Download DISSECTION OF AN EARTHWORM

DISSECTION OF AN EARTHWORM
In this activity you will dissect an earthworm so you can observe the digestive and circulatory
system.
Dissection is the scientific technique that allows you to separate one tissue from another.
Dissection of an organism is not simply a matter of cutting and slicing. The immediate aim of a
complete dissection is to separate the structures of one body system from the structures of the
other systems. You should see that each organism is a unique collection of intricate structures. Of
course, you will not see this unless you dissect with care and dexterity.
A dissection is complicated by several factors. First, most animal tissues are very soft. A
misplaced cut with scissors or scalpel can easily damage an important structure. Second,
sometimes the structures of one system seem jumbled with the structures of another. And, finally,
unless the animal is freshly killed, the preserving process can misshape and discolour some key
tissues.
Purpose: To exam the external and internal anatomy of the earthworm.
Materials:
preserved earthworm
dissecting tray
scalpel
dissecting kit
hand lens
Procedure:
External Anatomy
1. Put on your lab apron, gloves, and eye glasses or goggles.
2. Place a moistened paper towel in the bottom of the dissecting pan to prevent the earthworm
from drying out. Add water to the pan periodically to keep the animal moist.
3. Place an earthworm in the dissecting pan. Locate the anterior and posterior ends and the dorsal
and ventral surfaces. How do you distinguish between the anterior and posterior end?1 How
do you distinguish between the dorsal and ventral surfaces?2
4. Measure the length of your worm. Record this data in your observations.3 Estimate the number
of body segments. This can be done by counting the number of segments in 1 cm and then
multiplying by the length of the worm. Record the number of body segments. Make sure to
show your calculations .4
5. Locate the clitellum. It is about 31 segments from the anterior end. It produces the mucus that
makes a living worm so slippery. Describe the colour and shape of the clitellum.5
6. Examine the ventral surface of the worm. Rub your finger along the ventral surface to locate
the rows of bristles or setae. How many setae are on each segment?6
7. Look closely at the anterior end. The mouth is situated beneath an overhanging lip called the
prostronium. Use a hand lens to see the parts, then do a proper biological diagram of the
anterior end of the worm. Does the mouth run vertically or horizontally?7
8. Examine the posterior end of the worm. Use a hand lens to see the parts, then do a proper
biological diagram of the posterior end of the worm. Does the anal slit run vertically or
horizontally.8
Internal Anatomy
9. Stretch your earthworm out with the dorsal side up and the anterior end facing away from you.
10. Pinch up a fold of skin just in front of the clitellum. Snip with the scissors to make a small
shallow cut through the dorsal body wall. Make a very shallow cut into the surface of the
worm because the outer covering is extremely thin.
11. Lay the worm across your hand, dorsal side up. Insert the scissor point under the dorsal wall.
Cut slowly and carefully toward the anterior end, keeping the hidden scissor point from
digging into the body cavity. Be especially careful when you reach segment 1.
12. Now lay the worm straight out, incision side up, on the dissecting tray. Pin the anal and first
segment into the tray. Use the scalpel and forceps to separate the body wall from the internal
organs. You will have to cut through the septa, which are tissues that hold the body wall in
place and separate one segment from another. Pin down the body wall at regular intervals to
keep the internal organs exposed.
The Circulatory System
13. Examine the internal structure of the earthworm. Locate the aortic arches. How many are
there?9
14. Locate the dorsal blood vessel on the intestine. Describe the dorsal blood vessel.10
15. Tease away part of the intestine from the body wall on the ventral surface. You should see the
white nerve cord on the body wall. Look closely and you should see the ventral blood vessel
attached to the intestine. Describe the ventral blood vessel.11
The Digestive System
16. Locate the mouth. Follow the digestive system to the pharynx. Describe its colour and
structure.12
17. The esophagus is hard to see since the creamy-yellow seminal vesicles and the aortic arches
surround it completely. Carefully remove these structures. Describe the esophagus.13
18. Following the esophagus are two structures not found in the human digestive system. They
are the crop and the gizzard. Describe their shape and colour.14
19. Use a probe to gently push down on the crop and the gizzard. Describe any differences you
feel.15
20. Open the gizzard. Describe its contents.16
21. The remaining length of the digestive system is the intestine. Measure and record the length
of the intestine. What percentage of the total length of the worm does the intestine comprise?17
Describe the intestine.18
22. Cut open the intestine. Describe its contents.19
23. Draw a proper biological diagram of the anterior end of the internal anatomy of the
earthworm.
24. When you are finished, wrap your worm in paper towel and dispose of the tissue according to
the teacher’s instructions.
25. Wash the work area and your hands thoroughly.
Discussion Questions:
1. State and explain the function of the setae.
2. Why are the aortic arches of the earthworm not considered true hearts?
3. In which direction does blood flow in the dorsal and in the ventral blood vessel.
4. Based on your descriptions of the crop and gizzard, explain how their functions might explain
the differences you observed.
5. You examined the contents of the intestine. What might these contents be? Explain your
answer.
6. As you have observed, the intestine is well supplied with blood vessels. Explain why this is so.
EARTHWORM DISSECTION LAB
Read the Dissection booklet and follow the instructions to dissect your earthworm.
The booklet contains dissection instructions and, from time to time, an italicized instruction with
a small superscript numeral. When you come to such a sentence, write the numeral in your
notebook and respond to the statement or question. These will end up as your detailed set of
dissection notes.
Observations:
Answer all questions in italics.
Biological drawing of anterior end of worm
Biological drawing of posterior end of worm
Biological drawing of interior anatomy of worm
Discussion Questions:
1. State and explain the function of the setae.
2. Why are the aortic arches of the earthworm not considered true hearts?
3. In which direction does blood flow in the dorsal and in the ventral blood vessel.
4. Based on your descriptions of the crop and gizzard, explain how their functions might explain
the differences you observed.
5. You examined the contents of the intestine. What might these contents be? Explain your
answer.
6. As you have observed, the intestine is well supplied with blood vessels. Explain why this is so.
Dissection of an Earthworm Lab - Marking Sheet
Shared Work
Whole Group
Observation
Questions
1. Anterior vs. posterior end
2. Dorsal vs. ventral surface
3. Length of worm
4. Number of body segments
5. Clitellum description
6. Number of setae
7. Direction of mouth
8. Direction of anal slit
9. Number of aortic arches
10. Dorsal blood vessel description
11. Ventral blood vessel description
12. Pharynx colour and structure
13. Esophagus description
14. Crop/Gizzard shape and colour
15. Differences in crop/gizzard feel
16. Contents of gizzard
17. Intestine percentage
18. Intestine description
19. Contents of intestine
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Inquiry
Name:
Group Member
1
Communication
Name:
Group Member
2
Communication
Name:
Group Member
3
Communication
Title Page (C)
Typing Observation Questions 6-12
Anterior End diagram
Discussion Question 1 (I)
Discussion Question 4 (I)
Communication
/9
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Inquiry
Typing Observation Questions 13-19 (C)
Internal Anatomy Diagram
Discussion Question 5 (I)
Discussion Question 6 (I)
Communication
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Inquiry
Lab Formatting (C)
Typing Observation Questions 1-5
Posterior End Diagram
Discussion Question 2 (I)
Discussion Question 3 (I)
Communication
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Inquiry
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Total Marks
Inquiry
Communication
Group Member 1
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/9
Group Member 2
/40
/9
Group Member 3
/40
/9