Crustaceans, on the other hand, are most common to marine and estuarine environments. The most common marine arthropods are the decapod crustaceans, which include the crabs, shrimp and lobsters. The prefix deca means ten, and pod refers to legs, so these are ten-legged animals. 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.