Earthworms belong to phylum annelida. This phylum is part of the animal kingdom and includes marine, freshwater and terrestrial worms in addition to leeches.
The phylum annelida is characterized by its members' body segments. Body segments are advantageous to those animals that have them, making everything from development to movement easier thanks to the individualized functioning of each segment. These segments also make it easy for members of the annelida phylum to survive injury quite easily; that's why earthworms can continue living after having some segments removed. Earthworms, like their phylum mates, have a closed circulatory system, bilateral symmetry, a complete digestive system and a basic nervous system.