Biology 521 - Earthworm Dissection Kingdom Phylum Class Order Genus Species Animalia Annelida Oligochaeta Haplotaxida Lumbricus L. terrestris PRELAB: The earthworm is an excellent organism to study as an introduction to dissection because of its highly organized body. Its body segments (each ring is called a somite) are made of rings which can be used to locate specific organs (see the attached map in Figure 1). The earthworm has a tube within a tube body plan. The outer tube is the body wall and the inner tube is the digestive tract. It lives in moist, nutrient rich soil all around the world provided the soil is not too dry or sandy. They are primarily a nocturnal organism and will come out of their burrows at night. Earthworms exhibit well developed digestive, circulatory, excretory, reproductive, and nervous systems. Before coming to the lab, the following pages should be read: p. 238-239, p. 222, p. 282-283, p. 158, p.175-176. OBJECTIVES: In this lab, the student will 1. study the internal and external anatomy of the earthworm 2. take care to see and determine the function of each suggested structure 3. be prepared to identify structures and functions of earthworm anatomy. MATERIALS: preserved specimen, dissecting pan, scissors, probe, teasing needle, dissecting scope, colored pencils PROCEDURE: 1. External Anatomy 1. Examine the external features. The thick band near the anterior (segments 32-37) of the animal is called the clitellum. It separates the anterior end (about 1/3 the body length) from the posterior end. The clitellum secretes a mucous cocoon around fertilized eggs. The prostomium is the front tip of the worm which hangs over the earthworm s mouth. Notice the presence of tiny bristles called setae which are found on the ventral surface of the earthworm. Another difference between the dorsal and ventral surface is the fact that the ventral surface is lighter in color. 2. Besides the mouth and anus, there are other openings in the earthworm s body. Eggs are released on the ventral surface of segment 14 through two small pores which will require a dissecting scope to view. Sperm is released through two larger pores on the ventral surface of segment 15. These pores are more easily seen because of the swollen lips around each pore. Other pores are present but are difficult to observe. They will be discussed later with the systems to which they belong. Internal Anatomy Hold the earthworm ventral side down in your non-cutting hand between your thumb and index finger. With a fine pair of scissors, cut a small lateral (across its body) incision through the skin of the earthworm just anterior to the clitellum. Insert one blade of the scissors into the incision and extend the cut one quarter of the way around the tubular body. Do the same going the other direction. Insert one blade of the scissors to one side of the center of the dorsal surface and cut all the way to the mouth. Be sure to always pull up on the scissors as you cut so that you do not damage any of the internal organs. Using a teasing needle, gently tear the membranes that attach the organs to the body wall and pin the body wall to the tray using the 6-10 T pins provided. Insert the pins at a sharp angle to the dissecting pan so that they do not obstruct your view of the specimen.
