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Earthworm Dissection Lab
The earthworm is a segmented worm belonging to the phylum annelid. 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 tube-like 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. Straighten the earthworm out across the dissection tray placing the ventral side
down.
a. The ventral (belly) side appears flatter and lighter in color than the other
sides of the earthworm.
b. The dorsal (back) side is the worm’s rounded top and is darker in color.
2. Identify the anterior and posterior ends of the earthworm.
a. The anterior (head) end is slightly pointed and is closer to the clitellum.
b. The blunt posterior (tail) end features the anus, the end of the digestive
tract. This end is usually flatter than the head.
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3. Locate the mouth on the anterior end of the earthworm (segment 1). It will look
like a U. The flap of skin protecting the u-shaped mouth is called the
prostomium. Gently pull the prostomium back with your dissecting needle and
you will see the mouth opening.
4. 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.
5. Find the clitellum, the light-brown enlarged band around the body in segments
33-37. It forms a cocoon into which eggs are deposited during reproduction.
6. Find the anus. This is the opening from which waste material passes from the
earthworm’s digestive tract.
Record the following data:
a. Number of segments from anterior end (mouth) to clitellum:_________
b. Number of segments from mouth to anus: ______
c. Length of earthworm: _________cm
7. In the diagram below, label the following parts of the worm using the word
bank:
Anterior
Posterior
Clitellum
Mouth
Anus
Setae
Questions:
1. What are the functions of the following organs?
a. Clitellum- ___________________________________________________
b. Setae- ______________________________________________________
c. Prostomium- _________________________________________________
Part B: Internal Anatomy
8. 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.
9. 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!
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10. When the cut is complete, carefully open the body wall. Beginning at the
posterior end, use your probes to gently tear the septa (little thread like
structures that hold the skin to organs below it). 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
11. Study the digestive system first. The mouth is located in the first three
segments.
12. Locate a slight swelling, the muscular-walled pharynx posterior to the mouth.
After food enters the mouth, it goes into the muscular pharynx where it is
swallowed. You will see stringy things attached to either side of the pharynx
(pharyngeal muscles).
13. The esophagus leads from the pharynx. On top of the esophagus tube are the
hearts and reproductive organs so it may be difficult to see.
14. The esophagus carries food to the crop. In segments 15-16 you will find a thinwalled structure called the crop. The crop temporarily stores food.
15. 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.
16. With the probe, feel the difference in the walls of the crop and gizzard. Describe
here:
__________________________________________________
17. The intestine extends from the gizzard to the anus. Digestion of food continues
in the intestine and nutrients are absorbed.
18. 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: _________________________________________________
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Part D: Reproductive System
19. 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.
20. 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
Part E: Circulatory System
21. Find the dorsal blood vessel running down the back of your worm. This blood
vessel carries blood to the heart.
22. 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.
23. 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
24. 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.
25. 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.
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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.
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
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. An earthworm’s tubelike digestive system is similar to a “disassembly
line”. Describe 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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