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Earthworm Anatomy 1. Segmented BodyEarthworms are classified in the phylum Annelida or Annelids. Annelidain Latin means, “little rings.” The body of the earthworm is segmentedwhich looks like many little rings joined or fused together. Theearthworm is made of about 100-150 segments. The segmented body partsprovide important structural functions. Segmentation can help theearthworm move. Each segment or section has muscles and bristles calledsetae. The bristles or setae help anchor and control the worm whenmoving through soil. The bristles hold a section of the worm firmlyinto the ground while the other part of the body protrudes forward. Theearthworm uses segments to either contract or relax independently tocause the body to lengthen in one area or contract in other areas.Segmentation helps the worm to be flexible and strong in its movement.If each segment moved together without being independent, the earthwormwould be stationary.
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2.Digestive System The digestive system is partitioned into many regions, eachwith a certain function. The digestive system consists of the pharynx,the esophagus, the crop, the intestine and the gizzard. Food such assoil enters the earthworm’s mouth where it is swallowed by the pharynx.Then the soil passes through the esophagus, which has calciferousglands that release calcium carbonate to rid the earthworm’s body ofexcess calcium. After it passes through the esophagus, the food movesinto the crop where it is stored and then eventually moves into thegizzard. The gizzard uses stones that the earthworm eats to grind thefood completely. The food moves into the intestines as gland cells inthe intestine release fluids to aid in the digestive process. Theintestinal wall contains blood vessels where the digested food isabsorbed and transported to the rest of the body.
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3.Circulatory System Another important organ system is the circulatory system.The earthworm has a closed circulatory system. An earthworm circulatesblood exclusively through vessels. There are three main vessels thatsupply the blood to organs within the earthworm. These vessels are theaortic arches, dorsal blood vessels, and ventral blood vessels. Theaortic arches function like a human heart. There are five pairs ofaortic arches, which have the responsibility of pumping blood into thedorsal and ventral blood vessels. The dorsal blood vessels areresponsible for carrying blood to the front of the earthworm’s body.The ventral blood vessels are responsible for carrying blood to theback of the earthworm’s body.
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4.Respiratory System Earthworms do not have lungs. They breathe through theirskin. Oxygen and carbon dioxide pass through the earthworm’s skin bydiffusion. For diffusion to occur, the earthworm’s skin must be keptmoist.Body fluid and mucous is released to keep its skin moist. Earthwormstherefore, need to be in damp or moist soil. This is one reason whythey usually surface at night when it is possibly cooler and the“evaporating potential of the air is low.”(www.amonline.net.au/factsheets/earthworms.htm) Earthworms havedeveloped the ability to detect light even though they cannot see. Theyhave tissue located at the earthworm’s head that is sensitive to light.These tissues enable an earthworm to detect light and not surfaceduring the daytime where they could be affected by the sun.
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EarthwormReproduction Earthworms are hermaphrodites where each earthworm containsboth male and female sex organs. The male and female sex organs canproduce sperm and egg respectively in each earthworm. Althoughearthworms are hermaphrodites, most need a mate to reproduce. Duringmating, two worms line up inverted from each other so sperm can beexchanged. The earthworms each have two male openings and two spermreceptacles, which take in the sperm from another mate. The earthwormshave a pair of ovaries that produce eggs. The clitellum will form aslime tube around it, which will fill with an albuminous fluid. Theearthworm will move forward out of the slime tube. As the earthwormpasses through the slime tube, the tube will pass over the female porepicking up eggs. The tube will continue to move down the earthworm andpass over the male pore called the spermatheca which has the storedsperm called the spermatozoa. The eggs will fertilize and the slimetube will close off as the worm moves completely out of the tube. Theslime tube will form an “egg cocoon” and be put into the soil.


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Thefertilized eggs will develop and become young worms.
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