Mr. AG 03/16/09
Worm Dissection Lab
A. Purpose:
The objective of this experiment is to become more familiar with the
earthworm's anatomy through dissection. After the cell wall is cut the organs
will be visible. The numerous organs then can be studied as well as the rest of
the earthworm's anatomy. The student should be able to identify all the basic
organs on a diagram after completing this experiment.
The common earthworm is a member of genus Lumbricus
in the broader category of phylum Annelida (Wile). It is an invertebrate that
has a circulatory system that transports food and other necessary substances
throughout its body (Wile). They eat mostly dirt and can produce their own
weight in "castings" every 24 hours (Nature). They use a vacuum like
pharynx to suck up their food.
Apart from a complex circulatory system, earthworms all have a fascinating
means of locomotion. They have circular muscles that contract and get longer
and longitudinal muscles that get shorter, that either force the unanchored end
(the anterior) of the worm forward while the other end remains anchored or pull
the posterior end forward (Rosenoff). This turns into the squirming motion that
worms are known for. This also helps the worm push through the tough soil, as
well as the worm's eating of the soil.
Lastly, the sexual reproduction of the earthworm is fascinating. The earthworm
is hermaphroditic, which means it contains both male and female reproductive
organs, but it cannot sexually reproduce on its own (McGraw). Two worms will
come up alongside each other facing opposite directions and will engage in a
slime tube in which each worm injects their own sperm into each other (Wile).
This sperm eventually gets left in a cocoon produced by the worm in which eggs
are also deposited. These then fertilize and in a few weeks a new earthworm
emerges from the cocoon (Wile). Earthworms truly are complex organisms!
This experiment hopes to show a few things. First, it hopes to show that any attentive student can do a dissection as long as they follow the directions. Second, it hopes to show that organs and other internal structures are real, not just textbook pictures. Many students do not realize how creative and wise God is; He gives each organism all of the necessities of life, even a low organism like the segmented worm. So, not only does this experiment hope to show the anatomy of an earthworm, but it also hopes to show the power and might of God.
This topic is of interest to science because there are tons of segmented worms
in the world, and scientists like studying different organisms. Even a
"simple" creature like an earthworm has loads of fascinating internal
processes and organs. It is also of interest to science because scientists can
learn more about invertebrates by studying earthworms. Scientists can study a
broad range of topics in a single earthworm. For instance, they can study
feeding habits, their reproductive system, their habitat, and much more. By
studying just one aspect of a "simple" creature as a worm, perhaps
scientists will better understand that evolution is impossible and that there
is in fact a mighty God who created the universe.
Hypothesis: If the earthworm is dissected neatly, cautiously, and
according to the directions, then the student will be able to see and study the
reproductive organs in depth.
B. Equipment:
1. Dissecting tools and tray that came
with the dissection kit
2. Earthworm specimen
3. Magnifying glass
4. Laboratory notebook
5. Premade saline solution
C. Procedures:
1. Examine the earthworm specimen carefully. Lightly stroke the surface
until of the worm until bristles are felt. Those bristles are setae. Write in
laboratory notebook how many setae are on each of the worm's segments.
2. Using a magnifying glass, try to find the
nephridiopores, which are tiny holes near the bottom of many segments anterior
to and posterior to the clitellum.
3. Examine the clitellum. Write down how many segments there are in it in
the laboratory notebook.
4. Begin the dissection now. Place the specimen ventral side (the side
with the setae) down on the tray. Pin the anterior and posterior end to the
pad.
5. Cut through the body wall using the scissors. Start about an inch
posterior to the clitellum and just to the left of center. Being careful not to
cut anything but the body wall, and extend the cut all the way to the anterior
end.
6. Pull apart the edges of the cut and peer in. The intestine should be
visible. The space between the body wall and the intestine is called the body
cavity. Notice that the body cavity is separated by partitions that run from
the body wall to the intestine. These are called septa.
7. Using forceps (tweezers) and the probe (the tool with the metal
pointer), break the septa so that you can peel back the body wall.
8. Peel back the body wall on both sides of the cut and pin it down.
9. Now the internal structures should be visible. Make a drawing of the
dissected earthworm in the laboratory notebook. Identify the structures listed
below, and label them in the drawing in the notebook. Note any structures that
are not visible, as well as any unidentifiable organs.
10. Using Figure 11.7 and the picture on page 349 as guides, identify the
following digestive structures: the pharynx, the esophagus, the crop, the
gizzard, the intestine, the seminal vesicles, and the seminal receptacles.
11. Review the functions of these structures as described in the text.
12. Once again, using Figure 11.7 and the picture on page 349 as guides,
identify the following circulatory system structures: the dorsal blood vessel,
the aortic arches, and the ventral blood vessel. The seminal vesicles and septa
must be removed in order to see the arches clearly. Remove them only from the
left side of the earthworm and examine the aortic arches that are revealed.
They will look like tubes surrounding the esophagus. Use the probe to move
aside, but do not remove the intestine near the posterior end of the cut. This
should reveal the ventral blood vessel, which looks very similar to the dorsal
blood vessel.
13. Review the functions of these structures as described in the text.
14. Locate the nephridia. The best way to do this is to extend the cut
another two inches to the posterior. Without tearing the septa, use the probe
to lift up the intestine from this region and then use the magnifying glass to
look under the intestine and find the nephridia. They will be in all segments
except the first three and the last one, so there should be plenty to see. If
they are too hard to find, don't worry. They are the most difficult of the
earthworm's internal structures to find.
