Activity 37
Name
Period
Lab Partner
EXAMINING THE EARTHWORM
PURPOSE: To study an organism that possesses an "organ system" level of development.
To learn the meaning of anatomy terminology (dorsal, ventral, anterior, posterior, etc.)
To learn about annelid worms and the earthworm as an organism.
MATERIALS PER LAB STATION:
preserved earthworm
one dissecting tray
two dissecting probes
one pair dissecting scissors
one pair forceps
one razor blade
several dissecting pins
latex gloves (if the student chooses to bring them)
INTRODUCTION:
In the first dissection (the Ascaris) you examined an organism that showed degeneration. In other
words, many of its organs and organ systems were reduced in complexity or missing altogether. This
was an adaptation to its specialized environment (the intestines of mammals). For instance, the worm
did not have to have a complex digestive system, because it could easily absorb the food that was already
digested by the host's intestine. Now you will dissect a free-living segmented worm, the earthworm.
Rather than seeing degeneration, you will see how many systems are well-developed and rather complex.
Earthworms depend on their many systems to perform major life functions for the whole organism. You
may recall that this is the purpose of organ systems... to perform major life functions for the entire
organism. In this dissection you will examine many different organs of the earthworm, and will see how
they fit into various systems. You will also be introduced to another major type of worm... the
segmented worm. Segmented worms are classified in the phylum, Annelida. Leeches are another
example of segmented worms. While segmentation may seem little more than just having a ringed
appearance, it is much more important. Segmentation allowed for different regions of an organism to
specialize, with different segments or groups of segments taking on different functions for the whole
organism. While you will see little specialization in the earthworm, you will really see how different
segments specialize in our next dissection, the crayfish (Phylum Arthropoda).
Anatomy Terminology:
Before beginning a discussion of the anatomy of the earthworm, it is important for you to understand
some general terms. These terms are important, for they will direct you where to look for the various
organs in your specimen. "Anterior" refers to the "head-end" of an organism. In humans, the arms are
anterior to the legs. "Posterior" is the opposite of anterior and refers to the "tail-end" of an organism. In
humans, the heart is posterior to the brain. "Dorsal" refers to the backside of an organism. The dorsal
surface of an organism is the surface that tends to face the sky. In higher animals, the "backbone" is near
the dorsal surface. When considering these terms for humans, imagine the human crawling rather than
standing up. In humans, we would say that the shoulder blades are dorsal to the breastbone. "Ventral"
refers to the belly-side of an organism. The ventral surface of an organism is that surface which is
usually closest to the ground. In humans, the stomach is ventral to the spine. "Median" refers to the
midline of an organism. In humans, the spine is median to everything else. "Longitudinal" means
"running the length" of the organism. If you were told to make a longitudinal cut on the worm, you
would cut it from its mouth to its anus.
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External Features of the Earthworm:
In examining the external features of the earthworm, you should immediately notice the segmented
nature of the animal. The anterior end of the organism is thicker than the posterior, and contains most of
the important internal organs as you shall soon see. The earthworm is adapted to subterranean
(underground) life. It is streamlined, and has no major projections from its body that would hinder its
movement through the soil. On each segment, there are bristles, or setae. These setae are on the ventral
surface of the worm. The setae allow the worm to anchor itself in its burrow should a predator (like a
robin or other bird) try to pull it out. Mostly the setae provide traction for the worm so it can move
along.
Several segments back from the anterior end of the worm is a prominent swollen ring called the
clitellum. The clitellum has several gland cells that secrete mucus. When two worms move next to
each other to mate, they line up at their clitellums and secrete a thick layer of mucus. The sperm cells
are then passed from each worm to the other through this mucus. Mucus secreted from the clitellum is
also used to surround the eggs that are laid, protecting the developing embryos in a type of "cocoon"
before the little worms break free. Note that the clitellum is closer to the anterior end of the worm than
the posterior.
Internal Features of the Earthworm
The Digestive System
At the very anterior end of the earthworm is an opening called the mouth. The mouth is where the food
enters the worm. Earthworms eat leaves and other plant material that they pull down into their burrows.
