Biology
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27–2 Roundworms
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27–2 Roundworms
What Is a Roundworm?
What Is a Roundworm?
Most species of roundworms are free-living,
inhabiting soil, salt flats, aquatic sediments, and
water, from polar regions to the tropics.
Others are parasitic and live in hosts.
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27–2 Roundworms
What Is a Roundworm?
Roundworms are unsegmented worms
that have pseudocoeloms and digestive
systems with two openings—a mouth
and an anus.
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27–2 Roundworms
What Is a Roundworm?
This cavity is partially lined with tissue derived from
the mesoderm and is called a pseudocoelom,
meaning, “false coelom.”
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27–2 Roundworms
What Is a Roundworm?
Roundworms have a digestive tract with two
openings.
Food moves in one direction through the digestive
tract of roundworms.
Any food that is not digested leaves the body through
the anus.
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Form and Function in Roundworms
Feeding
Many free-living roundworms use grasping
mouthparts and spines to catch and eat other
small animals.
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27–2 Roundworms
Form and Function in Roundworms
Respiration, Circulation, and Excretion
Roundworms exchange gases and excrete
metabolic waste through their body walls.
They depend on diffusion to carry nutrients and
waste through their bodies.
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27–2 Roundworms
Form and Function in Roundworms
Movement
Fluid in the pseudocoelom and muscles
extending the length of their bodies function as a
hydrostatic skeleton.
Aquatic roundworms contract muscles to move like
snakes through the water.
Soil-dwelling roundworms push their way through
the soil by thrashing around.
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27–2 Roundworms
Form and Function in Roundworms
Reproduction
Roundworms reproduce sexually.
Most species have separate sexes.
Parasitic roundworms often have life cycles that
involve two or three different hosts or several
organs within a single host.
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27–2 Roundworms
Human Ascarid Reproduction
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27–2 Roundworms
Roundworms and Human Disease
Roundworms and Human Disease
Parasitic roundworms include
trichinosis-causing worms, filarial
worms, ascarid worms, and hookworms.
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27–2 Roundworms
Roundworms and Human Disease
Trichinosis-Causing Worms
Adult Trichinella worms live and mate in the
intestines of their hosts.
Larvae travel through the bloodstream and burrow
into organs and tissues.
Encysted juveniles
Muscle tissue
50 µm
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27–2 Roundworms
Roundworms and Human Disease
Trichinella completes its life cycle only when another
animal eats muscle tissue containing these cysts.
Humans can get trichinosis by eating raw or
incompletely cooked pork.
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27–2 Roundworms
Roundworms and Human Disease
Filarial Worms
Filarial worms are threadlike worms that live in
the blood and lymph vessels of birds and
mammals.
They are transmitted by mosquitoes.
Causes Elephantiasis
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27–2 Roundworms
Roundworms and Human Disease
Ascarid Worms
Ascaris lumbricoides is a serious parasite of
humans and many other vertebrate animals.
It absorbs digested food from the host’s small
intestine.
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27–2 Roundworms
Roundworms and Human Disease
Ascaris Life Cycle
1 Human ingests food or
water containing Ascaris
eggs.
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Roundworms and Human Disease
Ascaris Life Cycle
2 The eggs travel to
the small intestine
and develop into
larvae.
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Roundworms and Human Disease
Ascaris Life Cycle
3 Larvae enter blood
vessels and are
carried to the lungs
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Roundworms and Human Disease
Ascaris Life Cycle
4 Larvae are coughed
up and swallowed.
They then travel to
the small intestine
where they develop
to maturity
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Roundworms and Human Disease
Ascaris Life Cycle
5 Eggs are released
and leave the host
in faces.
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27–2 Roundworms
Ascarids
Ascarids taken from children of one village.
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27–2 Roundworms
Heartworm
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Roundworms and Human Disease
Hookworms
Hookworm eggs hatch and develop in the soil.
They use sharp toothlike plates and hooks to
burrow into the skin and enter the bloodstream.
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Roundworms and Human Disease
Hookworms travel through the blood of their host to
the lungs and down to the intestines.
There, they suck the host’s blood, causing weakness
and poor growth.
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27–2 Roundworms
Rare Nematode Attacks!
Gigantus wormius
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27–2
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In roundworms, the body cavity that forms
between the endoderm and mesoderm is the
a. ganglion.
b. hydrostatic skeleton.
c. pseudocoelom.
d. coelom.
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All of the following are parasitic roundworms
EXCEPT
a. tapeworms
b. filarial worms
c. hookworms
d. ascarid worms
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Characteristics of roundworms include a
digestive system with
a. one opening and a pseudocoelom.
b. one opening but no pseudocoelom.
c. two openings and a pseudocoelom.
d. two openings but no pseudocoelom.
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Gas exchange and excretion of metabolic
wastes in roundworms occurs
a. via a complex system of alveoli.
b. through their body walls by diffusion.
c. through excretory tubules.
d. by flame cells.
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The roundworms called ascarids cause harm by
a. causing serious body swelling.
b. burrowing into body tissues and causing
pain.
c. causing malnutrition.
d. causing internal bleeding.
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