The Earthworm Dissection
Name: _________________________________Partner #1:__________________________________
Partner #2: ______________________________ Partner #3: __________________________________
Purpose:
1. To examine the external and internal structures of the earthworm, Lumbricus terrestris.
2. To examine the increasing complexity through the phyla.
Materials:
Student #1 – Pick up: a) a dissecting pan b) Earthworm that has been rinsed in water
Student #2 – Pick up: instrument tray that includes:
a) 2 dull probes
b) 1 scalpel
c) 1 pair of scissors
d) 1 pair of tweezers or forceps
e) 12 dissecting pins (you can get more later if you want)
f) 1 ruler to measure length of organs
Student #3 – Pick up: a dissecting microscope
Other Equipment: You may want to wear disposable, surgical gloves. You may also want to wear a lab
apron or lab coat if you have one.
Helpful Hints:
1. Read all of the directions carefully before you begin to cut.
2. Cut upward with your scalpel or scissors to prevent cutting into the organs below the skin.
3. Cut only when you are directed to do so.
4. Use the dull probe to expose the structures.
PROCEDURES & OBSERVATIONS
Part 1. External Anatomy
a) Examine the earthworm. Note that the body of the animal is segmented. The segments are numbered
from anterior end to posterior end, and act as reference points during the dissection.
b) Locate the anterior end of the worm. You need to find the prostomium, the lip-like structure that
overhangs the mouth. The mouth is found on the ventral side of the first segment.
c) Locate the posterior end of the worm. You need to find the anus, a slit-like opening on the last
segment.
d) Determine the ventral and dorsal surfaces. The dorsal surface will be darker in colour and will feel
smooth compared to the ventral surface.
e) Hold the worm in your hand and feel it move around. Focus on feeling the bristly setae on each
segment.
f) Locate the thick, smooth structure called the clitellum. It is found closer to the anterior end and it
runs from segment 32 to 37.
Part 2. Dissection
a) Place the worm in the dissecting tray dorsal side up with the anterior end facing away from you. Fix
the worm into position by pushing a dissecting pin through the first and last segments.
b) Using the dissecting scissors or the scalpel, begin your cut just posterior to the clitellum. Cut forward
towards the anterior end, taking care not to cut too deeply. As you cut, carefully pin down the sides of
the body wall. Angle yours pins outward so you will have more room to work.
c) Notice the definite space between the body wall and the intestine (the long brown structure). This
space is called the coelom. The body wall and the digestive tract are connected between segments by
membranes called septa. As you cut, you will have to gently break these membranes so the worm will
open up better for pinning and observing. Try using a dull probe to break the septa.
d) Continue to cut and pin all the way to the anchor pin at the anterior end.
Part 3. The Digestive System
a) Use the diagrams and your notes to help you identify the various parts of the digestive system. Make
sure you use your dull probe to gently touch and point out the various structures.
b) The mouth of the earthworm opens into the muscular pharynx. Locate the pharynx (within the first 5
segments) and gently poke it with your probe.
Describe its texture or thickness and how it feels.
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Why would the pharynx have a thicker muscular wall than other structures?
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c) Posterior to the pharynx is a narrow tube called the esophagus, which extends for about 10 segments.
What is the function of the esophagus? ____________________________________________________
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d) The esophagus widens into the crop. Posterior to the crop is the gizzard. Use the probe to gently poke
the crop and gizzard to feel the relative textures and thickness. Describe the crop and give its function.
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Describe the gizzard and give its function. __________________________________________________
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e) From the gizzard, the food passes into the intestine. The intestine extends posterior to the anus.
Describe the intestine and identify its function.
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f) Draw a longitudinal view of the earthworm’s digestive tract. Label the mouth, pharynx,
esophagus, crop, gizzard, intestine and anus. Note: you don’t necessarily have to open the entire worm
to complete the sketch
Part 4. The Circulatory System
a) You will have to remove the large white structures that are a part of the reproductive system.
Using tweezers or forceps, remove the reproductive structures.
b) Locate the dorsal blood vessel that runs along the top of the intestine. Locate the ventral blood
vessel that runs underneath the intestine. You will have to push the intestine aside in order to find the
ventral blood vessel. Describe the dorsal and ventral blood vessels.
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c) In segments 6 to 11 the dorsal and ventral blood vessels are connected by 5 pairs of aortic arches.
They are fragile and may have been damaged during the dissection. How many hearts were you able to
find? ___________________ Describe the appearance of the 5 pairs of aortic arches.
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Describe the direction of the flow of blood in an earthworm. ___________________________________
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Is the earthworm circulatory system open or closed? ____________________ Explain. ______________
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d) Add the dorsal blood vessel, ventral blood vessel and 5 pairs of aortic arches to your diagram.
Part 5. The Nervous System
a) Locate the yellowish ventral nerve cord. You will have to push the intestine aside again in order to
locate the ventral nerve cord. Describe the appearance of the ventral nerve cord.
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b) Using the dissecting microscope, look at the nerve side branches in each segment. Describe the
appearance and function of these branches.
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b) Add the brain and ventral nerve cord to your diagram.
Part 6. The Excretory System
a) Remove a section of the intestine in order to locate the excretory organs called nephridia. Nephridia
are located in pairs on every segment, except for the first 3 segments and the last segment. You will
have to use the dissecting microscope to see the nephridia.
b) Describe the appearance of the nephridia. ________________________________________________
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c) What paired organs in our body contain similar structures to the nephridia in the earthworm?
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CLEAN UP!
If you are finished with the worm (LOOK AT QUESTION 5 BEFORE THROWING OUT), it can be thrown in
the garbage. The dissecting pan should be washed with soap and water and turned upside-down to
drain. The rest of the dissecting tools should also be washed with soap and water, dried and returned to
the appropriate area. When your dissecting pan is dry, place a paper towel in the bottom and make a
stack at the front of the class.
Discussion Questions:
1. Explain why the movement of the worm in your hand felt like a sort of tickle.
2. What is the function of the setae?
3. Briefly describe the process of cross-fertilization in earthworms.
4. Explain the process of how food is obtained and digested in the earthworm. Include all
structures involved.
5. How many segments does your worm have on its entire body? Compare with other groups.
Drawings: