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An earthworm is a soil-dwelling terrestrial invertebrate that belongs to the phylum Annelida. The term is the common name for the largest members of the class (or subclass, depending on the author) Oligochaeta. In classical systems, they were in the order of Opisthopora since the male pores opened posterior to the female pores, although the internal male segments are anterior to the female. Theoretical cladistic studies have placed them in the suborder Lumbricina of the order Haplotaxida, but this may change.[clarification needed] Other slang names for earthworms include "dew-worm", "rainworm", "nightcrawler", and "angleworm" (from its use as angling hookbaits). 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. The megadriles are characterized by a distinct clitellum (more extensive than that of microdriles) and a vascular system with true capillaries.[1]

Earthworms are commonly found in moist, compost-rich soil, eating a wide variety of organic matters,[2] which include detritus, living protozoa, rotifers, nematodes, bacteria, fungi and other microorganisms.[3] An earthworm's digestive system runs the length of its body.[4] They are one of nature's most important detritivores and coprophages, and also serve as food for many low-level consumers within the ecosystems.

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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.[8] Probably the longest worm on confirmed records is Amynthas mekongianus that extends up to 3 m (10 ft) [9] in the mud along the banks of the 4,350 km (2,703 mi) Mekong River in Southeast Asia.

From front to back, the basic shape of the earthworm is a cylindrical tube-in-a-tube, divided into a series of segments (called metameres) that compartmentalize the body. Furrows are generally[10] externally visible on the body demarking the segments; dorsal pores and nephridiopores exude a fluid that moistens and protects the worm's surface, allowing it to breathe. Except for the mouth and anal segments, each segment carries bristlelike hairs called lateral setae[11] used to anchor parts of the body during movement;[12] species may have four pairs of setae on each segment or more than eight sometimes forming a complete circle of setae per segment.[11] Special ventral setae are used to anchor mating earthworms by their penetration into the bodies of their mates.[13]

Generally, within a species, the number of segments found is consistent across specimens, and individuals are born with the number of segments they will have throughout their lives. The first body segment (segment number 1) features both the earthworm's mouth and, overhanging the mouth, a fleshy lobe called the prostomium, which seals the entrance when the worm is at rest, but is also used to feel and chemically sense the worm's surroundings. Some species of earthworm can even use the prehensile prostomium to grab and drag items such as grasses and leaves into their burrow.

An adult earthworm develops a belt-shaped glandular swelling, called the clitellum, which covers several segments toward the front part of the animal. This is part of the reproductive system and produces egg capsules. The posterior is most commonly cylindrical like the rest of the body, but depending on the species, it may also be quadrangular, octagonal, trapezoidal, or flattened. The last segment is called the periproct; the earthworm's anus, a short vertical slit, is found on this segment.[11]

Many earthworms can eject coelomic fluid through pores in the back in response to stress; the Australian Didymogaster sylvaticus (known as the "blue squirter earthworm") can squirt fluid as high as 30 cm (12 in).[19][17]

Touching an earthworm, which causes a "pressure" response as well as (often) a response to the dehydrating quality of the salt on human skin (toxic to earthworms), stimulates the subepidermal nerve plexus which connects to the intermuscular plexus and causes the longitudinal muscles to contract. This causes the writhing movements observed when a human picks up an earthworm. This behaviour is a reflex and does not require the CNS; it occurs even if the nerve cord is removed. Each segment of the earthworm has its own nerve plexus. The plexus of one segment is not connected directly to that of adjacent segments. The nerve cord is required to connect the nervous systems of the segments.[23]

The giant axons carry the fastest signals along the nerve cord. These are emergency signals that initiate reflex escape behaviours. The larger dorsal giant axon conducts signals the fastest, from the rear to the front of the animal. If the rear of the worm is touched, a signal is rapidly sent forwards causing the longitudinal muscles in each segment to contract. This causes the worm to shorten very quickly as an attempt to escape from a predator or other potential threat. The two medial giant axons connect with each other and send signals from the front to the rear. Stimulation of these causes the earthworm to very quickly retreat (perhaps contracting into its burrow to escape a bird).

