A protostome coelomate was the common ancestor of mollusks and annelids. The mollusks evolved from an ancestor resembling the chiton in body plan. The mollusks then evolved three main body plans; one clam-like, one like the octopus and squid, and one like snails and slugs. All three kinds of mollusks became, and still are, widespread in the sea. The gastropods (snails, etc.) and bivalves (clams, etc.) occupy both marine and fresh water bodies. The gastropods also occupy the land. The more highly evolved annelids exhibit segmentation, a trait that characterizes all advanced animals. The terrestrial annelid, the earthworm, is familiar to all. The tube within a tube construction is also characteristic of advanced animals. Less familiar is the bristled annelid, the polychaete, that is abundant in marine waters. The leech can be found in fresh and salt water and in terrestrial habitats.