EARTHWORM (Lumbricus terrestris)


DISSECTION

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Note: Students are expected to utilize this site as they dissect the earthworm.
BACKGROUND
EXTERNAL ANATOMYThis site will help greatly in your analysis of the external anatomy of the earthworm. Use it to help you identify parts of the worm.
1. Put on safety goggles, gloves, and a lab apron.2. Place earthworm in the dissecting tray and rinse off the excess preservative. Identify the dorsal side, which is the darker surface, and the ventral side, which is the lightened flattened bottom. Turn the worm ventral side up, as shown below.earthw1.gif
3. Use a hand lens as you observe all parts of the worm, externally and internally. Locate the clitellum, a saddle-like swelling on the dorsal surface. The clitellum produces a mucus sheath used to surround the worms during mating and is responsible for making the cocoon within which fertilized eggs are deposited.
The anterior of the animal is more cylindrical than the flattened posterior and is the closest to the clitellum. The ventral surface of the earthworm is usually a lighter colour than the dorsal surface. The mouth is located on the ventral surface of the first segment while the anus is found at the end of the last segment. Find the anterior end by locating the prostomium (lip), which is a fleshy lobe that extends over the mouth. The other end of the worm’s body is the posterior end, where the anus is located.

*Label the following anatomical sections on your worm diagram on your lab handout: clitellum, anterior, posterior, dorsal, ventral, and mouth.

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4. Run your finger over the ventral surface of the worm's body. Note the feel of the setae on the ventral surface of the worm. There are four pairs of setae arising from each segment except the first and the last.
*Lab question

INTERNAL ANATOMY


Use this site to help you identify internal structures on your worm.

The remainder of today’s lab will focus on the internal structures and organs of the earthworm. You will need to procure a dissecting pan, kit and pins. Place a pin through both the posterior and anterior ends of the worm which must be the dorsal side up in the wax-bottomed dissecting pan. Be sure to only pierce the prostomium of the worm at the anterior end. The prostomium is the extension of the first segment at the anterior tip. Any penetration further down will obscure internal organs.

Make a shallow initial incision with scissors about 1" behind the clitellum through just the body wall as seen below. Do not cut deep or the organs will be cut. Now continue cutting in the dorsal line forward, pinning out the flaps after severing the septa that join the intestine and the body wall. Notice the large body cavity (coelom) surrounding the organs. Use the diagram below to aid you in your dissection.

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1. Use the diagram below to locate and identify pairs of aortic arches, or hearts. Then find the dorsal blood vessel. Look for smaller blood vessels that branch from the dorsal blood vessel. The contractile dorsal blood vessel, in which the blood flows toward the anterior end, should be traced forward to segments 7 - 11 where it joins pairs of aortic arches or "hearts" (one pair in each segment). The aortic arches are contractile. They force blood under steady pressure into the ventral blood vessel. It distributes blood to most of the organs of the body.
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*Lab Question

2. Digestive System
The earthworm is an example of a foraging herbivorous annelid, obtaining food by eating its way through the soil and extracting nutrients from the soil as it passes through the digestive tract.

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3. The digestive system of the earthworm is the system you should be able to describe the best of all. This is one of the first creatures to exhibit a digestive system in which food takes a one-way path from beginning to end.
*LAB QUESTION

4. The gut is a tube running from the mouth to the anus. It is regionally differentiated, beginning at the anterior end in a buccal cavity (segments 1-3), followed by a pharynx (4-5).

*Note the striations of muscle fibers in the pharynx. You may have to pull the skin back further to see detail here. What do these striations suggest to you about what the pharynx can do?
*LAB QUESTION

5. The wall of the esophagus bears some small glands. The esophagus joins a thin-walled storage organ, the crop (15-16). Describe the feeling of the crop when probed. How does this relate to its function?
*LAB QUESTION

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6.Behind the crop is the grinding organ, called the gizzard (17-18). Poke this organ with your probe. What does it feel like? How does the feel of this organ relate to its function?
*LAB QUESTION

7.The intestine runs from the gizzard to the anus. Note that the intestine is constricted by each septum. These constrictions, together with a fold inside, increase the surface for secretion and absorption. Measure the length of the intestine in centimeters and record it here. Then measure the length of the entire earthworm. What is the percentage of total length of the worm that is taken up by the intestine? Why is this??
*LAB QUESTION

8. REPRODUCTIVE SYSTEM
The reproductive system consists of conspicuous and inconspicuous parts. Remember the earthworm is hermaphroditic.
Male System =>

The three pairs of large, whitish organs (9-12) are seminal vesicles in which the sperm ripen. By carefully removing the gut and circulatory system up to segment 5, the full extent of these organs may be seen. Testes are small and buried in the bottom of the vesicles. They will not be visible.

Female System =>

Two pairs of globular organs (9-10), the seminal receptacles store sperm received from another worm at the time of copulation. Tiny, conical ovaries hang, near the midline, from the anterior septum of segment 13 and the egg funnel and egg sac are in the posterior septum of the same segment. All specimens may not show these organs well. An oviduct leads from each egg sac to the openings seen on segment 14.

9. NERVOUS SYSTEM

To find organs of the nervous system, push aside the digestive and circulatory system organs. Use the diagram below to locate the ventral nerve cord. Trace the nerve cord forward to the nerve collar, which circles the pharynx. Find one pair of ganglia under the pharynx and another pair of ganglia above the pharynx. The ganglia above the pharynx serve as the brain of the earthworm.

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*LAB QUESTION