Download Earthworm Dissection – Ch 15

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Lab: Dissection of the Earthworm
Introduction: The earthworms are segmented worms and are members of the phylum Annelida. The word
“annelida” means "ringed" and refers to a series of rings or segments that make up the bodies of the members of
this phylum. The adult worm may have more than 100 of these segments. The earthworm is well adapted to its
terrestrial way of life. Its streamlined body helps it to burrow through the soil and it secretes a covering of
mucus to help it slide through the soil. The wet and slimy coating of mucus is also necessary for the exchange
of gases that occurs through the skin of the earthworm. As the earthworm moves through the soil, a muscular
pharynx aids in sucking in the dirt in its path. As this dirt moves through the worm internally, organic matter is
digested and used as a source of food.
Purpose:
1.
2.
3.
To observe the external and internal structures of a segmented worm.
To describe the appearance of the various structures found in the earthworm.
To name the organs that make up the various body systems.
Materials:
Preserved earthworm
Scalpel
Probe or teasing needle
Forceps
Safety goggles
Dissecting tray
Scissors
Dissecting pins
Hand lens
Laboratory apron
Safety Precautions:
1.
2.
3.
Safety goggles and laboratory apron are required for this lab.
You teacher may also require that you wear gloves.
Be careful when handling sharp instruments.
Procedure: The External Earthworm:
Directions: Rinse the earthworm with tap water and place in the dissection tray. Take a few minutes to
examine the external anatomy of the worm. Since the earthworm has bilateral symmetry, it has a dorsal,
ventral, anterior, and posterior side. Try to determine the location of each of these surfaces.
1.
Were you able to distinguish the anterior from the posterior end? How were you able to do this?
2.
Were you able to distinguish the dorsal from the ventral side? How were you able to do this?
3.
How many segments does this earthworm have?
4.
Are all of the segments exactly alike? Explain.
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Directions: Let’s check to see if you identified the surfaces correctly. The anterior end is rather pointed and
the posterior end is blunt. The dorsal side is rounded and is darker in color. The ventral side is flatter and is
lighter in color. Turn the worm so that the ventral side is facing you. Run your finger down the edge of the
ventral surface. You should be able to feel small, whisker-like structures along the ventral surface.
5.
What are these whisker-like structures called? What is their function?
6.
Use a hand lens to observe these structures. How many of these structures are located on each segment?
Directions: Near the anterior end is a segment of the worm that appears to be much larger than the other
segments. Locate this area.
7.
What is the name of this section of the earthworm?
8.
What is the purpose of this segment?
Directions: Find the anterior end by locating the prostomium (lip), which is a fleshy lobe that extends over the
mouth. The other end of the worm’s body is the posterior end, where the anus is located. The mouth and the
anus are the two openings of the tube-like digestive system.
Use a hand lens to observe the ventral surface. First, locate the seminal receptacles, which are located in the
grooves between segments 9 and 10 and between segments 10 and 11. Next, locate the openings of the
oviducts, which are located at segment 14. Finally, at segment 15, locate the openings of the sperm ducts.
(Note: These openings are very difficult to locate.)
9.
What is the purpose of the seminal receptacles?
10. What is the purpose of the oviducts?
11. What is the purpose of the sperm ducts?
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12. In the space below, make a drawing of the external earthworm. Locate and label all of the following
structures: prostomium, mouth, segments, clitellum, setae, sperm ducts, seminal receptacles, oviducts, and
anus.
Procedure: The Internal Earthworm:
Directions: Place the worm in the dissecting pan with the dorsal surface facing up. Stretch out the worm’s
body and pin it to the dissection pan using dissecting pins. Place one pin through the first segment and a second
pin through the last segment. Pin the worm firmly to the dissection tray.
Starting about one inch posterior to the clitellum, make a small slit
with scissors and begin to cut upwards toward the anterior end. Be
careful not to cut too deep. The major internal organs lie just
inside and the body wall of the earthworm is very thin. Be careful
as you move toward the anterior end, as the major organs are
located anterior to the clitellum.
