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Earthworm Dissection Lab Background: Among the most familiar invertebrate animals are the earthworms, members of the phylum Annelida. The word annelida means "ringed" and refers to a series of rings or segments that make up the bodies of the members of this phylum. Internally, septa, or dividing walls, are located between the segments. External segments are called metameres. There may be more than 100 segments in an adult worm. The clitellum is a swelling of the body found in sexually mature worms and is active in the formation of an egg capsule, or cocoon. Eggs are produced in the ovaries and pass out of the body through female genital pores. Sperm are produced in the testes and pass out through tiny male genital pores. During mating, sperm from one worm travel along the sperm grooves to the seminal receptacles of another worm. Fertilization of the eggs takes place outside the body as the cocoon moves forward over the body, picking up the eggs of one worm and the sperm of its mate. The pumping organs of the circulatory system are five aortic arches. Circulatory fluids travel from the arches through the ventral blood vessel to capillary beds in the body. The fluids then collect in the dorsal blood vessel and reenter the aortic arches. The earthworm takes in a mixture of soil and organic matter through its mouth, which is the beginning of the digestive tract. The mixture enters the pharynx, which is located in segments 1–6. The esophagus, in segments 6–13, acts as a passageway between the pharynx and the crop. The crop stores food temporarily. The mixture that the earthworm ingests is ground up in the gizzard. In the intestine, which extends over two-thirds of the body length, digestion and absorption take place. Soil particles and undigested organic matter pass out of the worm through the rectum and anus. The nervous system consists of the ventral nerve cord, which travels the length of the worm on the ventral side, and a series of ganglia, which are masses of tissue containing many nerve cells. The nerve collar surrounds the pharynx and consists of ganglia above and below the pharynx. Nervous impulses are responsible for movement and responses to stimuli. Each segment contains an enlargement, or ganglion, along the ventral nerve cord. Excretory functions are carried on by nephridia, which are found in pairs in each body segment. They appear as tiny white fibers on the dorsal body wall. The earthworm has no gills or lungs. Gases are exchanged between the circulatory system and the environment through the moist skin. Earthworm Dissection Earthworms are important helpers in the garden or field! Their tunneling mixes up the soil and brings rich soil to the surface. You can observe the organs of these tiny creatures by dissecting a preserved earthworm. Observation: External Anatomy 1. Find the anterior (front) end of the earthworm by locating the fleshy bump over its mouth, called the prostomium. The posterior (back) end has a small hole where solid waste is expelled, called the anus. The length of the worm is made up of many tiny segments, each separated by a thin wall called a septum. 2. About one-third of the way back from the mouth you should see a thicker and smoother section of the worm. This is called the clitellum, and it is involved in reproduction. 3. Notice that the earthworm has a rounded dorsal (back) surface and a flatter ventral (belly) surface. Usually the dorsal surface is darker than the ventral surface (though sometimes this is obscured in the preservation process). Lightly rub your finger along the ventral side toward the posterior end of the worm. You should feel a roughness caused by tiny bristles called setae. Using a magnifying glass, try to see the setae. 4. With your magnifying glass look for tiny pores on each segment. Liquid wastes are expelled through these pores. Near the front end of the worm you should see some larger pores that can be easily seen without magnification. These are genital pores and are important in reproduction. Dissection: Internal Anatomy 1. Lay the worm on your dissecting tray with its dorsal side facing up. Use dissection pins to secure each end on the tray. Start your dissection about an inch posterior to the clitellum. Lift up the skin with a pair of forceps and snip an opening with a pair of dissecting scissors. Insert the scissors into the opening and cut in a straight line all the way up through the mouth. Go slowly and be sure to cut just the skin--if you go too deep you may damage the internal organs. 2. Using the forceps and dissection pins, carefully pull apart the two flaps of skin and pin them flat on the tray. (You may need to drag a pin along the inside of the skin to sever the septum walls to make it easier to spread the skin.) 3. Look at the labeled picture to help you find the following features: Pharynx: This is the light-colored organ just inside the mouth. Its muscular contractions pass food on down to the esophagus. Hearts (or "aortic arches"): Behind the pharynx are five dark loops wrapped around the esophagus. Click for full-size pdf These are the blood vessels that serve as the hearts of the worm. Dorsal blood vessel: This is a dark line extending from the hearts over the top of the crop. Crop: Food from the esophagus is temporarily stored in the crop. Gizzard: Food comes from the crop into the gizzard, where it is ground up. Intestine: The intestine is the long tube extending from the gizzard all the way to the anus. Food is digested and absorbed here. Reproductive organs: The light colored tissue above and around the hearts are seminal vesicles. Other reproductive parts appear as small white organs on the ventral side of the hearts. Ventral Nerve Cord: With your forceps, gently push aside the intestine to view the long white nerve cord running along the length of the worm beneath it. 4. Optional: Finish cutting the rest of the worm open from the first incision through to the anus. Observe how the intestine and ventral nerve cord both continue through the entire length of the worm. Name(s)_______________________________ Date__________ Period_______ Earthworm Lab sheet 1. Which part of the worm uses muscular contractions to pass food from the mouth to the esophagus? 2. How many hearts are there in a worm? 3. Where is food stored temporarily? 4. What happens in the gizzard? 5. Where does the intestine start and end? 6. Where does digestion and absorption take place? 7. What happens to the undigested soil particles? 8. Your dissection of the earthworm did not go beyond segment 32.What will you observe if you dissect the remainder of the worm to its posterior end? 9. Label the parts of the earthworm you observed. 10. What are the similarities and differences between the human digestive system and the worm’s digestive system. (Answer should be a paragraph of at least 5-6 sentences or a chart) Rubric for Worm Dissection Categories Knowledge / Understanding -use appropriate terminology related to digestion Thinking and Investigation -conduct inquiries involving locating and describing the interrelationship between digestive organs in a worm Communication -expression and organization of ideas and information (e.g., clear expression, logical organization) Level 1 (50 - 59%) Level 2 (60 - 69%) Level 3 (70 - 79%) Level 4 (80 - 100%) -demonstrates limited understanding of terminology -demonstrates some understanding of terminology -demonstrates considerable understanding of terminology -demonstrates thorough understanding of terminology -conducts inquiries with limited effectiveness -conducts inquiries with some effectiveness -conducts inquiries with considerable effectiveness -conducts inquiries with a high degree of effectiveness -expresses and organizes ideas and information with limited effectiveness -expresses and organizes ideas and information with some effectiveness -expresses and organizes ideas and information with considerable effectiveness -expresses and organizes ideas and information with a high degree of effectiveness Demonstrates poor safety management skills (i.e. rarely wear s safety goggles, does not keep work area organized ...) Demonstrates fair safety management skills (i.e. occasionally wears safety goggles, does not keep work area organized . . . ) Demonstrates good safety management skills (i.e. always wears safety goggles, work area is usually neat and organized . . . ) Demonstrates excellent safety management skills (i.e. always wears safety goggles, work area is always neat and organized . . . ) Demonstrates poor laboratory skills Demonstrates fair laboratory skills Demonstrates good laboratory skills Demonstrates excellent laboratory skills Application Safety Laboratory Skills