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Biology
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27–2 Roundworms
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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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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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What Is a Roundworm?
This cavity is partially lined with tissue derived from the mesoderm and is called a pseudocoelom, meaning, “false coelom.”
Roundworms such as hookworms are unsegmented worms that have a pseudocoelom and a digestive system with a mouth and an anus. Roundworms develop from three germ layers, and a pseudocoelom forms between the endoderm and mesoderm layers.
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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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7. 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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8. 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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9. 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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10. 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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11. Human Ascarid Reproduction
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12. Roundworms and Human Disease
Parasitic roundworms include trichinosis-causing worms, filarial worms, ascarid worms, and hookworms.
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13. 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.
50 µm
Encysted juveniles
Muscle tissue
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14. 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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15. 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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16. 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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17. Roundworms and Human Disease
Ascaris Life Cycle 1
Human ingests food or water containing Ascaris eggs.
Ascaris lumbricoides fill the host’s intestine. These worms absorb the host’s digested food and can cause severe malnutrition. Blockage of the intestine can be severe enough that it causes death.
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18. Roundworms and Human Disease
Ascaris Life Cycle 2
The eggs travel to the small intestine and develop into larvae.
Ascaris lumbricoides fill the host’s intestine. These worms absorb the host’s digested food and can cause severe malnutrition. Blockage of the intestine can be severe enough that it causes death.
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19. Roundworms and Human Disease
Ascaris Life Cycle 3
Larvae enter blood vessels and are carried to the lungs
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20. 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
Ascaris lumbricoides fill the host’s intestine. These worms absorb the host’s digested food and can cause severe malnutrition. Blockage of the intestine can be severe enough that it causes death.
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21. Roundworms and Human Disease
Ascaris Life Cycle 5
Eggs are released and leave the host in faces.
Ascaris lumbricoides fill the host’s intestine. These worms absorb the host’s digested food and can cause severe malnutrition. Blockage of the intestine can be severe enough that it causes death.
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Ascarids
Ascarids taken from children of one village.
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Heartworm
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24. 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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25. 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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Rare Nematode Attacks!
Gigantus wormius
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27–2
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27–2
In roundworms, the body cavity that forms between the endoderm and mesoderm is the
ganglion.
hydrostatic skeleton.
pseudocoelom.
coelom.
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27–2
All of the following are parasitic roundworms EXCEPT
tapeworms
filarial worms
hookworms
ascarid worms
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27–2
Characteristics of roundworms include a digestive system with
one opening and a pseudocoelom.
one opening but no pseudocoelom.
two openings and a pseudocoelom.
two openings but no pseudocoelom.
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27–2
Gas exchange and excretion of metabolic wastes in roundworms occurs
via a complex system of alveoli.
through their body walls by diffusion.
through excretory tubules.
by flame cells.
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27–2
The roundworms called ascarids cause harm by
causing serious body swelling.
burrowing into body tissues and causing pain.
causing malnutrition.
causing internal bleeding.
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