Most mollusks have shells, and echinoderms have spiny skeletons.

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KEY CONCEPT
Most mollusks have shells,
and echinoderms have spiny
skeletons.
BEFORE, you learned
NOW, you will learn
• Body shape affects how
animals move and behave
• Cnidarians have radial symmetry and simple body systems
• Worms have bilateral symmetry
and segmented body systems
• About different types of
mollusks and their features
• About different types of
echinoderms and their features
VOCABULARY
THINK ABOUT
mollusk p. 136
gill p. 137
lung p. 137
echinoderm p. 139
How does a snail move?
Snails belong to a group of mollusks
called gastropods. The name means
“belly foot.” Snails are often put into
aquariums to clean up the algae that
can build up along the walls of the
tank. If you get a chance, look at a
snail moving along the glass walls of
an aquarium, observe how it uses its
foot to move. How would you describe
the action of the snail’s foot?
foot
Mollusks are soft-bodied animals.
VOCABULARY
Choose a strategy from
earlier chapters, such as a
word triangle, or one of
your own to take notes on
the term mollusk.
One characteristic that is shared by all mollusks is a soft body. Many
of these invertebrate animals also have an outer shell to protect their
body. Oysters, clams, snails, and mussels are all mollusks. So are octopuses, squids, and slugs. Mollusks live on land and in freshwater and
saltwater environments. You will read about three groups of mollusks:
bivalves, gastropods, and cephalopods.
Most mollusks have well-developed organ systems. They have
muscles, a digestive system, a respiratory system, a circulatory system,
and a nervous system with sensory organs. Mollusks reproduce sexually, and in most species there are distinct male and female organisms.
Two adaptations distinguish mollusks as a group. First, all mollusks
have a muscular foot. A mollusk’s head is actually attached to its foot.
Second, all mollusks have a mantle, a layer of folded skin that protects
its internal organs.
136 Unit 1: Diversity of Living Things
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Bivalves
Bivalves are named for a hard shell that is made up of
two matching halves. Clams, mussels, scallops, and oysters are all bivalves. The shell, when it is closed,
completely encloses the body. If you’ve ever seen a raw
oyster, you know that a bivalve’s body looks like a mass
of tissue. Bivalves do not have a distinct head, but they
do have a mouth and sensory organs. The scallop
shown in the photograph has light-sensitive organs that
look like tiny eyes.
Bivalves are filter feeders, they filter food from the
surrounding water. To move, a bivalve balances upright,
opens its shell, and extends its foot. The animal moves
by pushing the foot in and out. The foot is also used for
burrowing, digging down into the sand.
Bivalve
Most of this blue-eyed scallop’s body
is inside its two-part shell.
The invertebrates you’ve studied so far—sponges, cnidarians, and
worms—take in oxygen all along the surface of their bodies. A bivalve
takes in oxygen through a pair of gills. A gill is an organ that filters
dissolved oxygen from water. The gill is an adaptation that allows an
organism to take in a lot of oxygen in just one area of its body. It is
made up of many folds of tissue that create a large surface area. Blood
picks up the oxygen and moves it to the rest of the animal’s body. In
most bivalves, the gills also filter food from the water.
Check Your Reading
RESOURCE CENTER
CLASSZONE.COM
Discover more about
mollusks.
What are the two functions of gills and how do those functions
relate to where bivalves live?
Gastropods
Gastropods are the most diverse group of mollusks.
Some, such as snails and slugs, live on land. Many
live in water, for example, conches, whelks, and
periwinkles. Many gastropods are protected by a
spiral-shaped shell. To protect itself, a gastropod
withdraws into the shell.
The gastropod’s head is located at the end of its
foot. The head has eyes and specialized tentacles for
sensing. Many gastropods have a cutting mouth part,
called a radula, that shreds their food. Some gastropods
eat animals, but most feed on plants and algae.
Gastropods that live in water have gills. Some gastropods that live on land have lungs. A lung is an organ
that absorbs oxygen from the air. Like gills, lungs have
a large surface area.
Gastropod
This brown-lipped snail extends most of
its body out of its shell when it moves.
Chapter 4: Invertebrate Animals 137
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Mollusks and Echinoderms
How do mollusk shells compare with
echinoderm skeletons?
PROCEDURE
1
SKILL FOCUS
Observing
MATERIALS
Closely observe the mollusk shells and skeletons of sea stars and
sand dollars you are given.
2 Examine the shape and texture of each. Sort them by their characteristics.
Selection of
mollusk shells,
sea stars, and
sand dollars
TIME
15 minutes
WHAT DO YOU THINK?
How are the shells and skeletons the same?
How are they different?
CHALLENGE Based on your observations,
what can you infer about the bodies of living
mollusks, sea stars, and sand dollars?
Cephalopods
Cephalopods (SEHF-uh-luh-PAHDZ) live in saltwater environments.
Octopuses, squids, and chambered nautiluses are cephalopods. Among
mollusks, cephalopods have the most well-developed body systems.
Cephalopods have a brain and well-developed nerves.
