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 kit
dissecting tray hand lens
scalpel
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.