15. If the dissection has been a bit sloppy, these next steps might not turn
out too well. Using Figure 11.7 and the picture on the page 349 as guides, try
to find the ganglia that form the earthworm's "brain." They should be
just anterior to the pharynx. Follow the ventral nerve cord from the ganglia.
Note the small ganglion (which looks like a bulge) that appears at each
segment.
16. To get a better look at the reproductive structures as well as the
ventral nerve cord, remove a portion of the digestive track. Do so by cutting
across the intestine near the clitellum. Make a similar cut just posterior to
the pharynx. This cut out section is a portion of the digestive tract. Gently
pull that section (that portion of the intestine, gizzard, crop, and
esophagus), out of the earthworm.
17. Look for the ventral nerve cord. It might have been damaged when the
portion of the digestive tract was removed. The ganglia on the nerve cord
should be easier to see now.
18. Notice that the seminal vesicles and seminal receptacles are also
below this portion of the digestive tract, not just to
either side of it.
19. Dispose of the specimen.
20. Clean and dry the dissection tools, tray, and pins. Put everything
back in its proper place.
D. Observations:
1. The setae are prickly and invisible to the naked eye. It feels like
there is one seta per segment.
2. Nephridia are small but there is a plenteous supply of them.
3. The clitellum was of a lighter shade (of grayish-brown) than the rest
of the earthworm. There are 6 segments in the clitellum.
4. The specimen is extremely long and was tough to pin down diagonally.
There was approximately 2 inches of slack in the earthworm after it was pinned
down. Hence, not all of the posterior end was ventral
side down.
5. The cut is jagged and not quite straight. Nonetheless, nothing else
was sliced except for the body wall.
6. The specimen is tough to open because of all the septa. The intestine
is clearly seen as well as the septa and the body cavity.
7. Septa take a long time to cut and the first few cuts are difficult. No
organs get severed.
8. Successfully pinned the body wall down. The body wall eventually
ripped off of a few of them, however.
9. All the organs and structures are clearly visible. Drawing is precise
and clearly labeled.
10. The pharynx and the intestine are the easiest to identify. The
gizzard is darker than the crop and is more round. The seminal vesicles are
seen but the receptacles are smaller and tougher to identify. The esophagus is
the hardest to find, relatively speaking, since these organs are all visible.
11. Textbook is studied, especially Figure 11.7, in order to gain a
better understanding of each of these organs.
12. The dorsal blood vessel is a greenish-brown and is clearly seen. The
aortic arches are slightly harder to find because of the seminal vesicles getting
in the way. The ventral blood vessel is of a similar shade and is seen easily.
13. The functions of these circulatory system structures are studied,
with the focus on the aortic arches.
14. These definitely are the toughest to find. A few whitish circular
things are found but they do not appear to be nephridia. So, nephridia are not
located or seen.
15. Dissection has been sloppy but the saline solution helped clear up
some of the dirt and other waste products. The ganglia are clearly see but are
crushed and look like squished grapes. The actually look similar to the inside
of a grape as well.
16. The digestive track is hard to remove due to some septa that has not been cut efficiently. The cut out section of the
digestive track is dirty and filled with a soil like substance.
17. The nerve cord is seen. It is a darker shade compared to the other
organs. It is smaller than the blood vessels and is tubular. The ganglia were
seen before, so this did not really make a difference in seeing them any easier.
18. The seminal vesicles and the seminal receptacles indeed do surround
the digestive tract. The vesicles are much larger than the receptacles and are
a tannish-green color.
19. The specimen is neatly placed on a paper towel, wrapped up, and
thrown out.
20. Tools are cleaned with warm soapy water and are dried. They are
carefully placed back into their proper locations.
E. Conclusions:
The hypothesis stated that if the earthworm is dissected neatly, cautiously,
and according to the directions, then the student will be able to see and study
the reproductive organs in depth. The procedures were closely and meticulously
followed and the reproductive organs of the earthworm were seen. They were
closely observed and by using their physical appearance, location, and outside
sources much was learned about them. Specifically, the means of sexual
reproduction was studied in the textbook and on various websites. Therefore,
the hypothesis was correct.
This experiment could be improved by getting a longer sponge-board. The one
that was used was far too small and definitely hindered observation of the
earthworm. Another improvement would be to use a stronger magnifying glass, and
perhaps even use a microscope. Also, a more luminous room would aid this experiment
by allowing the organs to be seen better, especially the nephridia.
An idea for further research would be to study worms from different phyla such
as Platyhelminthes and Nematoda and to compare and contrast the major features
of each organism. For example, the planarian could be compared to the earthworm
and tapeworms could be compared to nematodes. Another idea for further research
would be to study the human circulatory system and understand the differences
between it and the earthworm's. Also, it would be fascinating to study the
differences of the open circulatory of the crayfish and the closed circulatory
system of the earthworm.
F. Bibliography:
McGraw Hill Higher Education. "Earthworm
Dissection"
Domain: http://www.mhhe.com
Document: /biosci/genbio/virtual_labs/BL_14/BL_14.html
Nature Watch. "Worm Watch-Ecology"
Domain: http://www.frogwatch.ca
Document: /english/wormwatch/about/ecology.html#reproduce
Rosenoff, Steven. Classroom/Internet Lecture.
March 2009.
Wile, Dr. Jay. L. and Durnell,
Marilyn F. Exploring Creation with Biology, 2nd
Ed. Apologia Educational Ministries, Inc. 2005