Parts of Earthworms
Earthworms are common first-time dissection experiments for classes from sixth grade on, because their anatomy is complex enough to demonstrate different systems, like a digestive system and a circulatory system, yet the organism is simple enough that organs can be easily located and defined. When describing an earthworm's physiology, it is easiest to begin at the anterior end closest to the mouth where most of the major organs are located and work back towards the posterior end where the anus is located.
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Exterior Anatomy
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The most obvious features of an earthworm's exterior physiology are the segments. A full grown earthworm will have 120 to 170 segments. Covering the segments are setae, which are tiny hairs the worm uses for movement. Each segment has four pairs of setae. The mouth is located at the anterior end of the earthworm and is covered by a flap of skin called the prostomium. The most obvious feature of the earthworm's body is the large pale section close to the anterior end. This is called the citellum and is used in the production of cocoons. Also located near the anterior of the worm on the bottom, or ventral, side of the worm are small openings called the sperm ducts. Further back towards the citellum are different small openings known as the genital setae. On the top of the worm, known as the dorsal side, is a large dark line. This is the dorsal blood vessel. A ventral blood vessel can be seen on the bottom side of the worm, but it is not as dark and obvious.
Reproductive System
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After the earthworm is opened, you can begin viewing the internal organs. Beginning from the citellum and working towards the mouth, the first thing you will see are the seminal vesicles. These produce sperm. The female reproductive organs are located just beneath the crop. To reproduce, earthworms rub together at the citellum and form a cocoon into which the male injects sperm.
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Digestive System
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Beneath the seminal vesicles, you will find the digestive system. The digestive system runs the entire length of the worm from the mouth to the anus, but its primary features can be seen next to the citellum. The mouth opens up to the pharynx, which is attached to the esophagus. The esophagus leads to the a bulge called the crop, the gizzard and then into the intestines. The intestines end at the anus.
Circulatory System
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The most prominent feature of the circulatory system is the dorsal blood vessel. The heart is found just above the esophagus immediately after the pharynx. An earthworm has five aortic arches comprising the heart. The ventral blood vessel is located beneath the digestive system.
Other Organs
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Posterior of the aortic arches is the nephridium. This is an organ that functions like a kidney. An earthworm also has a very small brain located just anterior of the the pharynx, near the mouth cavity.
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