Earthworms are living soil mills with a tube-within-a-tube body
Earthworms are terrestrial annelids—the larger oligochaetes—eating detritus, microbes, and algae in moist soils. A gut runs the body’s length; segments, setae, and dual fluid systems (coelom plus closed blood) structure the familiar crawler. Most are hermaphrodites that trade sperm.
Sizes span roughly ten millimeters to three meters; Lumbricus terrestris nears 360 mm, while Amynthas mekongianus ranks among the longest recorded. Nerve cords and paired ganglia coordinate each metamere; chemoreceptors crowd the mouth. Many Lumbricidae can reproduce parthenogenetically. Folk names include dew-worm, rainworm, nightcrawler and angleworm, the last from its use as fishing bait. Big terrestrial species are megadriles, marked by a prominent clitellum and true capillaries, as opposed to the small microdriles. Older schemes put them in the order Opisthopora because the male pores open behind the female ones; cladistic work has moved them to the suborder Lumbricina, though that may change.
Segment count is typically fixed at birth within a species; growth enlarges segments rather than adding them. Dorsal pores slime the cuticle. They rank among nature's chief detritivores and dung-eaters, and in turn feed wild boars, moles, blackbirds, centipedes, ground beetles and flatworms; even their faeces and mucus become food. Amynthas mekongianus stretches up to 3 metres in the mud beside the Mekong. Lacking bones, a worm holds its shape with fluid-filled coelom chambers that act as a hydrostatic skeleton, breathes through its skin, and moves using rings of circular and lengthwise muscle in each segment; similar muscles push food down the gut. The laboratory test worm Eisenia fetida is one of the species able to reproduce without mating.
The first segment carries the mouth under a fleshy lobe, the prostomium, which closes the opening at rest, senses the surroundings and in some species grabs leaves and grass to haul into the burrow. Most segments bear bristles called setae, anywhere from four pairs to a full ring, which anchor the body as it moves; special belly setae hold mating partners together. Adults grow a glandular belt, the clitellum, that produces egg capsules. Under stress many worms spray coelomic fluid from pores in the back, and Australia's blue squirter, Didymogaster sylvaticus, can shoot it about 30 centimetres.
Source: Earthworm
