How to Dissect a Worm
Typically, worm dissection in a biology class involves earthworms, one of a group of animals called annelids or segmented worms. Well-defined segments divide annelids' bodies both externally and internally. Each segment is similar, except for the anterior (head) and posterior (tail.) To dissect a worm, you must first distinguish between the anterior and the thinner posterior. Now you're ready to dissect.
Instructions
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Make the Initial Cuts
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Position the earthworm, dorsal side up, onto your dissecting tray. Pin down the anterior and posterior ends at the last segments. Lift the dorsal skin with forceps. At the forceps' position, insert scissors and cut an off-center line through to the anus.
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2
Hold the body with your forceps. Use a scalpel to cut through the septa, the thin wall separating each segment, on each intestinal side. Cut up to about one inch from the ****ellum or the smooth, thick collar about two-thirds up from the anus.
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3
Pin the entire body wall you've cut through to the dissecting tray with six to eight pins. Cut through the ****ellum just on the anterior side with a scissors. Cut through the septa, up to the head and pin the remaining body wall to your tray.
Find the Worm's Systems
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4
Locate all parts of the digestive system. The mouth's opening, at the anterior end, sits between the prostomium, a liplike structure. Follow the mouth to the pharynx, an expanded muscle, typically covered by three pairs of white vesicles. The worm's pharynx moves food through the esophagus to the crop, a larger round area, which temporarily stores food.
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5
Find the circulatory system's major vessels, which lie on top of the digestive tract. The ventral blood vessel lies below the digestive tract and connects with the dorsal vessels with many minor vessels that run around the digestive tract.
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6
Slice through the intestine with your scalpel near the ****ellum for a clear view into the reproductive system. With your scalpel, lift the intestinal wall toward the pharynx and cut it out. Note that the worm releases sperm from the seminal vesicles, which lie beside the crop. The worm's testes rest inside the seminal vesicles but can't be seen.
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Try to locate the worm's nervous system. Although hard to find, the ventral nerve cord runs along the worm's inner ventral surface. At the anterior end, this nerve cord divides around the pharynx front and then enlarges into the cerebral ganglia, two swellings considered primitive brains.
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8
Search for the worm's excretory system by looking in the fifth segment until the last. These segments contain excretory structures called metanephridia, which are coiled tubular structures. They lie beside the body wall and open to an exterior pore, called the nephridiopore.
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Tips & Warnings
When cutting the dorsal skin, be careful to only cut as deep as the skin so as not to cut the internal organs.
The worm has a closed circulatory system, meaning the blood circulates inside blood vessels.
Earthworms are monoecious, meaning they contain complete sets of male and female reproductive organs. But they do undergo cross fertilization during copulation.
