Eathworms Essay

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Earthworms (also known as angleworms) are any one of the over 1,800 species of terrestrial worms who are members of the phylum Annelida, class Oligochaeta. There are 17 native species and 13 introduced European species of earthworm in the United States. The most common earthworm in the U.S., L. terrestris, grows to about 25 centimetresa, although some species grow as long as 3.3 meters. Earthworms can be found almost everywhere where the moisture and makeup of the soil are appropriate to sustain them. Only a handful of the earthworm species are in the genus Lumbricus. There are 32 described native earthworms in the Pacific states alone. There are over 20 introduced lumbricids, plus many introduced megascolecids. This is an English language Wikipedia, not a US one, so US-only info should be used sparingly. Digestion and excretion The earthworm has a simple digestive system compared to other creatures. It possesses an esophagus, which serves the same purpose as it does in higher animals: to carry food. However, the worm esophagus leads to the crop and gizzard rather than to a stomach (which earthworms do not possess). The crop and gizzard combined serve the purpose of a human stomach. The crop holds food while the gizzard grinds it down mechanically. The gizzard leads into the intestine, which follows the length of the earthworm’s body to the anus. This passage from mouth -> esophagus -> crop -> gizzard -> anus constitutes the entire digestive system. The lumbricids have this pattern but it is not the only one. Most earthworms have the gizzard in the anteriormost segments, and many have no gizzard at all. The primary function of the esophageal region in most earthworms (including the lumbricids) is calcium carbonate secretion. The usual patterns are below. Mouth>pharynx>gizzard>esophagus>intestine Mouth>pharynx>esophagus>crop>gizzard>intestine
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