The annelids, also known as ringed worms or segmented worms, are a large invertebrate phylum, with over 17,000 modern species including ragworms, earthworms, and leeches. Various forms specialize in their respective ecologies; some in marine environments as distinct as tidal zones and hydrothermal vents, others in fresh water, and yet others in moist terrestrial environments.
The annelids are bilaterally symmetrical, triploblastic, coelomate organisms. They also have parapodia for locomotion. Although most textbooks still use the traditional division into polychaetes, oligochaetes, and leech-like species, research since 1997 has radically changed this scheme, viewing leeches as a sub-group of oligochaetes and oligochaetes as a sub-group of polychaetes. In addition, the Pogonophora, Echiura, and Sipuncula, previously regarded as separate phyla are now regarded as sub-groups of polychaetes. Annelids are considered members of the Lophotrochozoa, a "super-phylum" of protostomes that also includes molluscs, brachiopods, flatworms, and nemerteans.