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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: ______________________
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Describe the setae and how they make the worm well adapted to its habitat:
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What body characteristics do you observe that help classify this animal as an annelid?
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Describe the appearance and the purpose of the clitellum.
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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.
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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?
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What is the function of the pharynx in earthworms? /2
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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
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Pumping
ring vessels
“hearts”
2. Is this an example of a closed or open
circulatory system? How do you know? /2
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3. Describe the pathway that blood takes
through the parts of the earthworm’s
circulatory system /2
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4. What are nephridia? /1
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5. Why are earthworms described as hermaphrodites? /1
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6. Explain how an earthworm demonstrates cephalization. /1
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Conclusion:
1. Why does segmentation represent an important development in the animal phyla? /1
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2. After studying the internal structure of the earthworm, why do you think that a
circulatory system is needed? /2
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3. Why do you think earthworms usually come out to eat at night or on rainy days?
Explain. (hint: what function requires moisture…) /2
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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
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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
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