They also eat soil, digesting the organic material found in the soil. By eating the soil, they create
passageways throughout the soil. This helps to aerate the soil, which helps plant growth. It also helps
improve the drainage of the soil, another beneficial factor. After the mouth is a thick-walled muscular
organ that actually pulls the soil into the mouth by its muscular contractions. This organ is called the
pharynx. The food then passes through a long slender tube called the esophagus. The esophagus
extends to the 15th segment. The purpose of the esophagus is to pass food from the pharynx to the next
structure, the crop. Another benefit of the esophagus is that it has calciferous glands that secrete extra
calcium into the food. This calcium will be passed out of the worm, preventing the worm from getting
too much of it in its body. The next structure after the esophagus is a wide, thin-walled structure called
the crop. The crop serves as a storage sac. Immediately after the thin-walled crop is the thick-walled,
muscular gizzard. The gizzard's muscles grind the food thoroughly. From the gizzard the food passes
into the very long, thin intestine. The intestine secretes enzymes that chemically digest the food
particles, breaking them down into their building blocks, which can then be absorbed. Virtually all
chemical digestion of the food occurs in the intestine, along with virtually all absorption of the food
particles. The intestine continues all the way to the posterior end of the worm where the undigested
particles pass out the anus. This solid waste are called "castings" and they are incredibly important to
the quality of our soil. The castings have some undigested materials that will serve as plant "fertilizer",
adding to the organic material of soil. In addition, worms add nitrogen-waste products to the castings,
increasing the richness of the soil.
The Reproductive System
In segments 9, 10, and 12 the seminal vesicles can be found. These appear whitish-yellow in the
dissected specimens. The seminal vesicles produce the sperm cells. In segments 9 and 10, smaller
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whitish-yellow structures, the seminal receptacles, can be found. These receptacles store sperm
received from another worm during the mating process. Smaller ovaries are located under the seminal
vesicles, but they may be difficult to locate. The ovaries produce the eggs. From this discussion, it
should be obvious that worms possess both male and female reproductive organs. This feature makes
them hermaphrodites.
The Circulatory System
After the worm has been opened up on its dorsal side, the dorsal blood vessel should be obvious. This
is the large vessel that travels on the dorsal surface of the intestine. It appears dark because of the blood
it carries. If one looks closely, he can see that the dorsal blood vessel branches in each segment near the
intestine. Near the seminal vesicles, in segments 6-11, several hearts or aortic arches are visible. These
structures appear dark because of the blood they contain. There are 10 total (5 pairs) of aortic arches,
and they lie perpendicular to the direction of the esophagus, over which they lie. The aortic arches
extend around to the ventral side of the worm. While these structures are often referred to as hearts, they
are really little more than thickened blood vessels. However, they do have muscles that contract and
help circulate the blood. On the ventral surface of the worm (one would have to move the intestine aside
during the dissection to see it) lies another large blood vessel, the ventral blood vessel. Like the other
parts of the circulatory system, this vessel appears dark because of the blood it carries.
The circulatory system of the earthworm is an example of a closed circulatory system. This means that
the blood is always contained within a series of vessels. This is in contrast to the next organism we will
dissect, the crayfish, which has an open circulatory system. An open circulatory system can contain
some blood vessels, but the blood is not always contained within them. In other words, at some point
the blood leaves the heart and or vessels and simply bathes the tissues directly. The blood will often
slosh between the tissues when the organism moves, allowing it to deliver oxygen and or nutrients to the
cells, and to pick up waste products.
The Excretory System
In each segment (except the first three and the last), a pair of coiled, white, loose tubes lie along side of
the digestive tract. These are excretory organs called nephridia. They filter wastes from the body
cavity and excrete them through excretory pores.
The Nervous System
In the second to third segment, at the very anterior tip of the earthworm, is the brain or dorsal ganglion.
