Annelids may be aquatic (marine and fresh water) or terrestrial, free-living and sometimes parasitic. They exhibit organ-system level of body organisation and bilateral symmetry. They are triploblastic, metamerically segmented and coelomate animals. The body surface is distinctly marked into segments or metameres, hence the phylum name Annelida (Latin, annulus: little ring). They possess longitudinal and circular muscles that help in locomotion. Aquatic annelids like Nereis possess lateral appendages called parapodia, which help in swimming. A closed circulatory system is present. Nephridia (singular nephridium) help in osmoregulation and excretion. The neural system consists of paired ganglia connected by lateral nerves to a double ventral nerve cord. Nereis, an aquatic form, is dioecious, but earthworms and leeches are monoecious. Reproduction is sexual. Examples include Nereis, Pheretima (earthworm) and Hirudinaria (blood-sucking leech). Annelids are the first phylum in which metameric segmentation and a closed circulatory system appear together with a true coelom, and they are a frequent source of questions on nephridia, parapodia and the double ventral nerve cord. It is helpful to remember that, unlike the arthropods that follow, annelids lack jointed appendages, a chitinous exoskeleton and an open circulatory system.