Science High School Reviewer | Grade 4 Science | Life Science | Kingdoms of Life | Lesson 4: Classification of Animals

Scientists divide the animal kingdom into
two main groups: animals with backbones and animals without backbones.

4.1 Vertebrates – Animals with Backbones
Animals with backbones
are called vertebrates. Vertebrates are divided
into five classes—fish, amphibians, reptiles, birds,
and mammals.

Fish –

Fish are usually covered with scales. They live only
in water. Fish breathe mostly with gills. Fish are
cold-blooded and most lay eggs.

Amphibians –

Amphibians are covered with a smooth skin.
They can live both on land and in the water. They
breathe with lungs or gills or both. They are coldblooded.
Amphibians hatch from eggs.

Reptiles –

Reptiles are covered with scales. Most reptiles live
on land. Some can live in water. They use lungs to
breathe. Reptiles are cold-blooded. Reptiles usually
lay eggs instead of having live births.

Birds –

Birds are covered with feathers. They usually live
on land, but many birds spend much of their time
in water. Birds use lungs to breathe. They are
warm-blooded. All birds lay eggs.

Mammals –

All mammals have hair or fur. Most live on land,
but a few live in water. They use lungs to breathe.
Mammals are warm-blooded, they control their
body temperature. Most mammals have live births.

4.2 Invertebrates – Animals without backbones

Animals that do not have a backbone are called
invertebrates. Most of the animals in the world
are invertebrates. Some of these are jellyfish, worms,
insects, spiders, and lobsters. Some invertebrates,
such as jellyfish and worms, have no protection for
their soft bodies. Others, such as snails and clams,
have a soft body inside a hard shell.

Arthropods and More
Arthropods are animals with jointed legs.
They are the largest group of invertebrates. Insects,
spiders, crabs, and shrimp are all arthropods.
Their legs and bodies are divided into sections.
Arthropods are covered by a hard, lightweight
outer skin, or exoskeleton. Like a suit of armor,
the exoskeleton protects the animal inside.
Some other groups of invertebrates include
sponges, sea stars, worms, and mollusks.
Sponges are sea animals that have many pores.
Some worms, such as earthworms and leeches,
have bodies made of segments. Others, such as
tapeworms, have simpler bodies.