As you make your incision, fold back the skin flaps and pin them
to the tray. Continue to cut open the worm and pin back the skin
flaps until you have worked all the way up to the tip of the anterior
end, where the mouth is located. Continue in this manner until the
upper half of the worm has been opened and pinned back as seen
in the diagram to the right.
Let’s begin with the digestive system.
a) The mouth is located in the first three segments.
b) Just behind the mouth you should be able to locate a swelling,
which is the muscular pharynx. This should be seen in
segments 3 – 6.
c) Leading out of the pharynx is a thin walled and slender
esophagus. Look for this in segments 6 – 14.
d) The esophagus leads to a thin-walled crop, located in segments
15 and 16.
e) The crop empties into a strong and muscular gizzard. This
should be found in segments 17 and 18.
f) Leading out of the gizzard, you should see the intestine. This
extends down the remainder of the worm, ending with the anus.
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13. In the outline of the worm to the right, draw and label the structures that compose the digestive system of
the earthworm.
14. What is the function of the pharynx?
15. What is the function of the esophagus?
16. What is the function of the crop?
17. What is the function of the gizzard?
18. What is the function of the intestine?
19. With a teasing needle or problem, gently touch the crop and the gizzard. How are they different? How is
this difference in structure related to their function?
20. One reason both the roundworms and the segmented worms are considered more advanced than the
flatworms is the presence of the “tube-within-a-tube” digestive system. What is a “tube-within-a-tube”
digestive system? How is this different than the digestive system seen in the flatworms?
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Directions: Now try to locate the structures that compose the circulatory system.
a) To find the aortic arches, locate the region in between the pharynx and
the crop. You should be able to find 5 pairs of aortic arches that lie on
top of the esophagus.
b) Find the dorsal blood vessel that runs the entire length of the body.
c) Gently move aside the intestine to locate the ventral blood vessel.
d) Look for any smaller blood vessels that branch out from the dorsal
blood vessel.
21.
In the outline of the worm to the right, draw and label the structures
of the circulatory system.
22.
Does the earthworm have an open or a closed circulatory system?
Explain.
23.
What are the advantages to having this type of circulatory system?
24.
What is the purpose of the aortic arches?
Directions: Locate the structures of the nervous system.
a) Move aside the organs of the digestive and circulatory systems to locate the parts of the nervous system.
b) The brain is a white mass of tissue found in the third segment, just beneath the pharynx. Gently move aside
the pharynx and esophagus to expose the brain.
c) Also look for the ventral nerve cord that extends out of the brain down the length of the body.
d) With a hand lens, see if you can locate the ganglia that are found in each segment of the body. They will
appear as small masses that are attached to the ventral nerve cord.
25.
Define ganglion.
26.
Does the earthworm have any specialized sense organs?
Directions: Locate the structures of the excretory system.
a) Push aside the intestine in a region posterior to the clitellum.
b) With a hand lens, look for the white colored nephridia. There should be a pair of nephridia in each body
segment.
27.
What is the function of the nephridia?
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Directions: Locate the structures of the reproductive system.
a) An earthworm has both male and female reproductive organs.
b) Many of these structures are very difficult to find, but you should be
able to locate the seminal vesicles.
c) Use a hand lens and look in the area of segments 9 to 13.
d) The seminal vesicles are three-lobed, light in color, and found along
side the esophagus.
28. In the outline of the worm to the right, draw and label the following structures:
ganglia, brain, ventral nerve cord, nephridia, seminal vesicles.
29. What is the function of the seminal receptacles?
Final Observation Questions:
1.
Why was it important not to make a deep cut with the scissors as you began to
dissect your earthworm?
2.
How is the earthworm digestive system beneficial to other organisms?
3.
In what two ways is the earthworm body well suited for its terrestrial life?
4.
In what way is the earthworm poorly adapted to life on land?
5.
How does the earthworm breathe?
6.
The earthworm is hermaphroditic. What does this term mean?
Copyright © November 2012 Amy Brown (aka Science Stuff)
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Earthworm Dissection:
Answers and Teacher Guide
1.