The presence of a nervous system is essential for an animal to be able to experience nociception or pain. However, other physiological capacities are also required such as opioid sensitivity and central modulation of responses by analgesics.[24] Enkephalin and -endorphin-like substances have been found in earthworms. Injections of naloxone (an opioid antagonist) inhibit the escape responses of earthworms. This indicates that opioid substances play a role in sensory modulation, similar to that found in many vertebrates.[25]

The gut of the earthworm is a straight tube that extends from the worm's mouth to its anus. It is differentiated into an alimentary canal and associated glands which are embedded in the wall of the alimentary canal itself. The alimentary canal consists of a mouth, buccal cavity (generally running through the first one or two segments of the earthworm), pharynx (running generally about four segments in length), esophagus, crop, gizzard (usually), and intestine. [27]

At birth, earthworms emerge small but fully formed, lacking only their sex structures which develop in about 60 to 90 days. They attain full size in about one year. Scientists predict that the average lifespan under field conditions is four to eight years, while most garden varieties live only one to two years.

Several common earthworm species are mostly parthenogenetic, meaning that growth and development of embryos happens without fertilization.Among lumbricid earthworms, parthenogenesis arose from sexual relatives many times.[34] Parthenogenesis in some Aporrectodea trapezoides lineages arose 6.4 to 1.1 million years ago from sexual ancestors.[35] A few species exhibit pseudogamous parthogenesis, meaning that mating is necessary to stimulate reproduction, even though no male genetic material passes to the offspring.[36]

In Hormogaster samnitica and Hormogaster elisae transcriptome DNA libraries were sequenced and two sex pheromones, Attractin and Temptin, were detected in all tissue samples of both species.[37] Sex pheromones are probably important in earthworms because they live in an environment where chemical signaling may play a crucial role in attracting a partner and in facilitating outcrossing. Outcrossing would provide the benefit of masking the expression of deleterious recessive mutations in progeny[38] (see Complementation).

Copulation and reproduction are separate processes in earthworms. The mating pair overlap front ends ventrally and each exchanges sperm with the other. The clitellum becomes very reddish to pinkish in colour. Sometime after copulation, long after the worms have separated, the clitellum (behind the spermathecae) secretes material which forms a ring around the worm. The worm then backs out of the ring, and as it does so, it injects its own eggs and the other worm's sperm into it. Thus each worm becomes the genetic father of some of their offspring (due to its own sperm transferred to other earthworm) and the genetic mother (offsprings from its own egg cells) of the rest. As the worm slips out of the ring, the ends of the cocoon seal to form a vaguely onion-shaped incubator (cocoon) in which the embryonic worms develop. Hence fertilization is external. The cocoon is then deposited in the soil. After three weeks, 2 to 20 offspring hatch with an average of 4. Development is direct i.e. without formation of any larva.

Within the world of taxonomy, the stable 'Classical System' of Michaelsen (1900) and Stephenson (1930) was gradually eroded by the controversy over how to classify earthworms, such that Fender and McKey-Fender (1990) went so far as to say, "The family-level classification of the megascolecid earthworms is in chaos."[43] Over the years, many scientists have developed their own classification systems for earthworms, which led to confusion, and these systems have been and still continue to be revised and updated. The classification system used here which was developed by Blakemore (2000), is a modern reversion to the Classical System that is historically proven and widely accepted.[44]

Categorization of a megadrile earthworm into one of its taxonomic families under suborders Lumbricina and Moniligastrida is based on such features as the makeup of the clitellum, the location and disposition of the sex features (pores, prostatic glands, etc.), number of gizzards, and body shape.[44] Currently, over 6,000 species of terrestrial earthworms are named, as provided in a species name database,[45] but the number of synonyms is unknown.

From a total of around 7,000 species, only about 150 species are widely distributed around the world. These are the peregrine or cosmopolitan earthworms.[46]Of the 182 taxa of earthworms found in the United States and Canada, 60 (33%) are introduced species. e24fc04721