Crustaceans: Crabs, Lobsters, Shrimp & kin

At 67,000 species and counting, crustaceans are the arthropod-rulers of today's waters. Marine crustaceans include crabs, lobsters, shrimp, krill, crayfish, barnacles, and several other varieties. Though most species live in the oceans, some may traverse onto land to exploit other resources, and several have even evolved into completely terrestrial varieties like wood lice. Crustaceans are united by an exclusive stage in larval development called "nauplius," in which the larva uses its antennae for swimming, and the head possesses a single eye that is later reduced after the development of two compound eyes (though still retained into adulthood in some species). While most crustaceans are scavengers, some species of crabs and lobsters will actively hunt by using enormous claws on their front limbs to catch prey. The origin of the crustaceans is very ancient, reaching at least 505 million years ago (the Cambrian period) with a small species called Canadaspis perfecta.