The crustaceans (Crustacea) are a large group of arthropods, comprising approximately 52,000 described species , and are usually treated as a subphylum . They include various familiar animals, such as lobsters, crabs, shrimp, crayfish and barnacles. The majority are aquatic, living in either fresh water or marine environments, but a few groups have adapted to terrestrial life, such as terrestrial crabs, terrestrial hermit crabs and woodlice. The majority are motile, moving about independently, although a few taxa are parasitic and live attached to their hosts (including sea lice, fish lice, whale lice, tongue worms, and Cymothoa exigua, all of which may be referred to as "crustacean lice"), and adult barnacles live a sessile life — they are attached head-first to the substrate and cannot move independently.
The scientific study of crustaceans is known as carcinology. Other names for carcinology are malacostracology, crustaceology and crustalogy, and a scientist who works in carcinology is a carcinologist, crustaceologist or crustalogist.Information from Wikepedia - http://en.wikipedia.org/wiki/Crustacean