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Biology 11 Earthworm Dissection Lab Name: _________________ Date: __________________ Total: /35 Block: ____ Segmented worms belong to the phylum Annelida. The most common of all the segmented worms is the earthworm (Lumbricus terrestris). The earthworm is cylindrical in shape and tapered at both ends. Along the sides there are small bristle like structures called setae to help with forward movement through the soil. Its body is divided into many sections which enables each body segment to perform separate, specialized functions. For example, the area known as the clitellum is specialized to secrete a mucus ring into which sperm and egg are deposited during mating. Unlike other less complex organisms, the earthworm also has a space between the internal body organs and body wall called a coelom. The coelom creates space for several well developed body systems. Purpose: In this investigation you will study the external features and internal structures of the earthworm Materials: earthworm, scalpel, dissecting tray and pens, probe, lens, eye dropper Procedure and Pre-Lab Questions: 1. Turn the earthworm so that the darker & more rounded dorsal side is upwards. Observe your specimen. Make a labelled drawing in pencil below with the following terms: anterior, posterior, dorsal, ventral, mouth, clitellum, setae Phylum: ______________ Class: _________________ Scientific Name: ______________________ /5 Describe the setae and how they make the worm well adapted to its habitat: ___________________________________________________________________ ___________________________________________________________________ /2 What body characteristics do you observe that help classify this animal as an annelid? /1 _____________________________________________________________________________ Describe the appearance and the purpose of the clitellum. ______________________________________________________________________________ /2 Dissection Procedure- INTERNAL ANATOMY 2. Pin the ends of the worm to the tray. CAREFULLY make an incision from the mouth at the anterior end to about 5cm past the clitellum. Make your incision slightly to one side of the dorsal blood vessel (a dark line on the dorsal side) and make a small cut down each side where you began the incision. Pin the sides open. 3. Use a medicine dropper to wet the internal organs with water so they do not dry out. 4. Begin at the anterior end below the mouth-locate the brain. It can be recognized as two tiny white lobes found below the mouth in the third segment and in front of the pharynx (which is rounded & bulbous). 5. Now study the digestive structures. Identify the mouth, pharynx, esophagus, crop, gizzard, intestine and anus. Label the structures in bold on the diagram provided. /3 Gently press on the crop and then the gizzard. Can you feel any differences in muscularity? Consider their functions. Which structure you would expect to feel more muscular and why? /2 __________________________________________________ __________________________________________________ __________________________________________________ What is the function of the pharynx in earthworms? /2 __________________________________________________ __________________________________________________ 6. Next, find the dorsal blood vessel- a dark brown tube on top of the digestive tract. Examine the pumping ring vessels which form five dark brown, arch-like hearts over top of the esophagus. 7. Locate the ventral blood vessel by gently turning over the intestine- it is the brownish tube on the bottom of the intestine. 8. Look for the seminal vesicles- three pairs of large white structures on either side of the esophagus. These structures store sperm made by the testes until mating. Try and find the tiny seminal receptacles- two dot like structures anterior to the seminal vesicles These tiny sacs receive sperm exchanged during mating. The ovaries are beneath theses sacs but are very tiny and difficult to find! 9. With your probe, gently push the intestine to one side to expose the ventral nerve cord on the floor of the body cavity. It looks like a white thread and runs parallel to the ventral blood vessel. 10. Use a hand lens and try to identify the coiled nephridia along the body wall of the intestinal area. Discussion Questions Use the diagram below to help you answer the following questions. 1. Compare the sizes of the different digestive organs. Which organ is the largest? How does its size make it well suited for its function? /2 ___________________________________ ___________________________________ Pumping ring vessels “hearts” 2. Is this an example of a closed or open circulatory system? How do you know? /2 ___________________________________ ___________________________________ __________________________________ 3. Describe the pathway that blood takes through the parts of the earthworm’s circulatory system /2 ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ 4. What are nephridia? /1 ___________________________________ 5. Why are earthworms described as hermaphrodites? /1 ___________________________________________________________________ ___________________________________________________________________ 6. Explain how an earthworm demonstrates cephalization. /1 ___________________________________________________________________ ___________________________________________________________________ Conclusion: 1. Why does segmentation represent an important development in the animal phyla? /1 ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ 2. After studying the internal structure of the earthworm, why do you think that a circulatory system is needed? /2 ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ 3. Why do you think earthworms usually come out to eat at night or on rainy days? Explain. (hint: what function requires moisture…) /2 ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ 4. How do you think the removal of all earthworms in an area would affect a piece of agricultural land? Explain why you think this. /2 ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ 5. Do you think annelids such as earthworms can be considered to be more developed than roundworms eg Ascaris)? Consider how each group accomplishes their life functions and give two pieces of evidence to support your answer. /2 ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________