Sea mice, rag worms, bloodworms, palolo worms and lugworms are types of annelids in the Polychaeta class. Other annelids include earthworms, class Oligochaeta, and leeches, class Hirudinea. Annelids are named for their segmented bodies, which are divided into ring-like sections.
More than 15,000 species of annelids exist in land and aquatic habitats, including both freshwater and salt water. The smallest annelids are less than a millimeter long, while the largest species grow up to 10 feet.
The phylum name Annelida means "little ring" in reference to the tubular rings known as somites or annulations that shape the worm's body. As invertebrates, annelids have no rigid backbone structure inside their flexible body cavity, or coelom. An annelid's coelom is filled with fluid, and the organism's slithering motion is enabled by strong muscular walls that expand and contract.
Polychaetes are usually found in marine environments, where they either live as sedentary burrowers or freely travel through bodies of water. They often require a greater range of motion and many species have small, fleshy appendages known as parapodia extending from the sides of their bodies. Leeches live in water and soil, while most earthworms are land burrowers. With the exception of leeches, most annelids have tiny, hair-like bristles protruding from each body segment, which aid in motion and traction.