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Classifying Animals | Science Lessons for Grade 4 Students

Animals are a diverse group of organisms on earth. Among animals, there are those that have a backbone, called vertebrates, and those that do not have a backbone, called invertebrates.

Turtles, gorilla, parrot, lizard, hawk, bat, snake and hippopotamus are all examples of vertebrates.

Butterflies, crab, scorpion, star fish, ants, snails, earthworms, spider, ladybug and jellyfish are all examples of invertebrates.

Although most animals without a backbone are small in size, some can be very large. The giant squid, for example, can reach sizes of up to 13 metres. That is more than the length of a classroom!

Animals without a backbone (invertebrates)

Insects: the most common and diverse group of animals without backbones are insects. There are more kinds of insects than any other groups of animals. Common types of insects include butterflies, grasshoppers, bees, mosquitoes, beetles and ants.

How can you tell that an animal is an insect? What characteristics do insects have in common?

You can tell that an animal is insect by observing its body parts. An insect has six legs and two antennae. The body of an insect is made up of three main parts.

  • The head
  • The thorax
  • The abdomen

In the picture above of an insect, you can also see the legs, eyes, antennae and wings.

Insects have a hard outer body covering that protects their bodies. This is known as exoskeleton.

Another characteristic of insects is that they all reproduce by laying eggs. Young insects hatch from eggs.

Many insects can fly. Some insects, such as butterflies, have two pairs of wings. Some insects that have two pairs of wings include dragonflies, grasshoppers, butterflies, bees, wasps, true bugs and moths. Some other insects such as bees have one pair of wings. Other insects, such as most ants, do not have wings and cannot fly.

Animals with a backbone – vertebrates

Vertebrates or animals with a backbone can be classified into a number of groups as shown below:

  • Fish
  • Amphibians
  • Reptiles
  • Birds
  • Mammals


Fish are animals that live in water. They can be found in freshwater streams and lakes, in coral reefs, and in the deep ocean. Fish have special body parts called gills that help them to get the oxygen they need from water.

Most fish are covered with scales. The scales help to protect their bodies and allow them to move easily in water. Their fins help the fish to swim. Most fish reproduce by laying eggs.

Surgeon fish, lionfish, blacktip reef shark, clownfish, sardine and mackerel are examples of fish.




Amphibians are animals that spend part of their life in water and part of their life on land. (The word ‘amphibian’ comes from the Greek words ‘amphi’ meaning ‘both’ and ‘bios’ meaning ‘life’.)

Common amphibians include frogs, newts, toads and salamanders.



As young amphibians grow, their bodies change. Their tail gets smaller and they begin to develop legs and feet. The gills change into lungs so they can breathe in air.

Most amphibians have smooth, moist skin and usually live in moist places. Adult amphibians return to water to lay their eggs.


Animals such as snakes, lizards, crocodiles and turtles are classified as reptiles. Most reptiles live on land, but some, such as crocodiles, sea snakes and sea turtles, spend most of their time in water.

Reptiles have lungs to breathe in air. Reptiles that live in water need to come to the surface to breathe.

Most reptiles reproduce by laying eggs. Reptiles have dry skin covered with scales. The scales help to protect them from harm.




Birds are the only animals that have a body covered with feathers. Birds also have two wings, two legs and a beak. Birds reproduce by laying eggs. Most birds use their wings to fly. Some birds cannot fly. An emu, for example, has long powerful legs to walk and run on land. Penguins have small wings that work like fins to help them swim.

Birds have lungs to breath in air. Given below are examples of some common birds.




Mammals have hair or fur on their bodies. Most mammals live on land. Common examples include monkeys, bats, elephants and tigers. Some mammals, such as dolphins, seals and whales, live in water.

Mammals have lungs and get the oxygen they need by breathing in air. Mammals that live in water do not have gills like fish do. To get oxygen, they must come to the surface regularly to breathe.

Most mammals give birth to the young. Mothers feed milk to the young. However, some mammals such as anteaters and platypuses reproduce by laying eggs.