Download Earthworm Dissection

Earthworm Dissection
Earthworms belong to the phylum annelida and the class oligochaeta. The earthworm is a burrowing animal that literally
eats its way through the soil. As the soil passes through the earthworm’s digestive tract, decaying plant material is digested.
PROCEDURE
Part A – External Anatomy
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Place a well rinsed preserved earthworm in a dissecting pan.
Examine the ring-like segments that make up the length of the earthworm’s body.
Identify the anterior (head) and posterior (tail) end. At the anterior end is a small lobe on the ventral (lower) surface
called the prostomium which is used for digging and burrowing. Just behind the lobe is the mouth which opens into
a long, tube-like digestive system that ends at the anus. Note that the dorsal (upper) surface of the worm is a darker
color, whereas the ventral surface of the earthworm is lighter in color.
Rub the ventral surface of the earthworm in both directions with your finger. The bristles you feel are called setae.
The setae are used in locomotion and anchoring the worm in soil.
Examine the prominent band-like structure called the clitellum. It is located one-third of the way from the anterior
end of the worm’s body. This structure secretes mucus that is used in reproduction.
Each segment (except the first three and the last one) has nephridiopores. These small openings connect with
metanephridia, which are the primitive kidneys of the earthworm. Liquid wastes, which collect in the body cavity,
are excreted through these pores. Locate these pores.
Earthworms do not have lungs.
Make a lateral sketch of an earthworm in the space below. Label the anterior, posterior, a segment, the mouth, setae,
clitellum and a nephridiopore.
Part B – Internal anatomy
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Stretch out the earthworm in the dissecting pan so that the dorsal surface faces up. Place a pin as close as you can
to each end of the worm.
Make a shallow cut about 10 segments posterior to the clitellum and to the right or left of the dorsal blood vessel
that you can see as a dark line running down the “back.”
Continue cutting carefully towards the anterior of the worm, by spreading and pinning the skin and muscle to the
dissection pan as you go. You worm should like figure 1 below.
Examine the space between the body wall and the internal organs. This cavity is called the coelom (segmented
worms are eucoelomate) and is divided into compartments by septa. Remember, this cavity is filled with fluid in the
living earthworm and gives support to the body.
Figure 1
Locate and identify the following systems and anatomical structures that are printed in bold. Label them
on Figure 2.
Digestive System
The pharynx is just posterior to the opening for the mouth. The pharynx is an enlarged, muscular structure that helps
ingest food. The narrower esophagus leads from the pharynx to the thin-walled crop, an enlarged storage sac. Food
passes from the crop to the muscular gizzard. The earthworm swallows small stones that help grind food in the gizzard.
Ground up food passes to the intestine where chemical digestion and absorption occurs. The solid wastes pass out
through the anus (earthworms have complete digestive systems).
Respiratory System
Earthworms do not have lungs. Oxygen and carbon dioxide pass through the earthworm’s skin through diffusion. For
diffusion to occur, the skin of an earthworm must remain moist. Body fluid and mucous is released to keep the skin
moist.
Circulatory System
The earthworm has a closed circulatory system. Five dark vessels called aortic arches surround the esophagus. These
five vessels pump blood posteriorly through the ventral blood vessel located under the digestive system and anteriorly
through the dorsal blood vessel located above the digestive system. The ventral blood vessel supplies blood to the
posterior of the worm and the dorsal blood vessel supplies blood to the anterior of the worm.
Nervous System
The small white brain has two lobes and is located anterior and dorsal to the pharynx in the area of the third segment. A
nerve ring leaves the brain and connects to the white ventral nerve cord that runs the length of the worm. From this
nerve cord arise smaller nerves that go out to various structure in each segment of the worm’s body.
Excretory System
Each segment contains a pair of excretory structures called metanephridia. These coiled tubular structures, lying next to
the body wall, open to the exterior by a pore called a nephridiopore. Internally, they are connected to the septum of the
segment just anterior to them. Each nephridium collects fluid wastes from the segment anterior to the segment in which
it is located.
Reproductive System
Each earthworm has both male and female reproductive organs and is said to be a hermaphrodite. Earthworms, though
hermaphroditic, mate with other earthworms. They don’t fertilize their own eggs. The seminal vesicles are the three
large white structures on each side of the esophagus. These vesicles hold a worm’s own sperm. In mature specimens,
the lobed, sperm-producing testes are in segments ten and eleven below the seminal receptacles. The smaller round
white structures anterior to the testes are the seminal receptacles. They receive and store sperm from another
earthworm. The egg-producing ovaries are in segment 13, posterior to the testes. The testes and ovaries are very small
and will be difficult to find. Two worms come together along their ventral sides and release sperm cells from the seminal
vesicles into the seminal receptacles of the opposite worm. The animals separate and the clitellum of each worm then
secretes a mucus ring. The worm then "backs" out of this ring. As it slides over the anterior segments, it picks up eggs
from the oviducts in the 14th segment and then slides off the anterior end of the worm to form the egg cocoon, from
which the young eventually hatch.
Color the organs/systems in Figure 2 according to the following instructions.
Digestive system – yellow
Circulatory system – red
Nervous system – blue
Reproductive system – green
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Which surface of the earthworm is darker in color-dorsal or ventral?
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What is the function of the setae?
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In which direction (anterior or posterior) do they point?
List the sequence of structures that food passes through after being taken in by the earthworm.
4. Explain why an earthworm’s skin needs to remain moist.
5. The earthworm does not undergo self-fertilization, so what advantage does it have to be hermaphroditic?
6. Describe how the clitellum in used in helping an earthworm reproduce.
7. How does the coelom of an earthworm compare to the coelom of a flatworm and roundworm?
8. Name two other animals that are found in the phylum annelida (segmented worms)