27–2 Roundworms
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27–2 Roundworms
What Is a Roundworm?
What Is a Roundworm?
Phylum Nematoda
Round, unsegmented worms with tapering ends
ranging in size from microscopic to a meter in length.
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 develop from three germ layers
(trioploblastic) and do have a body cavity between
the endoderm and mesoderm tissues.
This cavity is only 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. This body plan is often called a “tubewithin-a-tube” model.
The inner tube is the digestive tract, and the outer
tube is the body wall.
Food moves in one direction through the digestive
tract of roundworms.
Any food that is not digested leaves the body through
the anus, the posterior opening of the digestive tract.
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27–2 Roundworms
Form and Function in Roundworms
Form and Function in Roundworms
Roundworms have specialized tissues and organ
systems that carry out essential physiological
functions.
Feeding
Many free-living roundworms use grasping
mouthparts and spines to catch and eat other
small animals.
Some soil dwelling and aquatic forms eat algae,
fungi, or pieces of decaying organic matter.
Others digest the bacteria and fungi that break
down dead animals and plants.
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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
Response
Roundworms have simple nervous systems,
consisting of several ganglia.
Several nerves extend from ganglia in the head
and run the length of the body.
These nerves transmit sensory information and
control movement.
They also have several sense organs to detect
chemicals given off by prey or hosts.
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27–2 Roundworms
Form and Function in Roundworms
Movement
Fluid in the pseudocoelom and muscles
(longitudinal only) extending the length of their
bodies function as a hydrostatic skeleton.
Side-to-side flipping causes squishing and
extending of the body as muscles are contracted
or relaxed.
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: dioecious.
Roundworms reproduce using internal fertilization.
Usually, the male deposits sperm inside the
female’s reproductive tract.
Parasitic roundworms often have life cycles that
involve two or three different hosts or several
organs within a single host.
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Roundworms and Human Disease
Roundworms and Human Disease
Parasitic roundworms include trichinosis-causing
worms, filarial worms, ascarid worms, and
hookworms.
Trichinosis-Causing Worms
Trichinosis is a disease caused by the roundworm
Trichinella.
Adult Trichinella worms live and mate in the
intestines of their hosts.
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27–2 Roundworms
Roundworms and Human Disease
Female worms carrying fertilized eggs burrow into the
intestinal wall and release larvae.
Larvae travel through the bloodstream and burrow
into organs and tissues.
The larvae form cysts and become inactive in the
host’s muscle tissue.
Trichinella completes its life cycle only when another
animal eats muscle tissue containing these cysts.
Two common hosts are rats and pigs.
Humans can get trichinosis by eating raw or
incompletely cooked pork.
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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 biting insects, especially
mosquitoes.
Large numbers of filarial worms may block the
passage of fluids within lymph vessels and cause
swelling.
This causes elephantiasis, where the affected part
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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 and causes malnutrition in more than 1
billion people worldwide.
Ascaris lumbricoides is commonly spread by eating
foods that are not washed properly, like vegetables
and fruits.
A single ascaris can reach a length of 50 cm in the
human intestine!
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Roundworms and Human Disease
Ascaris Life Cycle
1 Human ingests food or
water containing Ascaris
eggs.
3 Larvae enter blood
vessels and are
carried to the lungs
4 Larvae are coughed
up and swallowed.
They then travel to
the small intestine
where they develop
to maturity
2 The eggs travel to
the small intestine
and develop into
larvae.
5
Eggs are released
and leave the host
in feces.
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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.
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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Roundworms and Human Disease
Research on C. elegans
This worm lives on rotting vegetation.
It is extraordinary because its DNA was the first of
any multicellular animal’s to be completely
sequenced.
It is being used to study how eukaryotes became
multicellular and how differentiation and development
take place.
The research may also shed light on how genes
make multicellular organisms both similar to and
different from one another.
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27–2
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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27–2
All of the following are parasitic roundworms
EXCEPT
a. tapeworms
b. filarial worms
c. hookworms
d. ascarid worms
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27–2
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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27–2
Gas exchange and excretion of metabolic
wastes in roundworms occurs
a. via a complex system of alveoli.
b. through their body walls.
c. through excretory tubules.
d. by flame cells.
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27–2
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 weakness and poor growth.
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