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Earthworm Anatomy
The earthworm belongs to a group of animals called Annelids, or segmented
worms. It has special body parts for circulation, digestion, reproduction,
excretion, and it also has a simple nervous system.
The earthworm has five enlarged blood vessels which act as pumps for the
blood. These are called “hearts”.
The earthworm is adapted to a life of burrowing through the soil. Its streamline
shape helps it move through the soil. A coating of mucus secreted by the skin
lubricates the earthworm as it passes through the soil. The mucus coating also
helps oxygen pass through the earthworm’s skin while it is in the air or under the
water. Earthworms can only breathe through their skin. Earthworms must
remain moist in order to breathe; they dehydrate easily. Oxygen from the air
passes through its moist skin and carbon dioxide moves out through the skin.
The earthworm moves through the soil by sucking the soil in its path into its
mouth. As material passes through the tubelike digestive system, sand grains in
the gizzard help grind the food, which is then digested and absorbed in the
intestine. If you cut through the intestine of your earthworm you will see the soil
and decaying matter it has been eating.
It has been estimated that an acre of good soil contains over 50,000
earthworms. By their continuous tunneling and hunting the earthworms help to
turn over the soil and work rich materials into the soil. The earthworm enriches
and aerates the soil and farmers and gardeners consider it a good sign when
they see many earthworms in their soil.
Part A: External Anatomy
1. Rinse a preserved earthworm thoroughly with water to remove excess
preservative.
2. Place the earthworm in a dissecting tray. Turn the earthworm over and
observe the difference between the darker colored dorsal (on the back)
side, and the lighter colored ventral (on the belly) side.
3. Locate the slightly pointed anterior (head) end and the blunt posterior
(tail) end of the earthworm’s body. Use your probe to count the number
of segments in the earthworm. Record the number here. __________
4. Locate the mouth on the anterior end of the earthworm. It will look like
a U. The flap of skin coming down over the u-shaped mouth is called the
prostomium. Gently pull the prostomium back with your dissecting
needle and you will see the mouth opening.
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5. Run your finger lightly along the ventral surface of the earthworm from
anterior to posterior. The bristles that you feel are setae. The setae help
the worm move through the soil.
6. Find the clitellum, the light-brown enlarged band around the body. It
forms a cocoon into which eggs are deposited during reproduction. In
which segments is the clitellum located? _______________________
7. Find the anus. This is the opening from which waste material passes
from the earthworm’s digestive tract.
8. In the diagram below, label the following structures of the earthworm’s
external anatomy: mouth, setae, segments, clitellum, and anus.
Questions:
1. What are the functions of the following organs?
a. Clitellum- _____________________________________
b. Setae- __________________________________________
c. Anus- ___________________________________________
Part B: Internal Anatomy
9. Place the preserved earthworm in the dissecting tray with the dorsal
surface up. Locate the dorsal blood vessel, a dark line that runs along
the midline of the dorsal surface. The vessel runs from the anterior end
to the posterior end.
10. With the tip of the scissors, make a hole in the skin of the worm about 1
inch from the anus. Put the scissor tip in the hole and carefully begin to
snip forward through the skin. Make your cut just to the right of the
dorsal blood vessel. Cut the worm all the way up to the mouth. Keep the
cut as shallow as possible!
11. When the cut is complete, carefully open the body wall. Use your probe
to break the membranes that are holding the skin. Place dissecting pins
every few segments to hold open the body wall. Place the pins at an
angle so you can view the organs that lie between the pins.
Part C: Digestive System
12. Study the digestive system first. The mouth is located in the first three
segments.
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13. Locate a slight swelling, the muscular-walled pharynx posterior to the
mouth in segments 4 and 5. After food enters the mouth, it goes into the
muscular pharynx where it is swallowed.
14. Follow the tube-like esophagus to segment 15. On top of the esophagus
tube are the hearts and reproductive organs so it may be difficult to see.
15. The esophagus carries food to the crop. In segments 15-16 you will find
a thin-walled structure called the crop. The crop temporarily stores food.
16. Locate the gizzard, a grinding organ, behind the crop in segments 17-18.
The gizzard mixes food with sand from the soil and physically breaks the
food into smaller pieces.
17. With the probe, feel the difference in the walls of the crop and gizzard.
Describe here:
______________________________________________________
18. The intestine extends from the crop to the anus. Digestion of food
continues in the intestine and nutrients are absorbed.
19. The digestive system ends in the anus where waste passes out of the
earthworm’s body.
Questions:
1. What is the function of the following organs?
a. pharynx: ______________________________________________
b. crop: _________________________________________________
c. gizzard: _______________________________________________
d. intestine: ______________________________________________
e. anus: _________________________________________________
Part D: Reproductive System
20. Each earthworm has both male and female reproductive organs inside it.
In segments 9, 10, and 11 you will see some very white fan-shaped
structures that fold up over the esophagus and hearts. These are
seminal vesicles. They produce sperm in the worm.
21. The ovaries are way down under the esophagus and seminal vesicles in
the area of segment 12. Carefully push the esophagus over to the right
with your probe to uncover them. They are very small round white
structures. These are the female organs that produce the eggs.
Aortic arches
Dorsal blood vessel
Ventral blood vessel
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Part E: Circulatory System
22. Find the dorsal blood vessel running down the back of your worm. This
blood vessel carries blood to the heart.
23. Gently remove the white seminal vesicles by using your forceps to pull
them out. You should see some aortic arches starting at segment 7-12.
There are five aortic arches, or “hearts”. The aortic arches surround the
esophagus. These arches contract to force blood from one area of the
body to the other. You will probably not find all 5; they look like small
dark curved blood vessels.
24. In the posterior section use a probe to carefully move aside the intestine.
Locate the ventral blood vessel. The ventral blood vessel carries blood
toward the skin and intestine. The aortic arches connect the dorsal blood
vessel with the ventral blood vessel to form a closed circulatory system.
Part F: Nervous System
25. Try to locate the brain, which is a white mass of tissue found in the third
segment anterior and dorsal to the pharynx. The earthworm has a very
small brain, and chances are you cut through it when you opened the
worm.
26. Now find the ventral nerve cord. The ventral nerve cord runs along the
inner ventral surface from segment 3 to the last segment. It is white and
looks like a piece of thread.
Questions:
1. To what phylum do earthworms belong?
____________________________________________________________
2. How does the earthworm get oxygen?
____________________________________________________________
____________________________________________________________
3. What type of circulatory system does the earthworm have? What organs
make up the circulatory system?
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
4. Explain how an earthworm enriches and aerates the soil, thus improving it
for plant growth.
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
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Critical Thinking and Application:
5. Why is it important not to make a deep cut with the scissors when
dissecting an animal?
____________________________________________________________
____________________________________________________________
6. How might an earthworm’s lack of appendages (arms and legs) be an
adaptation for burrowing?
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
7. Describe two ways in which an earthworm’s body is adapted to life in the
soil.
____________________________________________________________
____________________________________________________________
____________________________________________________________
8. Imagine an assembly line for disassembling things instead of putting them
together. Compare an earthworm’s tubelike digestive system to such a
“disassembly line” by describing what each organ does to help with the
disassembling of food. What is the advantage of a tubelike digestive
system?
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
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Organ systems
Color code the organ systems for the earthworm using the following key:
Digestive System – Green






Mouth
Pharynx
Esophagus
Crop
Gizzard
Intestine
Circulatory System – Red



Dorsal blood vessel
Aortic arches
Ventral blood vessel
Reproductive System – Blue

Seminal vesicles
Nervous System – Yellow


Brain
Ventral nerve cord
Label all 12 parts on
the earthworm
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