Worm dissection prep

1. Getting Ready to Dis s ect Lumbricus terres tris !!

2. We have studied the organization of animals from cells, to tissues,
to organs, to systems, to organisms. To see how all the organs
form systems and how the systems form an organism, you will study
the anatomy of a common invertebrate, the earthworm (also called
the dew worm): Lumbricus terrestris. Earthworms are
invertebrates. That is, they do not have a backbone. Insects, sea
stars, spiders, jellyfish, and millipedes are other examples of
invertebrate animals.
Earthworms are important helpers in gardens and fields.
Their tunneling mixes up the soil and brings rich soil to the
surface. You can observe the organs of these tiny creatures
by dissecting one. Let’s answer some questions you may
have about dissection.

3. Why dissect?
Dissection is a valuable tool for visualizing the
anatomical structure of different animal classes and
species. We actually learn anatomy more easily
through dissections because we are simultaneously
engaging the sense of sight and touch along with
analytical thinking at the same time.

4. What tools do I need?
The basic dissection tools are a dissection
tray, pins, and scissors and two probes to
examine delicate parts. You will also have a
guide to show you how to dissect the
specimen. You will also need markers/colored
pencils/crayons (orange, light brown, red,
green, yellow, pink, purple)
,

5. How will we dispose of dissected
specimens?
When you have completed your dissection and have
had it checked, wrap all parts in a paper towel and
dispose of them in the container provided. DO NOT
dump any of the remains in the sink or take them out
of the room. Doing so will result in a detention. Wipe
out the dissecting tray with a Clorox wipe. Clean and
replace equipment.

6. How many people will be in my
group?
We will plan one specimen and one set of
tools per two students. This provides the best
learning situation. Students working in pairs
have ample opportunity to fully participate in
the dissection and to carefully examine the
specimen. You are also able to share and
discuss your observations. The pairs will be
decided by your wise, wonderful teacher!


7. Where will we do our dissection?
It is important to do your dissection on a
solid, uncluttered surface with plenty of
light. You will need to wipe off the table or
counter top you used with a Clorox wipe
also.

8. Important points:
As you dissect, proceed slowly and carefully.
Dissecting consists of carefully separating organs
and removing surrounding tissues to see an organ
clearly. Carelessness can damage structures and
make them difficult or impossible to find.
Follow directions exactly and do all steps in
the order indicated. As you do each step, it is
suggested that you check it off.
Do not cut out any organs unless specifically
directed to do so, but always remove enough of
the surrounding tissue to see an organ clearly.
A good dissection should reveal all organs
clearly enough so that a person could examine
the specimen and see the relationships and
connections between organs with no difficulty

9. Following are definitions of directional terms you
will encounter when reading dissecting instructions.
Anterior (toward the head) vs Posterior (toward the tail)
Dorsal (back) vs Ventral (front/belly)

11. Now, let’s look closely at an earthworm
and at your lab directions.
“DISSECTING AN EARTHWORM” LAB
PURPOSE: To dissect an earthworm and observe the organs
of the digestive system.
Preserved earthworm, dissecting pan, scissors,
MATERIALS: dissecting pins, forceps, paper towels, dissecting
probe, hand lens, markers or crayons (orange, light
brown, red, green, yellow, pink, purple)

12. PROCEDURES:
_____ 1. Obtain a dissecting kit and check to see that it has a scalpel,
scissors, forceps, metric ruler, and 2 dissecting probes.
_____ 2. Obtain an earthworm and place it in a dissecting pan.
_____ 3. Note that the body of the earthworm is divided into ringlike units, the
segments.
_____ 4. 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. The darker dorsal side is marked by a dark line running the
length of the animal. This is the dorsal blood vessel seen through the skin.
_____ 5. Find the anterior (front) end of the earthworm by locating the fleshy
bump over its mouth, called the prostomium. The opening is in segment number
1. Each succeeding segment is numbered consecutively throughout the length of
the worm.