The dorsal ganglion consists of two pin-point sized white structures that sit dorsally to the tip of the
pharynx. From each part of this ganglion, threadlike nerves travel around the pharynx to the ventral
side, where another ganglion exists. From this ganglion a long ventral nerve cord extends down the
whole length of the worm. The ventral nerve cord travels alongside the ventral blood vessel, but it is
thin and white, compared to the dark blood vessel. The intestine will have to be moved to the side to see
this structure. Smaller nerves branch off of the ventral nerve cord. It is important to note that this
organism and the next we will dissect, the crayfish, both have ventral nerve cords. The major nerve cord
of higher animals, like humans, is found dorsally (i.e.. our spinal cord is in our back, not our belly).
The Respiratory System
The earthworm has no lungs or other respiratory organs that you will see during dissection. The reason
for this is that the earthworm "breathes through its skin." The oxygen simply diffuses through the thin
skin into capillary beds, where the oxygen can then be transported to cells by the circulatory system. In
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order for oxygen or other gases to diffuse across the skin, the skin must be kept moist. Should the
earthworm dry out, it would die. Earthworms, then, do not "drown" if they remain submerged
underwater. After large rains, earthworms will frequently leave their burrows and come to the surface.
The reason for this is that the water that goes down their burrows loses its oxygen as it sifts through the
soil. Since the water that reaches them is so poor in oxygen, they come to the surface to get it. If the
water contained sufficient oxygen, however, they would not have to surface.
One final point, living on land presents major problems that must be overcome in order to survive. The
most common problem is in acquiring enough water, and then keeping it (avoiding drying out). While
many organisms have evolved thick, impermeable skin, to help prevent drying out, the earthworm has
not. It must diffuse oxygen through its skin, so it must remain thin and moist. So while earthworms are
terrestrial animals (they live on land), they have not really evolved physical characteristics to survive in
this harsh environment. Instead they evade the problems of living on land by their behavior. For
instance, they burrow in damp soil to avoid high temperatures and drying out. When they do come to the
surface, it is during cool, wet times.
Figure 1. "Internal Features of the Earthworm"
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PROCEDURE:
1. Remember the following points when performing the dissection....
a. Refer to the information in the "introduction" of this lab while performing the dissection,
especially
when looking at the different organs.
b. Read the instructions carefully BEFORE you begin to cut.
c. Identify the structures to be cut, before cutting.
d. Cut only when and where you are directed to do so.
2. Notice the definite segmentation of the earthworm provided to you by your instructor.
3. Examine the external features of your earthworm carefully. What color is the earthworm? Is it
darker or lighter in certain regions? Is it thicker and thinner in different regions? Is it smooth, rough?
Make observations on your lab answer sheet.
4. Run your finger along the dorsal and ventral surfaces of the worm, from anterior to posterior. You
should feel the setae. Make a note as to which surface they were on.
5. Place the specimen in the dissecting pan, dorsal side up.
6. Pin the posterior end of the worm to the pan, but DO NOT PIN the anterior end.
7. Take your dissecting probe, and in a spot about one inch behind the clitellum, scrape the probe back
and forth until you actually make a little cut into the worm. Keep going in a back-and-forth motion
until you have cut through the body wall of the worm. If you happen to see black material coming
from the scrape... STOP. This means you have scraped all the way into the intestine of the worm.
8. Now, using this little cut as an opening, take your dissecting scissors, and extend the cut all the way
to the very anterior end of the worm. Make sure that you do not jab the scissors downward into the
worm as you cut, otherwise you will damage the internal organs. Take your time with the incision, as
this is the most important step of the dissection. See Fig. 2.
Figure 2 "Making your incision"
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9. Separate the edges of the cut and pin down the sides of the body wall so the organs can be observed.
When pinning down the walls, do not place the pins straight up and down, and they will get in your
way. Instead, pin them at angles going away from the worm.
10. Locate the pharynx without destroying any structures that may be near or on it.
11. The esophagus extends after the pharynx, but may be difficult to see, as other organs are on top of it
(e.g.. aortic arches)
12. Locate the thin walled crop. It will appear somewhat dark from the soil inside it. Take your
dissecting probe and touch the crop, confirming that it is very thin.
