The 7,000 - 8,000 species of segmented worms constitute a major phylum of the animal kingdom which is divided into three classes. Marine worms (5,000 species) play an important role in turning over much of the marine sea-floor sediment and are important in the region of domestic outfall sewers. Earthworm (3,250 species) turn over and aerate the surface soil. Leeches form the third class. It is not known how many of these species are endangered by different forms of pollution.