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Worms
Objectives
• Identify the three main groups of worms.
• Identify the characteristics of each group of worms.
• Visually identify the body parts of a dissected segmented worm.
• Describe and explain the functions of the body parts of a segmented
worm.
The BIG Idea
• Most worms have complex body systems.
Key Concepts
• All worms have some characteristics in common.
• Most flatworms are parasites.
• Roundworms are a diverse group of worms.
• Segmented worms have bodies that are divided into individual
compartments called segments.
• Segmented worms have a closed circulatory system.
All worms have some characteristics in common.
•
Biologists classify worms into several phyla.
•
The three major groups of worms are flatworms, roundworms, and
segmented worms.
•
All worms are invertebrates and their bodies are narrow and long
without legs.
•
Worms are simplest organisms with a brain.
•
They also exhibit bilateral symmetry.
•
Both sexual and asexual reproduction are found in the worm phyla.
•
In many species of worms, there are separate male and female
animals.
•
However, some species of worm have both male and female sex organs.
•
A worm with both sets of reproductive organs cannot reproduce with
itself.
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•
Some worms reproduce asexually by splitting.
•
Some worms can be cut into small pieces and each piece will grow into
a new worm.
•
Earthworms do not have this ability. However, if you were to cut its
tail off, the worm could regrow the tail.
•
The ability to regrow body parts is called regeneration.
What are the three types of worms? Flatworms, roundworms,
and segmented worms.
Most flatworms are parasites.
•
Flatworms are worms with the simplest bodies.
•
Some flatworms can grow to be 10 to 12 meters long (that’s between
30-40 feet, the length of a school bus, for you non-metric people.)
•
However, most flatworms are too small to be seen.
•
Most flatworms are parasites that obtain their food from their hosts.
•
This can make the host weak and sick. Sometimes the worm will kill
the host. More often than not, the host will survive.
•
One type of flatworm is the tapeworm.
o Its body is adapted to absorbing food from the host’s digestive
system.
o It can live in many organisms and can grow in humans.
•
Another type of flatworm are the planarians
o It is not parasitic, what we call free-living.
o They are scavengers feeding on the dead or decaying material.
o They are also predators attacking any animals smaller than they
are.
o If you look closely at a planarian’s head, you will see two dots
called eye-spots Like Euglena, the eyespots help them detect
light.
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o A planarian’s head also has cells that pick up odors. This is how
it finds its food.
o The planarian feeds like a vacuum cleaner.
o The planarian inserts a feeding tube into its food.
o Digestive juices flow out into the food, where they begin to
break down the food while it is still outside the worm’s body.
o The planarian then sucks up the partly digested food.
o The food is then digested in a cavity in the worm.
Which type of flatworm is parasitic? Tapeworm
What is the function of the odor cells in a planarian? The odor
cell helps it find food.
Roundworms are a diverse group of worms.
•
Roundworms can live in nearly any moist environment.
•
In one square meter of damp sand, you will find about 1 million round
worms.
•
Unlike flatworms, roundworms have round bodies.
•
Many roundworms are carnivores or herbivores
•
Some are parasites such as the hookworm which can be found in pet
dogs.
•
Unlike cnidarians or flatworms, roundworms have a digestive system
that is like a tube, open at both ends.
•
Food enters at the animal’s mouth and exits through an opening called
the anus.
•
Food travels in one direction through the roundworm’s digestive
system.
•
The advantage of this type of digestive process is that it enables the
animal’s body to use foods efficiently by enabling it to abosrb large
amounts of the needed substances in food.
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How are flatworm bodies different from roundworm bodies?
They are flat and roundworms are round.
How is a roundworm’s digestive system different than a
cnidarian or flatworm digestive system? The roundworm has a
tube system: open at both eneds.
Segmented worms have bodies that are divided into individual
compartments called segments.
•
Segmented worms occupy nearly all environments, and most live in
burrows or tubes
•
The burrow helps the worm hide both from possible predators and
possible prey.
•
When you look at an earthworm, you notice that its body seems to
consist of a series of rings separated by grooves. We call these
segments.
•
On the outside, segments look nearly identical.
•
On the inside, some organs are repeated in most segments.
•
Other organs, however, such as the worm’s reproductive organs, are
found only in some segments.
•
Nerve cords run the length of the worm’s body.
•
Like roundworms, earthworms have a one-way digestive system with
two opening.
Segmented worms have a closed circulatory system.
•
In a close circulatory system, blood moves through the body within a
connected network of tubes called blood vessels.
•
An open circulatory system allows blood to “slosh” around the inside of
the body.
•
A closed system allows organisms to be larger.
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•
An earthworm has five pumping organs that act like hearts.
What are segments? A series of rings separated by grooves.
What type of circulatory system do earthworms have? Closed.
Earthworms are scavengers.
•
Earthworms are scavengers that eat decayed plant and animal remains
in the soil.
•
Night is a safe time for an earthworm to crawl on the surface,
because many worm predators are asleep then.
•
At night the air is damp, and this keeps the worm’s skin moist.
•
If a worm dries out, it will die, because it obtains oxygen through
moisture on its skin.
•
Well-developed muscles let an earthworm move through its burrow.
•
Stiff bristles stick out from each of the worm’s segments.
•
Earthworms are among the most helpful inhabitants of garden and
farm soil.
•
They benefit people by improving the soil in which plants grow.
•
Earthworm droppings make the soil more fertile
•
Earthworm tunnels loosen the soil and allow air, water, and plant
roots to move through it.
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