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52
Date.
Anatomy of the Earthworm
BACKGROUND
The earthworm is the best-known member of the phylum Annelida, the segmented worms.
Annelids are bilaterally symmetrical, and their bodies are divided into segments both externally and internally. They have a tube-within-a-tube
body structure. The outer tube is the
body wall, while the inner tube is the digestive tract. The c,wity between the outer and inner
tubes is the coelom.
OBJECTIVES
In this activity you will:
1. Study the external and internal anatomy of the earthworm.
2. Observe some characteristics of annelids.
MATERIALS
preserved earthworm (Lumbricus
dissecting tray
scalpel
dissecting scissors
straight pins
sp.)
hand lens or dissecting
forceps
probe
microscope
.
dissecting
needles
PROCEDURES AND OBSERVATIONS
Part I. External Anatomy of the Earthworm
a. Examinethe external structure of the earthworm.
The thickened region, the clitellum, is closer to
the anterior end of the animal. The clitellurrisecretes a cocoon around the fertilized eggs. The
upper, or dorsal, surface of the worm feels
smooth, while the lower, or ventral, surface feels
bristly. Determine which is the dorsal and which
is the ventral surface of the worm.
In addition to the openings in the first and last
segments of the body, the earthworm has sev~ral
othet types of openings found in most segments.
On the sides of most segments there are excretory
pores. On the ventral surface of se~ment 14 are
pores through which eggs are dischar~ed. On the
ventral surface of segment 15 are pores through
which sperm are discharged.
1. Why does the ventra"1surface feel bristly?
c. Using a hand lens or dissecting microscope,
examine each surface of the worm.
4. Can you see any of the openings described
above? If so, which ones?
2. How many bristles are on each segment?
b.
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Examine the anterior and posterior ends of the
worm.
3;"What
are the two openings you see?
~.
~\.~~,
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Part II. Internal Anatomy of the Earthworm
a. Place your earthworm in the dissecting tray with
the dorsal surface up and the anterior end facing
away from you. Place pins through .the first and
last segments to hold the worm in position. In
making an incision you must be careful to cut
only the body wall. If you cut too deeply, you will
damage the internal organs. The incision should
be slightly to one side of the midline. Using a
sharp scalpel,.make an incision from behind the
ditellum to the anus. Then turn the tray around
and extend the incision to the mouth. Holdingthe
body wall with your forceps, use a scalpel or pin
to cut the membranes that separate the segments
of the earthworm. Starting at the anterior end,
separate the body wall along the cut, and pin it
down, as shown in Figure 1.
FIGURE 1
forceps
1. How does the internal segmentation of the
earthworm compare with the external segmentation?
.
.,The mouth of the earthworm opens into the
muscular pharynx, which sucks food into the digestive tract. The pharynx is found within the
first 5 or so segments. See Figure 2. Posterior to
the pharynx is the esophagus, which extends for
about 10 segments. The esophagus is a narrow
tube that widens where it enters the crop, a
thin-wall orga.n in which food is stored temporarily. Posterior to the crop is the thick-walled gizzard, where food is broken down mechanically.
From the gizzard, food passes into the intestine,
which extends posteriorly to the anus.
b. Beginning at the anterior end of the worm (segment 1), identify the organs of the digestive systerti. Use a probe to feel the relative thicknesses
FIGURE2
seminal
receptacle
of the walls of the crop and gizzard. Use your
scalpel to make a cross section through the intes-
tine about half,wayalongits length.Examinethe
cu.tend of the intestine with a hand lens or dis-.
secting microscope.
dorsal blood vessel
. gizzard
ventral blood vessel
1
22B
5
10
15
20
25
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Name,
Date.
2. Draw a cross section of the earthworm
,
through the intestine. Label the body wall,
coelom, intestinal wall, and intestinal opening. If you can see the typhlosole, include it
in your drawing and label it.
The earthworm has a closed circulatory system. Blood is pumped through vessels by five
pairs of aortic arches, or hearts. The aortic arches
encircle the esophagus between sewnents 7 and
11. From the aortic arches, blood flows into the
ventral vessel, which nms beneath the organs of
the digestive tract. The ventral vessel branches
and divides into small~r vessels, eventually fomling capillaries that serve the cells of the animal.
The capillaries join, forming larger vesse Is. Blood
is returned to the aortic arches through the dorsal
vessel, whichnms
along the top of the digestive
tract
c. Identify the dorsal vessel/' which runs along the
top of the intestine. Follow it forward toward the
esophagus. Gently move aside any organs that
obscure your view so that you can see the aortic
arches around the esophagus: Lift the cut end of
the intestine so that you can see the ventral vessel, which runs along the ventral surface of the
.
digestive tract.
3. Describe the dorsal vessel,
arches, and the ventral vessel.
the aortic
I
4. Can you see branches of the dorsal and ven" tral vessels? If so, where?
Earthworms are hermaphroditic-they contain
both male and female reproductive structures.
However, self-fertilization does not occur. When
'earthworms mate, they exchange sperm, which
then fertilize the ~ggs produced by the ovaries.
Spe,rm are produced and stored in the seminal
vesicles. Sperm received from another animal in
mating are stored in the two pairs of seminal receptacles.
d. TI).emost visible parts of the reproductive system
are the pair of three-lobed seminal vesicles on
either side of the esophagus. The seminal receptacles are in segments 9 and 10. The two pairs of
testes are on the walls that separate segments 10
and 11, and the,ovaries areJn segment 13. Try to
identify the various parts of the reproductive system. Use a hand lens or dissecting microscope
where necessary.
5. List the parts of the reproductive system,
that you could see.
The excretory organs of earthworms E\re the
nephridia, which are tiny; coiled, white tubules.
Pairs of nephridia are' found in all segments except the first three and the last.
e. Using a hand lens or dissecting microscope, try to
identify a nephridium.
The earthworm has a central nervous system
made up of a brain and a pair of solid, ventral
nerve cords. The. brain is actually a pair of fused
gat:lglia,and the nerve cords enlarge into ganglia
in each segment A peripheral nervous system
consisting of nerves branching from the central
nervous system serves all part of the body.
f. The brain is a small mass of white tissue found on
the dorsal surface of the anterior end of the
pharynx. Extendingfrom the brain and running
around either side of the pharynx are nerve
cords. Gently move the pharynx and trace the
nerve cords. Beneath the pharynx is another pair
of ganglia. Extendingfrom these ganglia are the
pair of ventral nerve cords. Gently move any organs that are in the way, and identify as many
parts of the nervous system as you can.
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6. What parts of the nervous system could you
see?
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CONCLUSIONS AND APPLICATIONS
1. What advantage does hermaphroditism have for slow-moving organisms such as the earthworm?
2. In what ways does the internal structure of the earthworm show development of a specialized "head'
end?
3. In tenns of what you actually observed
structure of the earthwonn.
in your dissection,
describe
the tuoe-within-a-tube
body
4. How do you think the bristles on each segment function in locomotion?
5. List the general characteristics of annelids. (Use your textbook or an outside reference book to make
sure you have all the important characteristics.) Then describe how the structures and characteristics
of the earthworm compare with those of annelids in general.
( (