They have a pair of eyes near their mouth. The foot, which
surrounds the mouth, has tentacles for capturing prey. The
mantle is adapted to push water forcefully through a tubeshaped structure called a siphon. This produces a jet of
water that moves the animal. Gills take in oxygen, which is
picked up by blood vessels and pumped through the body
by three hearts.
Cephalopods
This Maori octopus has a welldeveloped head attached to a foot
with eight tentacles.
VISUALIZATION
CLASSZONE.COM
Watch how different
cephalopods move.
Octopuses and squids do not have protective shells.
They do have protective behaviors, however. Some can
change body color to match their surroundings. Some
release dark clouds of inklike fluid into the water, to confuse their predators. The lack of a shell lets them move
freely through the water.
The nautilus is the only cephalopod that has a shell. The shell is
made up of separate compartments, or chambers. The nautilus itself
lives in the outermost chamber. The inner chambers are filled with
gas, which makes the animal better able to float. The chambered shell
also provides the soft-bodied nautilus with protection from predators.
Check Your Reading
138 Unit 1: Diversity of Living Things
How is the foot of a cephalopod adapted for hunting?
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Mollusks show a range of adaptations.
You might not think that a clam would belong to the same
group as an octopus. These organisms look very different from
one another. They also interact with the environment in different ways. The great variety of mollusks on Earth today provides
a good example of how adaptations within a group can lead to
great diversity. A good example of this is the range of adaptations shown in the shape and function of a mollusk’s foot.
The foot of the bivalve is a simple muscular structure that
moves in and out of its shell. The foot allows a bivalve to crawl
along the ocean floor and to bury itself in the sand.
Gastropods have a head at the end of the foot, which runs the
length of the body. Muscles in the foot produce ripples that
allow the gastropod to glide over a surface as it searches for food.
In cephalopods, the foot has tentacles to pull food into its mouth.
The tentacles also help some cephalopods move along the ocean floor.
foot
COMPARE How does the
foot of this clam compare
with the foot of an
octopus?
Echinoderms have unusual adaptations.
Echinoderms are
a group of invertebrates that live in the ocean. In
their adult form, their bodies have radial symmetry. Sea stars, sea
urchins, sea cucumbers, and sand dollars belong to this group.
Echinoderms feed off the ocean floor as they move along. An echinoderm’s mouth is located at the center of the body, on the underside.
Some echinoderms, such as sea urchins and sand dollars, filter food
from their surroundings. Others, such as sea stars, are active predators
that feed on clams, snails, and even other echinoderms.
Spines and Skeletons
Echinoderm means “spiny-skinned.” Some of the more familiar
echinoderms have long, sharp spines, like the sea urchin in the
photograph at the bottom of this page. However, some echinoderm
species, such as sea cucumbers, have spines that are very small.
COMBINATION NOTES
Remember to take notes
and make sketches for
the main idea:
Echinoderms have unusual adaptations.
This purple sea urchin has
very obvious spines.
One unusual adaptation that echinoderms have is a type
of skeleton. Remember that echinoderms are invertebrates, they have no bone tissue. The echinoderm
skeleton is made up of a network of stiff, hard
plates. The plates lie just under the surface of the
echinoderm’s skin. Some echinoderms, such as
sea stars, have skeletons with loosely connected
plates and flexible arms. In other echinoderms, such
as sand dollars, the plates grow close together, so
the skeleton does not allow for much flexibility.
139
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tube feet
This sea star has captured a bivalve and is using its
tube feet to open the shell.
This sea star’s arms have been pulled up to show how
its tube feet are attached to the bivalve’s shell.
Water Vascular System and Tube Feet
Another adaptation that is unique to echinoderms is a water vascular
system. This system is made up of water-filled tubes that radiate out
from the center of the echinoderm’s body. Tiny openings along the
upper surface of the echinoderm’s body feed water into these tubes.
At the base of the tubes is a series of tube feet.
Muscles attached to the top of each tube can close the tube off,
producing suction at the base of the tube. The tube feet stick to the
ocean floor, allowing the echinoderm to pull itself along. The tube feet
can also be used for hunting prey. For example, a sea star can surround a clam or oyster with its body, as shown in the photograph on
the left. The tube feet pull the shell open. Then, the sea star’s stomach
is pushed out through its mouth and into the bivalve’s shell, where it
begins to digest the bivalve’s body. Not all echinoderms eat other animals. Some, like sea urchins, feed off algae on the ocean floor.
KEY CONCEPTS
CRITICAL THINKING
1. What two features do all
mollusks have?
4. Analyze For mollusks and
echinoderms, what are the
advantages and disadvantages
of having a shell or spiny
skeleton?
2. What are two features all
echinoderms have?
3. What are two functions of
tube feet in echinoderms?
140 Unit 1: Diversity of Living Things
5. Compare and Contrast
Compare the foot of mollusks
with the tube feet of echinoderms.
CHALLENGE
6. Analyze Animals with lungs
or gills can be larger than
animals that take in oxygen
through their skin. What feature do both lungs and gills
have that affect the amount
of oxygen they can absorb?
What role does the circulatory
system play?