Download earthworm dissection

EARTHWORM DISSECTION
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 cavity between the
outer and inner tubes is the coelom. In this activity you will
study the external and internal anatomy of the earthworm, and observe some
characteristics of annelids.
PART 1
PROCEDURES AND OBSERVATIONS
1) Examine the external structure of the earthworm. The thickened region, the
clitellum, is closer to the anterior end of the animal. The clitellum secretes a cocoon
around the fertilized eggs. The prostomium is the anterior tip of the earthworm.
The upper, or dorsal, surface of the worm feels smooth, while the lower, or ventral,
surface feels rough because of the setae, or bristles, which is the ventral surface of
the worm.
Count and record the number of segments between the prostomium and the clitellum.
2) Examine the anterior and posterior ends of the worm.
What are the two openings you see?
In addition to the openings in the first and last segments of the body, the earthworm
has several other types of openings. On the sides of most segments there are
excretory pores. On the ventral surface of segment 14 are pores through which eggs
are discharged. On the ventral surface of segment 15 are pores through which
sperm are discharged
3) Using a hand lens or dissecting microscope, examine each surface of the worm.
Can you see any of the openings described above? If so, which ones?
PART 2
1) Place your earthworm in the dissecting tray with the dorsal surface up and the
anterior end facing away from you. Place dissecting needles 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 or dissecting scissors, make an incision from behind the clitellum to the anus.
CAUTION: Handle the scalpel and scissors with caution throughout this lab
activity. Then turn the tray around and extend the incision to the mouth. Holding
the body wall with your forceps, use a scalpel or dissecting needle 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.
The mouth of the earthworm opens into the muscular pharynx, which sucks food
into the digestive tract. The pharynx is found within the first five or so segments.
Posterior to the pharynx is the esophagus, which extends for about ten segments.
The esophagus is a narrow tube that widens where it enters the crop, a thin-walled
organ in which food is temporarily stored. 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.
2. Beginning at the anterior end of the worm (segment one), identify the organs of
the digestive system. Use a probe to feel the relative thicknesses of the walls of the
crop and gizzard. Use your scalpel to make a cross-sectional cut through the
intestine about half way along its length. Make sure you cut only through the
intestine; do not cut any other body part. Examine the cut end of the intestine with a
hand lens or dissecting microscope. 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 segments seven and eleven. From the aortic
arches, blood flows into the ventral vessel, which runs beneath the organs of the
digestive tract. The ventral vessel branches and divides into smaller vessels,
eventually forming capillaries that serve the cells of the animal. The capillaries join,
forming larger vessels. Blood is returned to the aortic arches through the dorsal
vessel, which runs along the top of the digestive tract.
3) 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. Earthworms are hermaphroditic-they contain both
male and female reproductive structures. However, self-fertilization does not occur.
When earthworms mate, they exchange sperm, which later will fertilize the eggs
produced by the ovaries. Sperm are produced and stored in the seminal vesicles.
Sperm received from another animal in mating are stored in the two pairs of
seminal receptacles.
4) The most 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 nine and ten. The two pairs of testes are on the walls that separate
segments ten and eleven, and the ovaries are in segment thirteen. Try to identify the
various parts of the reproductive system. Use a hand lens or dissecting microscope
where necessary. The excretory organs of earthworms are the nephridia, which are
tiny, coiled, white tubules. Pairs of nephridia are found in all segments except the
first three and the last.
5) 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 cords. The brain is actually a pair of fused ganglia, 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 parts of the body.
6) The brain is a small mass of white tissue found on the dorsal surface of the
anterior end of the pharynx. Extending from 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. Extending from 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.
QUESTIONS TO PONDER
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) What is the proper name for the "tube-within-a-tube" body structure of the
earthworm?
4) How do you think the setae on each segment function in locomotion?
Modified from:
http://www.edu.pe.ca/montaguehigh/tpk9.htm
Montague Regional High School
Canada
Worm Terms
Include these terms in your Lab Report:
Anterior & Posterior
Aortic Arches
Brain
Clitellum
Coelum
Crop
Dorsal & Ventral
Dorsal Blood Vessel
Esophagus
Ganglia
Gizzard
Intestine
Nephridia
Ovary
Pharynx
Prostomium
Seminal Receptical
Seminal Vesicle
Setae
Testis
Ventral Blood Vessel
Ventral Nerve Cord