You may wish to have your students use gloves when carrying out the dissection. I have found that most
of my students prefer not to wear the gloves. If gloves will not be worm, be sure to have the students rinse
the worm before handling.
2.
The very large earthworms used for dissection can be ordered from any biological supply company.
3.
If you are new to the dissection of the earthworm, these photos will help guide you in locating many of the
structures that are commonly found during a dissection of the earthworm.
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Answers to Questions:
1.
Student answers will vary. Most students will identify the posterior and anterior ends correctly. The
anterior end is more pointed and the posterior end is more blunt and rounded. The clitellum is found
nearest the anterior end.
2.
Student answers will vary. Students will have more difficulty in determining which side is ventral and
which side is dorsal.
3.
Student answers will vary. There may be in excess of 100 segments per worm.
4.
No, all of the segments are not exactly alike. Some segments, like those found in the clitellum area, are
much larger than others.
5.
The bristle-like structures are called setae. They help to anchor the worm in the soil and aid in
locomotion.
6.
These may be difficult to see. There should be 2 pairs of setae per body segment.
7.
The section is called the clitellum.
8.
The clitellum is the section of the worm that functions in reproduction by secreting the cocoon into which
eggs are deposited.
9.
The seminal receptacles receive sperm from another earthworm during reproduction.
10.
The oviducts are pores through which eggs are released.
11.
The sperm duct openings are pores through which sperm is released.
12.
Student drawing of external earthworm.
13.
Student drawing of digestive system with organs drawn and correctly labeled.
14.
Soil is sucked into the earthworm body through this muscular pharynx.
15.
The esophagus is the passageway from the pharynx to the crop.
16.
The crop is a thin-walled, temporary storage area for food and dirt.
17.
The gizzard is strong and muscular and is used to grind up organic matter.
18.
In the intestine, digested organic materials are absorbed into the bloodstream.
19.
The crop will feel very soft. The gizzard will feel hard. The gizzard is very muscular and is the grinding
organ of the digestive system.
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20.
This type of digestive system is one that has two openings, a mouth at one end and an anus at the other
end. The digestive system is a tube shaped structure housed inside a tubular shaped body. The flatworms
have a digestive system with only one opening. Both food and wastes enter and leave through the same
opening.
21.
Student drawing of the circulatory system.
22.
The earthworm has a closed circulatory system. The blood is contained inside a system of blood vessels.
23.
A closed circulatory is much more efficient in delivering food and oxygen to all cells of the body and in
removing wastes and carbon dioxide from the cells.
24.
The aortic arches are “pseudohearts”. They are pumping organs and pump blood throughout the body.
25.
A ganglion is a small mass of nervous tissue.
26.
No, the earthworm can respond to basic stimuli, but they do not have any sense organs (eyes, ears, etc.)
27.
Nephridia are excretory tubules found just under the skin. They eliminate wastes and excess water.
28.
Student drawing of nervous system, excretory organs, and seminal receptacles.
29.
The seminal receptacles receive sperm from another earthworm during reproduction.
Final Observation Questions:
1.
The body wall of the earthworm is very thin. If the student cuts too deeply, they will destroy the internal
organs that are found just beneath the skin.
2.
Earthworms fertilize the soil by taking in dirt and organic matter, and eliminating wastes. The burrows
made by earthworms allow air to penetrate into the soil, bringing oxygen to plant roots and other
organisms that live in the soil. Earthworms loosen the soil, which allows water to seep in.
3.
Its streamlined body helps it to burrow through the soil and it secretes a covering of mucus to help it slide
through the soil.
4.
The earthworm has no respiratory system. It must exchange gases through the skin.
5.
The earthworm exchanges gases through the skin. The wet and slimy coating of mucus is also necessary
for the exchange of gases that occurs through the skin of the earthworm.
6.
The term hermaphroditic means that the earthworm has both male and female reproductive organs.
Created by Amy Brown – Science Stuff
Copyright © November 2012 Amy Brown (aka Science Stuff)
All rights reserved by author.
This document is for your classroom use only.
This document may not be electronically distributed or posted to a web site.
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