13. Locate the thick-walled gizzard, immediately following the crop. It will appear more yellow
because
of the muscle layers. Touch it with the probe confirming that it is thick and tough.
14. Locate the intestine. It is the narrow tube that food passes through after the gizzard. It extends the
rest of the length of the worm.
15. Cut through a piece of the intestine and look at it in cross section. You should see that one of the
walls folds in rather drastically. This increases the surface area of the intestine.
16. Locate the seminal vesicles in segments 9, 10, and 12 of the worm. They should appear to be larger
yellow structures.
17. Locate the seminal receptacles in segments 9 and 10. They should appear to be significantly smaller
than the seminal vesicles, but still yellow.
18. Locate the ovaries, if possible. Consult Figure #1 of this lab to help in the location. These
structures are more difficult to locate, however.
19. Locate the dorsal blood vessel. It is the dark blood vessel that runs in the medial surface of the
intestine.
20. Remove the seminal vesicles from one side of you worm by plucking them out with you forceps.
21. Locate the aortic arches (hearts) in segments 6-11 in your worm. With the seminal vesicles
removed,
they should be more visible. There are 5 pairs of arches which lie perpendicular to the digestive
tract.
22. Using your dissecting probe, push the intestine to the side so you can see structures that are ventral
to
the digestive tract.
a. Locate the ventral blood vessel. (It should appear dark)
b. Locate the ventral nerve cord. (it should appear as a white thread near the ventral blood
vessel.)
23. Using a dissecting microscope, if necessary, locate the nephridia. Nephridia are VERY tiny white
coiled tube-like structures in each segment along the digestive tract (except the first three and the
last segment)
24. At the beginning of the pharynx, and the very anterior end of your worm, locate the ganglion. This
will appear as two pin-point sized, light structures sitting side-by-side. You may actually see
threadlike nerves traveling out of the ganglion.
25. Clean up your lab station according to your instructor's guidelines. Do not throw worms or worm
parts in the sinks or the garbage, but rather put them in the container provided by your instructor.
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PRELAB QUESTIONS: In order to answer these questions you will need to read through the entire lab:
introduction, procedure, and materials list. After you have carefully read all of the lab, answer these
questions on another piece of paper and be prepared to turn the questions in at the beginning of class.
1. What is the purpose of organ systems?
2. In which phylum are segmented worms listed?
3. Besides the earthworm, give another example of a segmented worm.
4. Is your chin anterior to your tailbone?
5. On which surface is your "belly-button"?
6. What secretes the mucus that surrounds the eggs that are laid by the worms?
7. Describe what the ventral nerve cord will look like and where you will find it.
8. Can worms survive under water, even if the water is well-oxygenated?
9. Which end of the worm will be pinned down before any cuts are made?
10. Will you begin your first incision posterior or anterior to the clitellum?
ANALYSIS QUESTIONS:
Answer the following questions in complete sentences. Put your answers
in the space provided.
Make observations here:
1. Label the four parts of this diagram.
A=_______________
B= _______________
C= _______________
D=_______________
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2. Name the organ in the earthworm that stores the food.___________________
3. Name the organ in the HUMAN that stores food. ____________________
4. Name the organ in the earthworm that grinds the food. ____________________
5.
6.
7.
8.
Name the organ in the abdomen of a HUMAN that grinds food. ___________________
Where does most chemical digestion occur in the earthworm? ____________________
Where does most chemical digestion occur in the HUMAN? ______________________
What type of circulatory system does the earthworm have? Explain.
9. Why is it beneficial that the earthworm's intestine is so long and has a major fold? Explain.
10. Earthworms have both male and female reproductive organs. Rather than fertilizing their own eggs
with their own sperm, they mate with another worm, exchanging sperm that will be used to fertilize
their eggs. Why is this advantageous, when it would seem simpler for earthworms to have evolved
a method of fertilizing their own eggs with their own sperm?
11. What term describes organisms that have both male and female reproductive organs?
12. What functions do the setae serve?
13. If earthworms had thick tough skin they might be able to live on the surface of land, but if there
were no other changes, earthworms would die if they had this kind of skin. Explain.
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