CHAPTER 9
LESSON 3
Introduction to Animals
Chordates
Key Concepts
• What characteristics do
chordates have in common?
• What is the difference
between vertebrate and
invertebrate chordates?
• How do the groups of
vertebrate chordates differ?
What do you think? Read the two statements below and decide
whether you agree or disagree with them. Place an A in the Before column
if you agree with the statement or a D if you disagree. After you’ve read
this lesson, reread the statements to see if you have changed your mind.
Before
Statement
After
5. All young mammals take in milk from their
mothers.
6. Birds are the only animals that lay shelled eggs.
What is a chordate?
Think of the zoo you read about at the beginning of this
chapter. It’s likely that most of the animals at the zoo were
chordates. It also is likely that most of the animals at the
zoo were mammals, like you. Chordates are animals that are
grouped in the phylum Chordata. Mammals are chordates.
There are two types of chordates—vertebrate chordates
and invertebrate chordates. A vertebrate is an animal with
a backbone; an invertebrate chordate has no backbone. All
chordates have four traits in common: a notochord, a tail,
a nerve cord, and pharyngeal pouches. Each of these four
traits exists at some time during the life of a chordate.
Key Concept Check
1. Name characteristics
that all chordates share.
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Introduction to Animals
You are a chordate. When you were developing in the
womb, you had a notochord. A notochord is a flexible rod-shaped
structure that supports the body of a developing chordate. Your
backbone replaced the notochord. You also had a tail. What
is left of your tail is your tailbone. Before you had a brain
and a spinal cord, you had a nerve cord. You also had
pharyngeal (fuh run JEE uhl) pouches. Pharyngeal pouches
are grooves along the side of a developing chordate. Your pharyngeal
pouches developed into parts of your ears, head, and neck.
Fish have pharyngeal slits that provide support for gills.
These characteristics are evidence that you and other
chordates have ancestors in common.
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Sticky Notes As you read,
use sticky notes to mark
information that you do not
understand. Read the text
carefully a second time. If you
still need help, write a list of
questions to ask your teacher.
Invertebrate Chordates
As you have read, some chordates never develop
backbones. These animals are called invertebrate chordates.
What did the ancestor of chordates look like?
Lancelet
Lancelets
The earliest chordates
probably looked similar to
the lancelet in the figure
at right. Lancelets are
small animals that burrow
in the sand just off ocean
shores. Lancelets grow to
only 5 cm in length.
Lancelets can swim, but
they often sit in the sand
and catch food particles
floating by. Lancelets have
all four chordate traits, as
shown in the figure.
Pharyngeal
pouches
te
tebra
Inver ates
d
r
Cho
Both
Verteb
rat
Chorda e
tes
Nerve cord
Notochord
Visual Check
Tail
Tunicates
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Make a horizontal three-tab
Venn book to compare and
contrast the different types
of chordates.
2. Circle the names of the
traits that indicate the
lancelet is a chordate.
Adult tunicates look like sponges. Like sponges, adult
tunicates live in the sea, attached to rocks or other fixed
objects. However, adult tunicates have organized tissues and
internal structures such as organs. They also have all the
characteristics of chordates at some time in their lives.
Before a tunicate becomes an adult, it looks and acts like a
tadpole. Young tunicates can swim, and they have all four
chordate traits.
Lancelets look like fish more than tunicates do. Therefore,
scientists once thought lancelets were more closely related to
vertebrates than tunicates were. But when scientists studied
the DNA of all three groups, they discovered the opposite
to be true. Vertebrate DNA shares more similarities with
tunicate DNA than it does with lancelet DNA.
Vertebrate Chordates
Most of the animals you are familiar with probably are
vertebrate chordates. This group includes cats, dogs, fish,
snakes, frogs, and birds. Recall that vertebrates are animals
with backbones. Most vertebrates also have jaws. As the
bodies and skeletons of vertebrates continued to adapt,
vertebrates became better at catching food and avoiding
being eaten.
Reading Essentials
Key Concept Check
3. Differentiate What is
the difference between
invertebrate and vertebrate
chordates?
Introduction to Animals
155
Fish
Interpreting Tables
4. Identify Which group
of fish does not have paired
fins?
You might think of a goldfish and a shark as fish, but
what about a sea horse? All are fish and have traits that make
them fish. All fish live in water and use gills for breathing.
Gills are organs that exchange carbon dioxide in the water for oxygen.
All fish have powerful tails, and most fish have paired fins.
