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There are many different kinds of animals in the world. About 95% of them are invertebrates, that is to say they do not have internal skeletons with backbones. Many invertebrates have shells, others have external skeletons. So these invertebrates are soft inside and hard outside. Invertebrates include insects, which make 80 per cent of all the animals in the world.

We know of about one million different kinds of insects, and scientists think there may be the same number still waiting to be discovered.

About 5 per cent of the world’s animals are vertebrates. Vertebrates are soft outside and hard inside. They have internal skeletons with backbones. The main groups of vertebrates are fish, amphibians, reptiles, birds and mammals.

Fish, which live in water, are the largest group. Examples of two kinds of fish are trout and sharks. But not all vertebrates that live in the water are fish. Whales, for instance, are mammals, although they look like fish.

Amphibians live between land and water. A frog is a typical amphibian.

Reptiles, for example snakes, are cold-blooded animals, which live on land. For a hundred and sixty million years large reptiles, dinosaurs, were the most animals on the earth.

Birds are like reptiles in many ways but they are warm-blooded. They are the only vertebrates, which can fly, except for bats. Although they look rather like birds, bats are actually mammals. There are big differences between various kinds of birds. Compare an eagle with a duck, for example.

Mammals are warm-blooded like birds, but unlike birds, mammals do not lay eggs; they grow their babies inside them, and when the babies are born, their mothers feed them on milk.

Date: 2015-04-23; view: 871; Нарушение авторских прав