Eden High School
Grade 10 Academic Science (SNC 2D1)
The Earthworm Dissection
External Anatomy
1. Rinse your specimen with water, and pat dry.
2. Using the hand lens examine your earthworm and determine the dorsal (top) and ventral (bottom) sides.
3. Locate the dark line that runs down the dorsal side of the worm, this is the dorsal blood vessel. The ventral blood
vessel can be seen on the underside of the worm, though it is usually not as dark.
4. Locate the worm's mouth and anus. Note if they run vertically or horizontally.
5. Note the swelling of the earthworm near its anterior side - this is the clitellum.
6. Locate the spiky setae along the ventral surface. Whatpreservative.
would they be
use for?
Identify
the dorsal side, which is the worm’s rounded top,
the ventral side, which is its flattened bottom. Turn the worm ventral si
up, as shown in the diagram below.
3. Use a hand lens as you observe all parts of the worm, externally and
internally. Locate the conspicuous clitellum, a saddle-like swelling on the
dorsal surface. The clitellum produces a mucus sheath used to surround
Internal Anatomy
1. Place the specimen in the dissecting pan DORSAL sideworms
up. during mating and is responsible for making the cocoon 4within whic
Biology
fertilized eggs are deposited. The anterior of the animal is more cylindr
E arthworm
Dissection
L ab
2. Locate the clitellum and insert the tip of the scissors about 3 cm posterior
or to the back
of it.
than the flattened posterior and is the closest to the clitellum. The ven
3. Cut carefully all the way up to the head. Try to keep the
scissors
up, and isonly
surface
of pointed
the earthworm
usually a lighter colour than the dorsal
cut through the skin.
surface.
mouth
is its
located
5. W rapThe
the worm
and all
parts in aon the ventral surface of the first segme
paper
towel.
the worm
in theend of the last segment. Find the anterior
the
anus
isPlace
found
at the
4. Spread the skin of the worm out, use a probe or teasingwhile
needle
to
gently
tear
the
septa
designated garbage can.
end by locating the prostomium (lip), which is a fleshy lobe that extends
(little thread like structures that hold the skin to organs below it)
over6. the
Thearea
other
end of the worm’s body is the posterior end,
Cleanmouth.
up your work
and wash
before
leaving the lab.
5. Place pins in the skin at a 45 degree angle to hold it apart.
where your
thehands
anus
is located.
Reproductive System
1. The first structures you probably see are the seminal vesicles. They are cream
colored and located toward the anterior of the worm. These are used for producing
sperm. 2. Use tweezers to remove these white structures from over the top of the digestive
4. Locate the clitellum, which extends from segment 33 to segment 37.
system that lies underneath it.
for the worm’s setae, which are the minute bristle-like spines located on
every segment except the first and last one. Run your fingers over the
ventral surface of the earthworm’s body. You should be able to feel
Circulatory system
bristle-like
setae along
used for
1. The dorsal blood vessel appears as a dark brownish-red
vessel running
the locomotion
intestine. The heart (or aortic arches) can be found over
the esophagus
(just
5. Refer
again to the
diagram of the ventral view of the worm to locate
posterior to the pharynx). Carefully tease away the tissues
to expose
the arches
identify
the external
parts of
of its reproductive system. Find the pair of
sperm grooves
that
extend
the heart, the run across the worm. If you are careful enough,
you can
expose
all from
5 of the clitellum to about segment 15, wher
them.
2. The ventral blood vessel is opposite the dorsal blood vessel, and cannot be seen at
this time because the digestive system covers it.
Pictures: Modern Biology, Holt & Carolina Biological Supply, Earthworm BioKit.
Digestive System
1. The digestive system starts at the mouth. Trace the organs all the way to the anus and identify each on the worm.
2. Find the mouth opening, the first part after the mouth is the pharynx, you will see stringy things attached to either side
of the pharynx (pharyngeal muscles). 3. The esophagus leads from the pharynx but you probably won’t be able to see it, since it lies underneath the heart. You will find a two structures close to the clitellum. First in the order is the crop, followed by the gizzard. The
gizzard leads to the intestine which is as long as the worm and ends at the anus.
4. Use your scissors to cut open the crop and the gizzard. In which organ would you expect the contents to be more
ground up? In which would it hold larger amounts of food
5. Label each of the following organs on the drawing: Mouth, Pharynx,
Esophagus, Crop, Gizzard, Intestine, Anus.
Questions:
1. How can you tell the difference between the dorsal and ventral sides?
2. How can you tell the posterior and anterior ends apart?
3. Does the mouth run horizontally or vertically? Does the anal opening
run vertically or horizontally?
4. Compare the shape of the posterior 15 segments with the anterior 15 segments. Suggest a reason for the difference.
5. What might their function of the setae be?
6. What is the function of the aortic arches?
7. What is the colour of the intestine? Why is the intestine well supplied with blood?
8. Describe the contents of the digestive system. Explain.