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Earthworms

Earthworm is the common name for the larger members of the Oligochaeta in the phylum Annelida. Folk names for earthworm include "dew-worm", "night crawler" and "angleworm". 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. One often sees earthworms come to the surface in large numbers after a rainstorm. There are two 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. However, earthworms can survive underwater for several weeks if there is oxygen in it, so this theory is rejected by some. Secondly, 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. Since the relative humidity is higher during and after rain, they do not become dehydrated. This is a dangerous activity in the daytime, since earthworms die quickly when exposed to direct sunlight with its strong UV content, and are more vulnerable to predators such as birds. Earthworms are hermaphrodites (both female and male organs within the same individual) but generally 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 sexual structures, which develop later. Some earthworm species are mostly parthenogenetic, in which case the male structures and spermathecae may become abnormal, or missing.

Earthworms
Earthworms








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