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27–2 Roundworms
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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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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.”
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. This body plan is often called a “tube-within-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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5. 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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6. 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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7. 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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8. 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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9. 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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10. 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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11. 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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12. 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 of the body swells enormously with blocked fluid.
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13. 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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14. 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
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. 4
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.
Eggs are released and leave the host in feces. 5
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15. 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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16. 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
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.
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 weakness and poor growth.
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