Earthworms are hermaphrodites: each individual carries both male and female sex organs. As invertebrates, they lack either an internal skeleton or exoskeleton, but maintain their structure with fluid-filled coelom chambers that function as a hydrostatic skeleton.
"Earthworm" is the common name for the largest members of Oligochaeta (which is either a class or a subclass depending on the author). In classical systems, they were placed in the order Opisthopora, on the basis of the male pores opening posterior to the female pores, 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 (which translates to "big worms"), as opposed to the microdriles ("small worms") in the semiaquatic families Tubificidae, Lumbricidae, 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.
Depending on the species, an adult earthworm can be from 10 mm (0.39 in) long and 1 mm (0.039 in) wide to 3 m (9.8 ft) long and over 25 mm (0.98 in) wide, but the typical Lumbricus terrestris grows to about 360 mm (14 in) long. Probably the longest worm on confirmed records is Amynthas mekongianus that extends up to 3 m (10 ft) in the mud along the banks of the 4,350 km (2,703 mi) Mekong River in Southeast Asia.