It is almost impossible to generalize about the ecology and life history of arthropods; they are simply too diverse. Arthropods first appeared in the sea; trilobites were all marine. Most crustaceans are also marine, but one crustacean group has succesfully invaded the land, the Isopoda, a.k.a. "pillbugs" or "roly-polies." The vast majority of living arthropods belong to groups that are specialized for a land-dwelling existence.