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KEY CONCEPT
Cnidarians and worms
have different body plans.
BEFORE, you learned
NOW, you will learn
• Invertebrates are a diverse
group of animals
• Sponges are sessile organisms
• Sponges meet their needs with
simple bodies and specialized
cells
• About body systems in
cnidarians
• About body symmetry and
feeding patterns
• About body systems in worms
VOCABULARY
EXPLORE Worm Movement
cnidarian p. 128
tentacle p. 128
mobile p. 130
How does body shape affect movement?
PROCEDURE
1
Put a thin layer of soil on the tray and
gently place the worm on it. Use the spray
bottle to keep the soil and your hands moist.
2 Draw a sketch of the worm and try to
MATERIALS
•
•
•
•
worm
tray
soil
spray bottle filled
with distilled water
identify the parts of its body.
3 Record your observations of its movement.
4 Follow your teacher’s instructions in handling the worm
and materials at the end of the lab. Wash your hands.
WHAT DO YOU THINK?
• How does the shape of a worm’s body
affect its movement?
• How would you describe a worm to
someone who has never seen one?
FPO
Cnidarians have simple body systems.
COMBINATION NOTES
Make notes and diagrams for the main idea:
Cnidarians have simple
body systems.
Cnidarians (ny-DAIR-ee-uhnz)
are invertebrates. Like sponges, cnidarians are found only in water. This group includes jellyfish, corals, sea
anenomes, and small freshwater organisms called hydras. Most cnidarians feed on small plankton, fish, and clams. Many cnidarians are sessile
for most of their lives. Like sponges, cnidarians have adaptations that
allow them to pull food in from the water that surrounds them.
All cnidarians have tentacles, fingerlike extensions of their body
that reach into the water. Other animals have tentacles, but the tentacles
of cnidarians have specialized stinging cells. The tentacles, with their
stinging cells, are an adaptation that enables cnidarians to capture prey.
128 Unit 1: Diversity of Living Things
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Stinging Cells in Cnidarians
Jellyfish have specialized cells on their tentacles.
Stinging Cell
Each stinging cell contains a
nematocyst, a capsule with
a coiled filament inside.
trigger
coiled
filament
released
filament
nematocyst
stinging cell
tentacles
Each stinging cell has a nematocyst (NEHM-uh-tuh-SIHST),
a capsule that holds a barbed filament. The filament is like a tiny hollow
tube coiled up inside the capsule. When prey comes into contact with
the stinger cell, the filament is released. Sometimes this stinger wraps
itself around the prey. In most species of cnidarians, the stinger stabs
the prey and releases a poison from its tip. These stingers are what produce the sting of a jellyfish. Stinging cells have a second function. They
protect cnidarians from predators.
Check Your Reading
reading tip
Refer to the diagram above
as you read the description
of a nematocyst and its
structure.
Describe how the structure of a nematocyst allows it to
function in capturing food and providing protection.
Tissues and Body Systems
A cnidarian’s body is made up of flexible layers of tissue. These tissues, along with specialized cells, make up its body systems. The
tissues are organized around a central opening where food is taken in
and wastes are released. The tentacles bring the prey into this opening.
The opening leads into a cavity, a gut, where the food is digested.
Cnidarians have a simple muscle system for movement. Even
though cnidarians are sessile during most of their lives, they still move
their bodies. Cnidarians bend from side to side and extend their
tentacles. Adult jellyfish swim. The movement is produced by muscle
cells that run around and along the sides of its body. When the muscle
cells shorten, or contract, they produce movement.
Chapter 4: Invertebrate Animals 129
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When cnidarians move, they interact with the environment. They
sense and respond to the prey that come in contact with their
tentacles. This behavior is the result of a simple nervous system.
Cnidarians have a network of nerve cells, a nerve net that extends
throughout their bodies.
VOCABULARY
Choose a strategy from earlier chapters, such as a four
square, or one of your own
to take notes on the term
mobile.
Reproduction
Cnidarians reproduce both sexually and asexually, and water plays a role
in both processes. Buds produced by asexual reproduction are carried
away from the sessile parent by water. In sexual reproduction, sperm are
carried to the egg. Fertilization results in a free-swimming larva. The
larva, if it survives, develops into an adult.
Jellyfish are cnidarians with a life cycle that includes several stages.
A jellyfish’s body, or form, is different at each stage, as you can see in
the diagram below. When a jellyfish larva settles on the ocean floor, it
grows into a form called a polyp. The polyp, which is sessile, develops
disk-shaped buds that stack up like a pile of plates. The buds, once
they are released, are called medusas. Each medusa is an adult jellyfish. In the medusa stage, jellyfish are mobile, which means they can
move their bodies from place to place.
Jellyfish Life Cycle
Young Medusa
Jellyfish have a two-part life cycle that includes both a mobile
and a sessile form.
1
Adult female medusa
releases fertilized eggs.
Column of Discs
3
Disks begin to form
and later separate.
freeswimming
larva
late
polyp
2
polyp
130 Unit 1: Diversity of Living Things
Larva settles on
ocean floor.
