Annelids live all over the world in all types of ecosystems, preferring to burrow in soil, sand or sediment both on land and in the ocean. An estimated 17,000 species of annelids are known to exist worldwide.
Annelids are also known as segmented worms. The phylum includes leeches, earthworms, ragworms, tubeworms and other wormlike, segmented invertebrates. They are characterized by their bilateral symmetry, parapods used for locomotion, segmented bodies separating the multiple organ systems and closed circulatory system. Some annelids have functional jaws used for obtaining food, while others are filter feeders and still others lack a gut entirely and rely on beneficial bacteria for nutrients.