1. Digestive System: a) Follow the organs of the digestive tract from mouth to anus. Use the map in Figure 1 and the diagrams from your text to identify the mouth, pharynx, esophagus, crop, gizzard, intestine, and anus. b) Cut out a small section of the intestine (about 1mm in width). Clean the dirt out of it with a few drops of water and place it on a petri dish. Examine the cross section to observe the intestinal lumen (the cavity food goes through), intestinal wall, and the typhlosole. Make a proper scientific drawing of the earthworm intestinal cross section in the space provided on page 3. c) In Figure 2, label the parts of the digestive tract and color them yellow. d) Complete Table 1 on the digestive system of the earthworm. 2. Circulatory System: Examine the closed circulatory system of the earthworm. Blood is pumped through vessels by five pairs of aortic arches (similar to hearts) which surround the esophagus. Blood flows from the arches into the ventral vessel which runs underneath the digestive tract. The vessel branches to form smaller vessels, then capillaries that serve all cells of the body. The capillaries join to former larger vessels which empty blood into the dorsal vessel which runs along the top of the digestive system and returns blood to the aortic arches. The dorsal vessel will contract slightly to move blood back to the aortic arches. a) Identify the parts of the circulatory system in your specimen. Label these parts in Figure 2 and color them red. 3. Excretory System: The excretory organs in the earthworms are tiny, white tubes called nephridia. They are found in pairs in all segments except the first three and the last one. Use a dissecting scope to identify a nephridium. Each nephridium empties through a pore in the body wall called the nephridiopore. a) Label the parts of the excretory system in Figure 2 and color them green. 4. Reproductive System: Examine the reproductive structures of the earthworm. The earthworm is hermaphroditic. It has both male and female reproductive structures however, they rarely fertilize their own eggs. Earthworms reproduce by exchanging and storing the sperm from another individual. The sperm is produced in 2 pairs of testes found in segments 10 and 11. However, the testes are quite small and will be difficult to observe. The immature sperm are stored in the seminal vesicles found in segments 9-14. They are the 3 pairs of lobed structures which wrap around the esophagus. The sperm matures and is stored in the seminal vesicles until copulation occurs. During copulation, two earthworms lie side by side and exchange sperm. This sperm from the other individual is stored in two pairs of seminal receptacles found in segments 9 and 10. The openings to these structures are found between segments 9 and 10 and between segments 10 and 11. These are quite small and will be hard to observe. It is this sperm which will fertilize the eggs. One pair of ovaries are found in segment 13 which produce eggs that are discharged through 2 pores in segment 14. The ovaries are also very difficult to see. a) Identify the structures of the reproductive system in your specimen. Label these structures in Figure 2 and color them blue. 5. Nervous Systems: The central nervous system of the earthworm consists of a brain and a ventral nerve cord. The brain actually consists of 2 ganglia fused together and is found at the anterior end of the pharynx on the dorsal side. Two nerve cords branch off the brain, encircle the pharynx and join to form the ventral nerve cord which runs along the body wall below the digestive tract. The ventral nerve cord slightly widens within each segment. Each widened segment is called a ganglion and has lateral nerves extending from it into the somite. These ganglia and lateral nerves make up the peripheral nervous system. a) Identify the parts of the nervous systems in your specimen. Label these structures in Figure 2 and color them black.
