Lab
NAME ________________
Worm Dissection DAY 1- External Anatomy
Background: Earthworms are extremely important helpers in the garden or field.
Their tunneling loosens the soil so oxygen and water can penetrate easier. It also mixes
the nutrients from the surface and the minerals from below to make the fertile topsoil
that plants need in order to grow.
Purpose: To locate and observe the different parts of an earthworm.
Part 1: Observation of EXTERNAL anatomy:
1. Find the anterior (front) end of the earthworm by locating the fleshy bump over
its mouth, called the prostomium.
2. The posterior (back) end has a small hole where solid waste is expelled, called
the anus.
3. The length of the worm is made up of many tiny segments each separated by a
thin wall called a septum.
4. About 1/3 of the way back from the mouth, you should see a thicker and
smoother section of the worm. This is called the clitellum, and it is involved in
reproduction.
5. Notice that the earthworm has a rounded dorsal (back) surface and a flatter
ventral (belly) surface. Usually the dorsal surface is darker than the ventral
surface. Lightly rub your finger along the ventral side toward the posterior end of
the worm. You should feel a roughness caused by tiny bristles called setae. See if
you can locate these.
6. Using your hand lens, look for tiny pores on each segment. Liquid wastes are
expelled through these pores. Near the front end of the worm, you should see
larger pores. These are used during reproduction.
7. After observing all of the external anatomy, label the following parts of the worm
below:
prostomium, segment, clitellum, anus, anterior end, and posterior
end
8. Now based on our discussion and the explanation on the front page, come up
with a short definition of each term below (it may only be a few words):
ANTERIOR
POSTERIOR
ANUS
CLITELLUM
VENTRAL
DORSAL
PORES
PROSTOMIUM
Worm Dissection Day 2 – INTERNAL ANATOMY
1. Lay the worm on the tray with its dorsal (back) side facing up.
2. Use your dissecting pins to secure each end on the tray.
3. Start your dissection about an inch below the clitellum. Lift up the skin with a pair of forceps
and snip an opening with a pair of dissecting scissors.
4. Insert the scissors into the opening and cut in a straight line all the way up through the mouth.
5. Go slowly and be sure you just cut the skin. Don’t go too deep.
6. Using the forceps and dissection pins, carefully pull apart the 2 flaps of skin and pin them flat
on the tray.
7. Look at the labeled picture to help you locate the following features:
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
Pharynx
Esophagus
Crop
Gizzard
Intestine
Hearts
Dorsal blood vessel
Ventral nerve cord
Ganglia
Seminal vesicles / seminal receptacles
INTERNAL ANATOMY KEY TERMS
PHARYNX
ESOPHAGUS
CROP
GIZZARD
INTESTINE
HEARTS
DORSAL BLOOD VESSEL
VENTRAL NERVE CORD
GANGLIA
SEMINAL VESICLES /
SEMINAL RECEPTACLES