The annelids, ringed worms or segmented worms, are a large phylum, with over 17,000 species that include leeches, ragworms, and earthworms. The species exist in and have adapted to various ecologies - some in environments such as tidal zones and hydrothermal vents, others in fresh water, and yet others in moist terrestrial environments. The annelids are triploblastic, bilaterally symmetrical, coelomate, invertebrate organisms. They also have parapodia for locomotion.