22,000 species of segmented worms are found in marine environments. Found from tidal zones to hydrothermal vents, Annelida their respiratory systems are through the skin. However, many polychaetes have gills just as molluscas.
Polychaete
Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae. Polychaetes as a class are robust and widespread, with species that live in the coldest ocean temperatures of the abyssal plain, to forms which tolerate the extreme high temperatures near hydrothermal vents. Polychaetes occur throughout the Earth's oceans at all depths, from forms that live as plankton near the surface.
Clitellata
The Clitellata are a class of annelid worms, characterized by having a clitellum - the 'collar' that forms a reproductive cocoon during part of their life cycles. The clitellates comprise around 8,000 species. Unlike the class of Polychaeta, they do not have parapodia and their heads are less developed.
Oligochaeta
This class is made up of many types of aquatic and terrestrial worms, including various earthworms. With around 10,000 known species, the Oligochaeta make up about half of the phylum Annelida. These worms usually have few setae (chaetae) or "bristles" on their outer body surfaces, and lack parapodia, unlike polychaeta