Earthworms

When an earthworm is dissected, the intestine can be seen in the cross-section anterior to the gizzard. An earthworm’s digestive system includes the gizzard, intestine, crop, esophagus, and pharynx. When an earthworm eats, organic matter along with soil enters its mouth and goes through the pharynx. Next it passes through the esophagus, and is then stored in the crop before moving into the gizzard. In the gizzard, the soil eaten by the earthworm grinds its food completely. Finally, the food travels into the intestines. The intestines release solutions to help the digestive process. The blood vessels, inside the wall of the intestine, absorb the food digested and send nutrients to the rest of the body.
Aortic arches are in 5 sets of 2 and work similar to a human heart by pumping blood. These arches pump the blood through the ventral and dorsal blood vessels. The circulatory system of the earthworm is a closed circulatory system where blood circulates through vessels. The aortic arches, dorsal blood vessels, and ventral blood vessels are the three main vessels supplying blood to organs. The front of the body receives blood from the dorsal blood vessels and blood is sent to the back of the body by the ventral blood vessels.
During mating, earthworms interchange sperm. All earthworms have a pair of male openings and a pair of sperm receptacles. Once a mate is found, they exchange sperm alongside each other with their heads facing towards opposite directions. They have sperm receptacles that take in the sperm during mating. Earthworms not only have sperm, but they also have two ovaries that produce eggs. Therefore they are hermaphroditic and have both male and female organs. Most earthworms reproduce with a mate, while few can without a mate. During mating, the clitellum forms a slime tube around the two earthworms. As they move forward, eggs are picked up as the slime tube passes over the female pore and then passes over the male pore which has stored sperm in it. This fertilizes the eggs. The eggs will be stored in a “egg cocoon” and will rest in the soil until they develop into young worms.
Blood is enclosed within vessels at all times in a closed circulatory system. The vessels are of different sizes and have varying thickness of their walls. In this type of system, In the closed system, blood is driven through vessels by the heart, and the body cavity is not filled with blood. Arthropods have an open circulatory system. This type of system a pump blood into a hemocoel and diffuses back to the circulatory system among cells. The body cavity receives the blood from the heart and surrounds the tissues in it.
Earthworms have a front and a back end but they do not have a well-defined head. The front end is the prostomium and is used to push their way into the ground since it is pointed. Once inside the soil, the earthworm gathers nutrients with their mouth which is behind the prostomium. At the posterior end of the earthworm the anus is situated. This is the departure of food that was not broken down and absorbed by its body. The undigested organic matter and leftover soil particles pass out of the worm through the anus in a process called casting.
The earthworms are in the phylum Annelida or Annelids. Annelida in Latin means, “little rings.” It has a segmented body which resembles little rings fused together. The earthworm’s segmented body part helps it to move. As it moves the muscles and setae, which are like bristles, anchor and control it as it moves through the soil. The setae hold segments of the worm securely into the ground while the rest of the body projects forward. The earthworm can contract or relax its segments independently which causes the body to stretch in one area or contract in others. The segments on an earthworm help its movements to be strong and flexible.

References Mader, S. S. (2009). Essentials of Biology, Second Edition,. The Mcgraw-Hill Companies.
University of Pennsylvannia. (n.d.). Earthworms. Retrieved May 22, 2012, from http://www.sas.upenn.edu/~rlenet/Earthworms.html