoskeleton, and have two pairs of antennae. True crabs differ from other decapods in that their abdomen folds up under the carapace rather than extending like those of shrimp and lobsters. Hermit crabs are an exception because they lack a hard calcified abdominal shell. They utilize the abandoned shells of snails to house themselves for protection. As they grow they must continually move to larger and larger shells to accommodate their increased size.
Barnacles are also crustaceans.
Similar to shrimp in many ways, they differ from most other crustaceans by living within a hard, cone-shaped, calcareous shell. As larvae they drift in ocean currents and eventually cement themselves head first onto a hard object. Their feet are modified and feather-like, and are used to sweep fine particles into their mouths. The horseshoe crab is not really a crab at all. It is more closely related to spiders and scorpions.