13. _____ 6. The posterior (back) end has a small hole where solid waste is
expelled, called the anus. This is used for the removal of digestive waste and soil
which the worm has ingested.
*The length of the worm is made up of many tiny segments,
each separated by a thin wall called a septum.
_____ 7. About one-third of the way back from the mouth,
surrounding segments 32-35, you should see a thick, smooth belt of
the worm. This is called the clitellum, and is involved in
reproduction.
_____ 8. 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 (se`etee). Using a hand lens, try to
see the setae. These aid in the locomotion of the worm by
gripping the soil as the worm pulls itself along.

14. _____ 9. Look for the openings on segment 15. These are the
openings to the seminal vesicles. Sperm cells leave the body of the
worm through these openings.
_____ 10. In segment 14 are the openings of the oviducts. Egg
cells pass through these openings on their way to being fertilized.
They may be too small to see with your eyes alone.
_____ 11. Between segments 9-10 and 10-11 are the openings of
the seminal receptacles, the organs which receive sperm cells
during mating. The earthworm is a hermaphrodite, that is, it has
organs of both sexes. During copulation, or mating, sperm cells
are transferred from the seminal vesicles of one worm into the
seminal receptacles of another worm.

15. Let the dissecting begin…
______1. Place the earthworm with its top (darker) side up in the
dissection pan.
______ 2. Use dissection pins to secure each end on the tray.
Start your dissection about an inch posterior to the clitellum. Lift up the skin
with a pair of forceps and snip an opening with a pair of dissecting scissors.
Insert the scissors into the opening and cut in a straight line all the way up
through the mouth. Go slowly and make short shallow cuts and be sure to
cut just the skin so you do not damage the organs inside.

16. __________3. Using the forceps and dissection pins, carefully pull
apart the two flaps of skin and pin them flat on the tray. You may
need to drag a pin along the inside of the skin to sever the septum
walls to make it easier to spread the skin.
Pin at an angle rather than straight up and down. After you have pinned
from the clitellum to the mouth, check with your teacher before going on.
__________ 4. Look at the labeled picture to help you find the following
features. Use the coloring guide to color the internal parts.

18. Observe the main tube that runs down the center of
the worm. This is the digestive system.
__________ 5. Find the pharynx, a wide tube at the beginning of the
digestive system. Color it orange in the drawing.
__________ 6. On top of the pharynx is the ganglion, the worm’s equivalent to
a brain. It is a very tiny white structure which has two tiny lobes. On the
drawing, color it light brown.
__________ 7. Following the pharynx is the esophagus, a tube which runs
to about segment 14. Color it red .
__________ 8. The esophagus leads into the crop, an enlarged organ for
temporary food storage. Color it green .
__________ 9. The crop is followed by the gizzard, an organ which grinds
food. Color it yellow .
__________ 10. The remaining organ of digestion is the intestine which
extends to the posterior end of the worm. It is the place where digestion and
absorption of the food takes place. Color it purple .

19. __________ 11. Surrounding the esophagus near the anterior end
(segments 3-11), are a series of dark-colored loops. These are the
aortic arches. They are sometimes called the earthworm’s hearts, but
actually they only assist in pumping the blood. They are fragile and
may be broken apart in your specimen, but you can usually find some of
them. On the drawing, color the aortic arches pink.
If time permits:
Reproductive organs: the light colored tissue above and around the hearts
are seminal vesicles. Other reproductive parts appear as small white organs
on the ventral side of the hearts.
Ventral nerve cord: with your forceps, gently push aside the intestine to view
the long white nerve cord running along the length of the worm beneath it.

21. Have your teacher grade your dissection (see below)
_____ /25 for a successfully-dissected worm
-____ points deducted for _____________________________
_____ Total Score

22. Clean up time…
Remove all dissection pins and wrap the
earthworm in the paper towel and put it in
the designated bucket. Clean up all of the
equipment and return all instruments to your
CLEAN dissection tray.

23. Final thoughts…
How many segments does your earthworm have? ______________
Check the organs that you were able to observe. (not graded)
____ crop ___esophagus ___ gizzard ___ganglia ___anus
___ 5 pairs of hearts _____ pharynx _____ intestine
_____ reproductive structures

24. I knew you could do it!!!!!!!