Earthworm
Earthworm is the common reference for the larger members of the Oligochaeta (which is either a class or subclass depending on the author) in the phylum Annelida. more...
In classical systems they were placed in the order Opisthopora, on the basis of the male pores opening to the outside of body posterior to the female pores, even though the male segments are anterior to the female. Cladistic studies have supported placing them instead in the Haplotaxida, which also includes the family Haplotaxidae. Folk names for earthworm include "dew-worm"，"night crawler" and "angleworm."
Earthworms are also called megadriles (or big worms), as opposed to the microdriles, which include the families Tubificidae, Lumbriculidae, and Enchytraeidae, among others. The haplotaxids have been traditionally considered microdriles. The megadriles are characterized by having a multilayered clitellum (which is much more obvious than the single-layered one of the microdriles), a vascular system with true capillaries, and male pores behind the female pores.
Overview
There are over 2,200 species known worldwide, existing everywhere but Arctic and arid climates. They range in size from two centimeters (less than one inch) to over three meters (almost ten feet) in the Giant Gippsland Earthworm. Amongst the main earthworm species commonly found in the soil are the red coloured Lumbricus terrestris, which dwells close to and leaves its deposits on the surface, whilst the greyish blue Allolobophora caliginosa is deeper burrowing.
In temperate zone areas, the most commonly seen earthworms are lumbricids (Lumbricidae), mostly due to the recent rapid spread of a relatively small number of European species, but there are several other families, e.g. Megascolecidae, Sparganophilidae, Glossoscolecidae, Haplotaxidae, and others. These other families are often very different from the lumbricids in behavior, physiology and habitat.
Anatomy
Earthworms have a closed circulatory system. They have two main blood vessels that extend through the length of their body: a ventral blood vessel which leads the blood to the posterior end, and a dorsal blood vessel which leads to the anterior end. The dorsal vessel is contractile and pumps blood forward, where it is pumped into the ventral vessel by a series of "hearts" which vary in number in the different taxa. A typical lumbricid will have 5 pairs of hearts. The blood is distributed from the ventral vessel into capillaries on the body wall and other organs and into a vascular sinus in the gut wall where gases and nutrients are exchanged. This arrangement may be complicated in the various groups by suboesophageal, supraoesophageal, parietal and neural vessels, but the basic arrangement holds in all earthworms.
Reproduction
Earthworms are hermaphrodites (both female and male organs within the same individual) but cannot fertilize their own eggs. They have testes, seminal vesicles and male pores which produce, store and release the sperm, and ovaries and ovipores. However, they also have one or more pairs of spermathecae (depending on the species) that are internal sacs which receive and store sperm from the other worm in copulation. Copulation and reproduction are separate processes in earthworms. The mating pair overlap front ends ventrally and each exchanges sperm with the other. The cocoon, or egg case, is secreted by the clitellum, the external glandular band which is near the front of the worm, but behind the spermathecae. Some indefinite time after copulation, long after the worms have separated, the clitellum secretes the cocoon which forms a ring around the worm. The worm then backs out of the ring, and as it does so, injects its own eggs and the other worm's sperm into it. As the worm slips out, the ends of the cocoon seal to form a vaguely lemon-shaped incubator (cocoon) in which the embryonic worms develop. They emerge as small, but fully formed earthworms, except for lacking the sexual structures, which develop later. Some earthworm species are mostly parthenogenetic, in which case the male structures and spermathecae may become abnormal, or missing.
Behavior
One often sees earthworms come to the surface in large numbers after a rainstorm. There are three theories for this behavior. The first is that the waterlogged soil has insufficient oxygen for the worms, therefore, earthworms come to the surface to get the oxygen they need and breathe more easily. Secondly, some species (notably Lumbricus terrestris) come to the surface to mate. This behavior is, however, limited to a few species. Thirdly, the worms may be using the moist conditions on the surface to travel more quickly than they can underground, thus colonizing new areas more quickly. This is in any event a dangerous activity in the daytime, since earthworms die quickly when exposed to direct sunlight with its strong UV content.
Locomotion and importance to soil
Earthworms travel underground by the means of waves of muscular contractions which alternately shorten and lengthen the body. The shortened part is anchored to the surrounding soil by tiny claw-like bristles (setae) set along its segmented length. The whole process is aided by the secretion of a slimy lubricating mucous. In more compacted soils the earthworm actually eats its way through the soil, cutting a passage with its muscular pharynx and dragging the rest of the body along. The ingested soil is ground up, digested, and the waste deposited behind the worm. This process aerates and mixes the soil, and is often considered greatly helpful by gardeners and farmers. In addition, many earthworms will come to the surface and graze on the higher concentrations of organic matter there, mixing it with the mineral soil. Because a high level of organic matter is associated with soil fertility, an abundance of earthworms is a happy sight for the organic gardener. In fact as long ago as 1881 Charles Darwin wrote:
"It may be doubted whether there are any other animals which have played so important a part in the history of the world, as have these lowly creatures"
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