The majority of aquatic oligochaetes are small, slender worms, whose organs can be seen through the transparent body wall. They burrow into the sediment or live among the vegetation mostly in shallow, freshwater environments. Some are transitional between terrestrial and aquatic habitats, inhabiting swamps, mud or the borders of water bodies. About two hundred species are marine, mostly in the families Enchytraeidae and Naididae; these are found largely in the tidal and shallow subtidal zones, but a few are found at abyssal depths.