Lesson 3
How Are Animals Classified?
Fast Fact
A Sticky Subject Banana slugs are animals without a backbone. They creep along on a
cushion of slimy mucus. In the Investigate, you will compare models of an animal
with a backbone and one without a backbone.
Backbones
Materials
 newspaper
 ruler
 modeling clay
 straight drinking straw
Procedure
1. Cover your desk with newspaper. Work in pairs to make models to compare animals
with and without a backbone.
2. Make a base 5 cm in diameter and 3 cm high, using modeling clay.
3. Next, cover a straight drinking straw with modeling clay. The clay should have a
diameter of 3 cm to represent the body. The straw represents the backbone.
4. Poke the body into the base. You can build the base up around it to support it. The
base represents legs. You can add arms and a head, too.
5. Repeat Steps 2-4 without the straw.
6. Compare the way the models stand.
Draw Conclusions
1. What do you observe when you compare the two models?
2. Inquiry Skill Scientists plan simple investigations to test ideas. What steps would
you follow to find out if the thickness of the backbone is important?
Investigate Further
Find a garden snail. Observe it. Record how it moves. What can you infer about
whether this animal has a backbone?
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Reading in Science
VOCABULARY
vertebrate p. 50
invertebrate p. 52
SCIENCE CONCEPTS
 which animals are vertebrates and which animals are invertebrates
 how vertebrates and invertebrates are structured
READING FOCUS SKILL
MAIN IDEA AND DETAILS Look for characteristics of vertebrates and
invertebrates.
Main Idea
detail detail detail
Vertebrates
How many animal pets can you name? What about wild animals? The animal kingdom
is a large and diverse group of organisms. There are many different kinds of
animals. The animal kingdom is divided into two groups—animals with a backbone
and animals without a backbone.
Animals with a backbone belong to a group called vertebrates. You are a vertebrate. All
vertebrates have bonesinside their bodies that make up a skeleton.
Vertebrates are the most complex of all the animals. They have a system of nerves that
carry messages to and from the brain. They have a system that brings nutrients,
water, and oxygen in the blood to every cell of the body. The respiratory system
brings oxygen to the blood and removes carbon dioxide. There is a system for
digesting food and a system of muscles to move the body. There is a reproductive
system.
A kangaroo is a mammal. A kangaroo mother nurses her baby while it's inside her
pouch.
A fish is a vertebrate, too. Its backbone supports it as it swims.
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An iguana is a reptile. It suns itself to keep its body warm. 4
A Like all birds, a hummingbird has light, strong bones.
A salamander is an amphibian. Amphibians can live out of the water but must
return to water to lay their eggs.
There are five groups of vertebrates—mammals, birds, reptiles, amphibians, and fish.
Fish live in water. Their gills take oxygen from the water.
Amphibians have two lives—one in water and one on land. Most amphibians begin life
in water. They grow lungs and legs and often lose their tails before they can live on
land. They have smooth skin and lay eggs in water. Frogs, toads, and salamanders
are amphibians.
Reptiles live on land. They don't have gills. Their bodies are the same temperature as
what is around them. This is also true of fish and amphibians.
Reptiles lay eggs on land. Lizards, turtles, and snakes are reptiles.
Most birds can fly. They're covered with feathers, which keep them warm and dry and
help them fly. Birds are warm-blooded. This means that their bodies stay at the
same temperature no matter what the temperature around them is. All birds lay
eggs.
You are a mammal. Female mammals nurse their young. All mammals are warmblooded and have hair or fur. Most bear their young live.
MAIN IDEA AND DETAILS How are vertebrates complex?
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---see pictures
A snail is a mollusk. It uses its shell for protection.
A tarantula is an arachnid (uh•RAK•nid). It has eight legs and no antennae.
A jellyfish stings its prey with tiny poison darts from its tentacles.
A Christmas tree worm lives in a tube it builds on coral reefs. It filters food into its
body with its tentacles.
Invertebrates
Animals without a backbone belong to a group called invertebrates. There are more than
a million kinds of invertebrates, but scientists think that many more are yet to be
found.
Many invertebrates live in the oceans. Water supports the bodies of underwater
invertebrates.
Invertebrates have been divided into groups based on their body structures. Many
groups have very simple body plans. Sponges are simple organisms with only a few
kinds of cells. Members of the group that includes jellyfish and sea anemones are
sac-like organisms with a place for digesting food. Some corals are communities of
tiny animals.
Mollusks are more complex. They include snails, slugs, mussels, and clams. A mollusk
has a soft body and a muscular foot. It also has nerves that send messages through
the body.
There are several groups of worms. Earthworms and fan worms have a brain and a
system of nerves. An earthworm has five "hearts" that pump blood to all its parts.
Arthropods are the largest group of invertebrates. They live almost everywhere on land
and in water. All arthropods have jointed legs. They
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have a rigid body covering, called an exoskeleton, that covers the outside of their
bodies. The exoskeleton doesn't grow. So when an arthropod grows, it molts, or
sheds the old exoskeleton and grows a new one.
Insects are the largest group of arthropods. Crustaceans (kruhs•rAy•shuhnz), such as
crabs and shrimps, are also arthropods. Spiders and scorpions are arthropods that
belong to a subgroup called arachnids.
A grasshopper is a typical insect because it has six legs and three main body parts.
Crabs, which are crustaceans, walk on jointed legs and shed their shells as they
grow.
MAIN IDEA AND DETAILS What supports the bodies of ocean invertebrates?
Math in Science Interpret Data
Based on these circle graphs, what statement could you make about insects?
---see charts
Insta-Lab
What Kinds of Animals?
Make a list of as many different kinds of animals in your community as you can. Then
classify the animals. You can make up your own groups.
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Science Up Close
A Typical Insect
A typical insect has an exoskeleton to protect its body. The body is divided into a
head, a thorax, and an abdomen. An insect has six legs and antennae.
Antennae can detect smells and sounds. Insects use antennae to find each other.
An insect's eyes have many lenses. They are called compound eyes.
The head has eyes and mouth parts. The mouth parts of some insects are adapted
for chewing. Some are adapted for sucking.
The thorax is divided into three parts. A pair of legs is attached to each part of the
thorax. If the insect has wings, the wings are also attached to the thorax.
The six legs are jointed.
The abdomen contains most of the insect's body systems. An insect breathes
through the sides of its abdomen.
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1. MAIN IDEA AND DETAILS Copy this chart, and fill in the boxes.
---see chart
2. SUMMARIZE Write a summary of this lesson by using the lesson vocabulary
words in a paragraph.
3. DRAW CONCLUSIONS Why do you think a snake can move faster and in more
directions than a slug?
4. VOCABULARY Use the vocabulary words as headings for a table. List at least
three animals under each heading.
Test Prep
5. Critical Thinking If you found a newborn squirrel and took it to a vet, what would
the vet feed it? Why would the vet choose that food?
6. Which animal has an exoskeleton?
A. fish
B. grasshopper
C. jellyfish
D. snail
Writing
Expository Writing
Write a description of an animal you learned about in this chapter. Without naming the
animal, see if a classmate can identify it.
Math
Using Fractions
There were 100 arthropods in a garden. If 75 percent of them were insects, how many
insects were there? How many other arthropods were in the garden?
Social Studies
Useful Insects
Research an insect that is used by farmers to control insect pests. Write a report about
this insect.
For more links and activities, go to www.hspscience.com
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