An earthworm is a tube-shaped, segmented animal that is commonly found living in soil. Its digestive system runs straight through its body, it conducts respiration through the cuticle covering its skin, and it has a simple, closed blood circulatory system. Earthworms are hermaphrodites--each individual carries both male and female sex organs. As an invertebrate it lacks a skeleton, but an earthworm maintains its structure with fluid-filled chambers functioning like a hydro-skeleton.
"Earthworm" is the common name for the largest members of Oligochaeta (which is either a class or subclass depending on the author) in the phylum Annelida. In classical systems they were placed in the order Opisthopora, on the basis of the male pores opening posterior to the female pores, even though the internal male segments are anterior to the female. Theoretical cladistic studies have placed them instead in the suborder Lumbricina of the order Haplotaxida, but this may again soon change. Folk names for the earthworm include "dew-worm", "Rainworm", "night crawler" and "angleworm" (due to its use as fishing bait).
Larger terrestrial earthworms are also called megadriles (or big worms), as opposed to the microdriles (or small worms) in the semi-aquatic families Tubificidae, Lumbriculidae, and Enchytraeidae, among others. The megadriles are characterized by having a distinct clitellum (which is more extensive than that of microdriles) and a vascular system with true capillaries.
Read more about Earthworm: Reproduction, Regeneration, Locomotion and Importance To Soil, Benefits, Earthworms As An Invasive Species, Special Habitats, Economic Impact, Taxonomy and Distribution