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Radial Symmetry
Bilateral Symmetry
mouth
mouth
Animals with a circular body shape, such as this
sea anemone, have radial symmetry.
Animals with identical right and left sides, such
as this butterfly, have bilateral symmetry.
Animals have different body plans.
Scientists sometimes use the term body plan to describe the shape of
an animal’s body. Most cnidarians have a body plan with radial symmetry. This means the body is organized around a central point, a
mouthlike opening that leads into a gut. You can see from the diagram
of the jellyfish life cycle on page 130 that both the polyp and medusa
have radial symmetry.
A radial body plan allows a sessile organism, such as the sea
anenome shown in the photograph above, to capture food from any
direction. A radial body plan also affects how a mobile animal moves.
A jellyfish medusa moves forward by pushing down on the water. It
has to stop moving to change direction.
Most animals, including worms, butterflies, birds, and humans,
have a body plan with bilateral symmetry. One half looks just like the
other, as you can see in the photograph of the butterfly above. You can
recognize a bilaterally symmetrical shape because there is only one
way to draw a line dividing it into two equal halves.
Animals with bilateral symmetry have a forward end where the
mouth is located. This is the animal’s head. The animal moves forward, head first, in search of food. A bilateral body shape works well
in animals that are mobile. Food enters at one end, and is processed as
it moves through the body. Once all the nutrients have been absorbed,
the remaining wastes are released at the other end.
Check Your Reading
reading tip
The root of the word radial
means “ray,” like the spoke
of a wheel. The roots of the
word bilateral are bi-,
meaning “two,” and lateral,
meaning “side.”
Describe how radial symmetry and bilateral symmetry affect an
animal’s feeding behaviors.
Chapter 4: Invertebrate Animals 131
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Most worms have complex body systems.
Some worms have simple bodies. Others have well-developed body
systems. Worms have a tube-shaped body, with bilateral symmetry. In
many worms, food enters at one end and is processed as it moves
through a digestive tract. Worms take in oxygen, dissolved in water,
through their skin. Because of this, worms must live in moist environments. Many live in water.
RESOURCE CENTER
CLASSZONE.COM
Learn more about the
many types of worms.
Segmented Worms
Segmented worms have bodies that are divided into individual
compartments, or segments. These worms are referred to as annelids
(AN-uh-lihdz), which means “ringed animals.” One annelid you might
be familiar with is the earthworm. As the diagram below shows, an
earthworm’s segments can be seen on the outside of its body.
An earthworm has organs that are organized into body systems.
The digestive system of an earthworm includes organs for digestion
and food storage. It connects to the excretory system, which removes
waste. Earthworms pass soil through their digestive system. They
digest decayed pieces of plant and animal matter from the soil and
excrete what’s left over. A worm’s feeding and burrowing activity adds
nutrients and oxygen to the soil.
Check Your Reading
Name two body systems found in earthworms.
Inside an Earthworm
An earthworm has organs and body systems.
brain
Digestive
System
mouth
nerve
cord
digestive
tract
hearts
blood
vessels
muscle
tissue
132 Unit 1: Diversity of Living Things
excretory
organ
Nervous
System
Circulatory
System
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Earthworms have several layers of muscle tissue in their
body wall. Hairlike bristles on the segments help to anchor
a worm in the soil as it moves. The nervous system
includes a brain and a nerve cord that runs through the
body. An earthworm can detect strong light and vibrations
in the soil. These stimuli signal danger to a worm. An
earthworm also has a circulatory system. It is made up of
several hearts that pump blood through blood vessels.
Some annelids reproduce asexually, while others reproduce sexually. There are no distinct male or female worms.
Earthworms, for example, carry both male and female reproductive structures. To reproduce, two worms exchange
sperm. The sperm fertilize eggs the worms carry in their
bodies. The eggs are laid and later hatch into larvae.
Three Types of Worms
segmented worm
Flatworms and Roundworms
Flatworms are the worms with the simplest bodies. Some
are so small and flat that they move with cilia, not muscles.
These flatworms absorb nutrients directly through the
skin. Many flatworms live as parasites, feeding off other
organisms. For example, tapeworms are flatworms that
infect humans and other animals. The tapeworm has no
need for a digestive system because it gets digested nutrients from its host.
Roundworms are found just about everywhere on
Earth. The bodies of roundworms are more complex than
those of flatworms. They have muscles to move with, and a
nervous system and digestive system. Some roundworms
are important decomposers on land and in the water.
KEY CONCEPTS
CRITICAL THINKING
1. What adaptation do cnidarians
have for capturing prey?
4. Predict How might having
sense organs located at the
front end of the body benefit
an animal?
2. What is the difference
between radial symmetry and
bilateral symmetry?
3. Pick two systems found in an
earthworm and describe how
they work together.
5. Infer Would the food of a jellyfish medusa be different from
the food of a polyp? Support
your answer.
flatworm
roundworm
Segmented worms, flatworms, and
roundworms are the most common
worms on Earth.
CHALLENGE
6. Compare and Contrast
Describe how the different
body symmetries of cnidarians
and segmented worms affect
their movement and feeding
behaviors.
Chapter 4: Invertebrate Animals 133