Crustaceans are arthropods, like insects, but unlike their cousins, have primarily adapted to life in the oceans. The term "crustcean" comes from the Latin word crusta meaning "crust, shell, or hard surface." Like other arthropods, crustaceans have a hard shell, often thicker than its insect cousins, and mandibles used to handle and consume food. Crustaceans are distinguished from other arthropods by being a monophyletic group (descending from a common ancestor) and possessing biramous (branching) limbs.
Crustaceans include many familiar animals — lobsters, shrimp, barnacles, crabs, and crayfish. There are also terrestrial crustaceans, such as terrestrial crabs, woodlice, and terrestrial hermit crabs. There are billions of woodlice in a typical forest, and some Pacific islands are literally swarming with terrestrial crabs. Some, like the coconut crab, are huge, with a leg span of 2 m (6 ft) and a weight up to 4 kg (9 lb). The coconut crab is the largest living land arthropod, capable of crushing coconuts with a single hammering motion of its claws. It even consumes rats occasionally, and will attack a human if threatened, though no deaths have ever been reported.
There are some less familiar crustaceans. One is the giant isopod, Bathynomus giganteus, foot-long relatives of woodlice that slowly cross the ocean floors, eating detritus. Giant isopods, accustomed to the relatively desert-like environment of the deep ocean floors, are capable of going for two entire months without food. These animals were first discovered by French zoologist Alphonse Milne-Edwards in 1879, after he fished up a specimen from the Gulf of Mexico. At the time, the discovery was lauded by both scientists and the public, and it helped prove that the ocean floors were not entirely devoid of life. Yet, to this day, there are many people who have never heard of the giant isopod, and upon viewing images of giant isopods, consider them to be models or the result of photoshopping.
Another class of unfamiliar crustaceans are crustacean lice, which infect every imaginable ocean creature. The hideous whale louse, which is found in the skin lesions, genital folds, nostrils, and eyes of whales, can reach up to an inch in size.