The table below shows the three major groups of fish.
Groups of Fish
Jawless Fish Lampreys are jawless fish. The skeleton of
jawless fish is made of cartilage. The tip of your nose
and the flaps of your ears are made of cartilage. Some
jawless fish get their nutrition from other fish. They
have a circle of teeth that attach to the sides of other
fish and make a wound. They then slowly suck out
blood and other body fluid from the fish.
Sharks and Rays Most of the skeleton of sharks and
rays is made of cartilage. However, shark skulls are
made of bone. Sharks have paired fins. They are fast
swimmers and also have powerful jaws. Their jaws
make them dangerous predators of other animals,
especially other fish.
Amphibians
Reading Check
5. Summarize the
characteristics of amphibians.
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Introduction to Animals
In Canada in 2004, scientists discovered the fossil of an
animal that lived in shallow water long ago. The animal had
both gills and lungs, a flexible neck, and fins with arm and
hand bones. The fins could help the animal move in the
water and on land. These scientists may have discovered one
of the first tetrapods. A tetrapod is a vertebrate animal with
four limbs.
Amphibians are a group of tetrapods that live on land but
depend on water to survive and reproduce. Amphibian means
“both ways of life.” Most amphibians lay their eggs in water.
Young amphibians, such as tadpoles, have gills and must
spend most of their time in water. Most adults have lungs
for breathing on land. However, amphibian skin is thin and
moist. On land, amphibians must live in moist habitats to
keep their bodies from drying out.
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Bony Fish All other fish have a bony skeleton, as well
as paired fins and jaws. Bony fish, such as goldfish,
also have a special sac called a swim bladder that the
fish can fill with gas. This helps the fish move up and
down in the water. Sea horses are unique bony fish
because the males carry the young in their bodies as
they develop.
Types of Amphibians There are three types of amphibians.
Salamanders and newts have tails and move by bending
their bodies side-to-side. Frogs and toads do not have tails
as adults. They have long legs that enable them to jump.
Caecilians (sih SIHL yuhnz) are a group of amphibians that
do not have legs. They look similar to earthworms and
move by twisting their bodies back and forth like a snake.
Survival Scientists are concerned about the survival of
amphibians. Many amphibian populations have become
smaller since 1980. Some types of amphibians have not been
seen for years. Scientists hypothesize that the size of the
amphibian population is decreasing because of disease,
climate change, herbicides, and habitat destruction.
Reptiles
Lizards and snakes, turtles, and alligators and crocodiles
are the three most common groups of reptiles. Most reptiles
live on land, and all have lungs for breathing. Scales on their
skin prevent reptiles from drying out. Reptiles do not lay
their eggs in water. Most reptiles lay shelled eggs that don’t
dry out. Inside the egg is amnion, a protective membrane that
surrounds the embryo. An egg with an amnion is called an
amniotic egg, as shown in the figure below. Reptiles, birds,
and mammals have amniotic eggs.
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Amniotic Egg
Math Skills
The size of two organisms
might be the same, but one
floats while the other sinks.
This is because the organisms
have different densities. The
formula for density is
mass
density = ______
volume
For example, what is the
density of a chicken’s leg
bone that has a mass of 5.5 g
and a volume of 5.0 cm3?
Replace the terms in the
formula with the given
values.
5.5 g
5.0 cm
density = ______3
Solve the problem.
5.5 g
1.1 g
______
= ____
5.0 cm3
cm3
6. Use a Formula A piece
of a cow’s leg bone with a
volume of 10 cm3 has a mass
of 18 g. What is the density
of the bone?
Amnion
Embryo
Shell
Visual Check
7. Identify What surrounds
the developing embryo?
Yolk sac
Lizards and Snakes One reptile group includes both lizards
and snakes. Most lizards are small and would fit in your
hand. However, one lizard, the Komodo dragon, can grow to
3 m in length. Snakes are legless reptiles. Many snakes are
small. Some are several meters long. All snakes eat other
animals. When snakes catch their prey, they can crush them
or bite and poison them. Either way, most snakes swallow
their prey whole!
Reading Essentials
Introduction to Animals
157
Turtles These animals are best known for their protective
shells. Some turtles can live in the desert. Others, such as
snapping turtles and sea turtles, live mostly in water.
Alligators and Crocodiles The third group of reptiles,
Reading Check
alligators and crocodiles, lives in warm parts of the world.
They live in or near water but lay their eggs in nests on the
shore. They are fierce hunters and can move quickly for
short distances.