Questions: 1. Why is it important for the skin of the earthworm to remain moist at all times? /1 2. Why do earthworms come out of their burrows after a heavy rain? /1 3. Describe the location and position of the setae. How do they aid in locomotion? /2 4. Complete Table 1 on the digestive system. /6 5. Describe the function and direction of blood flow of the following structures: aortic arches, ventral vessel, dorsal vessel. How are the dorsal and ventral blood vessels connected elsewhere besides the aortic arches? /4 6. Why is an earthworm s blood red like a human s? Explain completely. /2 7. Why is hermaphroditism an advantage for this species? /2 8. Describe the major parts of a nephridium and how it works. /3 9. Describe the structures and functions of the earthworm s two nervous systems. /3 10. Why is the earthworm an important component of the ecosystem? Cross Section of the Earthworm s Intestine
Segm ent Digestive System Circulatory System Excretory System Segm ent Reproductive System Nervous System 1 Mouth Dorsal & Ventral Vessels 1 2 Pharynx Dorsal & Ventral Vessels 2 3 Pharynx Dorsal & Ventral Vessels 3 Brain 4 Pharynx Dorsal & Ventral Vessels Nephridia 4 5 Pharynx Dorsal & Ventral Vessels Nephridia 5 Ventral Nerve Cord 6 Pharynx Dorsal & Ventral Vessels Nephridia 6 Ventral Nerve Cord 7 Esophagus Aortic Arches Dorsal & Ventr al Vessels Nephr idia 7 Ventral Nerve Cord 8 Esophagus Aortic Arches Dorsal & Ventr al Vessels Nephr idia 8 Ventral Nerve Cord 9 Esophagus Aortic Arches Dorsal & Ventral Vessels Nephridia 9 Seminal Receptacle Seminal Vesicle Ventral Nerve Cord 10 Esophagus Aortic Arches Dorsal & Ventral Vessels Nephridia 10 Testes Seminal Receptacle Seminal Vesicle Ventral Nerve Cord 11 Esophagus Aortic Arches Dorsal & Ventral Vessels Nephridia 11 Testes Seminal Vesicle Ventral Nerve Cord 12 Esophagus Dorsal & Ventral Vessels Nephridia 12 Seminal Vesicle Ventral Nerve Cord 13 Esophagus Dorsal & Ventral Vessels Nephridia 13 Ovaries Seminal Vesicle Ventral Nerve Cord 14 Esophagus Dorsal & Ventral Vessels Nephridia 14 Egg Pores Seminal Vesicle Ventral Nerve Cord 15 Crop Dorsal & Ventral Vessels Nephridia 15 Sperm Ventral Nerve Cord 16 Crop Dorsal & Ventral Vessels Nephridia 16 Ventral Nerve Cord 17 Crop Dorsal & Ventral Vessels Nephridia 17 18 Gizzard Dorsal & Ventral Vessels Nephridia 18 Ventral Nerve Cord 19 Gizzard Dorsal & Ventral Vessels Nephridia 19 Ventral Nerve Cord 20 Gizzard Dorsal & Ventral Vessels Nephridia 20 Ventral Nerve Cord 21 Intestine Dorsal & Ventral Vessels Nephridia 21 Ventral Nerve Cord 22 Intestine Dorsal & Ventral Vessels Nephridia 22 Ventral Nerve Cord 23 Intestine Dorsal & Ventral Vessels Nephridia 23 Ventral Nerve Cord 24 Intestine Dorsal & Ventral Vessels Nephridia 24 Ventral Nerve Cord 25 Intestine Dorsal & Ventral Vessels Nephridia 25 Ventral Nerve Cord 26 Intestine Dorsal & Ventral Vessels Nephridia 26 Ventral Nerve Cord 27 Intestine Dorsal & Ventral Vessels Nephridia 27 Ventral Nerve Cord 28 Intestine Dorsal & Ventral Vessels Nephridia 28 Ventral Nerve Cord 29 Intestine Dorsal & Ventral Vessels Nephridia 29 Ventral Nerve Cord 30 Intestine Dorsal & Ventral Vessels Nephridia 30 Ventral Nerve Cord 31 Intestine Dorsal & Ventral Vessels Nephridia 31 Ventral Nerve Cord 32 Intestine Dorsal & Ventral Vessels Nephridia 32 Clitelum Ventral Nerve Cord 33 Intestine Dorsal & Ventral Vessels Nephridia 33 Clitelum Ventral Nerve Cord 34 Intestine Dorsal & Ventral Vessels Nephridia 34 Clitelum Ventral Nerve Cord 35 Intestine Dorsal & Ventral Vessels Nephridia 35 Clitelum Ventral Nerve Cord 36 Intestine Dorsal & Ventral Vessels Nephridia 36 Clitelum Ventral Nerve Cord 37 Intestine Dorsal & Ventral Vessels Nephridia 37 Clitelum Ventral Nerve Cord 38 Intestine Dorsal & Ventral Vessels Nephridia 38 Ventral Nerve Cord 39 Intestine Dorsal & Ventral Vessels Nephridia 39 Ventral Nerve Cord 109 Intestine Dorsal & Ventral Vessels Nephridia 109 Ventral Nerve Cord 110 Intestine Dorsal & Ventral Vessels Nephridia 110 Ventral Nerve Cord Last Anus Dorsal & Ventral Vessels Last Figure 1. Segment map of organs for Lumbricus terrestris.
Figure 2. Labelled and colored systems of Lumbricus terrestris. This diagram has been adapted from Investigating Living Systems Kaskill et. al., 1994.
Table 1. Digestive system of Lumbricus terrestris. structure type of digestion intracellular/extracellular mechanical/chemical description of structure and its function