8. Name three
How Reptiles Stay Warm Reptiles are ectotherms, animals that
characteristics of alligators
and crocodiles.
heat their bodies from heat in their environments. Warming, or
basking, in sunlight is a behavioral adaptation of ectotherms.
A reptile with a warm body can move faster and catch prey
more easily. Reptiles move to cool, dark places to conserve
energy when food is scarce.
Birds
Reading Check
9. Identify What trait is
unique to birds?
10. Describe the different
shapes of bird wings, and
explain how the shapes help
the birds survive.
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Introduction to Animals
Birds have many adaptations that enable them to fly. A
bird does not have a urinary bladder. A full bladder would
weigh a bird down. Instead, birds concentrate their urine into
crystals. The crystals are the white part of bird droppings.
Birds have bones that are nearly hollow and filled with air.
This makes the bones of birds lighter than the bones of
other vertebrates.
Wings and feathers are birds’ major adaptations for flight.
A bird’s wings are connected to powerful chest muscles. Bird
wings come in different shapes. Long, narrow wings enable a
seagull to soar on long flights. The short, broad wings of a
sparrow enable quick changes of direction to catch food or
escape an enemy.
Feathers also keep birds warm. Unlike reptiles, birds are
endotherms, animals that generate their body heat from the inside.
This enables birds to live in cold habitats. However, birds
require much energy to keep their body temperatures high.
Like you, birds shiver when they get cold. Shivering muscles
help produce more body heat.
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Reading Check
Can you name a unique trait of birds? Did you think of
flight? Many insects fly, and so do certain mammals. Some
birds, such as penguins and emus, don’t fly. Maybe you
thought of wings. But most insects and some mammals also
have wings. The one trait that makes birds different from all
other animals is their feathers.
Mammals
Maybe the main reason to go to the zoo is to see the
mammals. Lemurs, lions, alpacas, and apes all are mammals.
You are a mammal, too. All mammals have hair and mammary
glands, special tissues that produce milk for young mammals. Like
birds, mammals are endotherms. The hair of mammals is an
adaptation that helps keep them warm. Milk production also
is an adaptation. The milk helps the young grow and survive
when they are too young to find their own food. There are
three groups of mammals: monotremes, marsupials, and
placental mammals.
Monotremes One type of mammal, the monotreme, lays
eggs. After the eggs hatch, the mother’s milk nourishes the
young. These mammals include the platypus and the
echidna.
Reading Check
11. Explain How does
hair benefit a mammal?
Marsupials Mammals that raise their young in pouches are
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
called marsupials. The young are not fully developed when
born. After birth, they crawl through their mother’s hair into
a pouch. Here they can feed on their mother’s milk and
continue to grow. Most marsupials are native to, or live in,
Australia. Many marsupials resemble mammals that live in
North America. There are marsupial squirrels, marsupial
mice, and marsupial moles. The only marsupial native to
North America is the opossum.
Placental Mammals The third group of mammals is called
placental mammals. A placental mammal has a structure
called a placenta. The young are attached to the placenta as
they grow inside the mother. Of the three types of
mammals, you are probably most familiar with different
kinds of placental mammals, such as dogs, cats, horses,
cows, and humans.
Key Concept Check
12. Contrast How do
the groups of vertebrate
chordates differ?
Reading Essentials
Introduction to Animals
159
Mini Glossary
amnion: a protective membrane that surrounds the embryo
mammary gland: special tissue that produces milk for young
mammals
ectotherm: an animal that heats its body from heat in its
notochord: a flexible, rod-shaped structure that supports the
environment
body of a developing chordate
endotherm: an animal that generates its body heat from
pharyngeal (fuh run JEE uhl) pouch: a groove along the
the inside
side of a developing chordate
gill: an organ that exchanges carbon dioxide in the water for
oxygen
1. Review the terms and their definitions in the Mini Glossary. Write a sentence that explains
how mammary glands help mammals survive.
2. In the graphic organizer, identify one unique trait shared by all fish. Then do the same
for mammals and birds.
Chordate
Mammal
Bird
Unique trait:
Unique trait:
Unique trait:
3. No lizards live in Antarctica. Based on what you learned about lizards in the lesson, give
one reason for this.
What do you think
Reread the statements at the beginning of the
lesson. Fill in the After column with an A if you
agree with the statement or a D if you disagree.
Did you change your mind?
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Introduction to Animals
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END OF
LESSON
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Fish