The very first land animals were called invertebrates. Invertebrates have been around for more than 600 million years. Invertebrates are animals without backbones. There are many types of invertebrates which include insects, arachnids, crustaceans, centipedes, millipedes, worms, and molluscs. Invertebrates are are cold-blooded which means that their body temperature depends on the temperature of their environment. Invertebrate animals make up the majority of the world's known species.

The first animals with backbones are called vertebrates. Vertebrates began 500 million years ago as creatures known as lancelets and tunicates. Lancelets look like tadpoles without heads. Vertebrates include: mammals, amphibians, reptiles, birds and fish. Vertebrates account for less than three percent of the world's known species. Vertebrates have highly developed brains that help to regulate their body functions.

Mammals are warm-blooded and most always have hair. Mammals have hair to keep them warm. There are about 4,000 mammals. Mammals give birth to live young and feed them milk which is produced in the mammary glands which is where mammals get their name. Some mammals hibernate which means they sleep during the winter. Some of the mammals that hibernate are mice, ground squirrels, chipmunks, bats and bears. The most common type of mammals are the rodents which would include rats, mice and squirrels. The fastest mammal is the streamlined cheetah which can go over 62 miles per hour.

Birds have feathers for two main reasons - to help them fly and to keep them warm. A bird's feathers are made of keratin which is the same thing that our hair and fingernails are made from. There are around 8,600 species of birds. Birds build nests to protect their eggs and babies from predators and threatening weather and also to keep them warm. Some birds migrate (fly to other areas) when the seasons change so that they can find the food they need and a better climate at different times of the year. The ostrich lays the biggest egg of all birds. Birds sing as a way of attracting other birds of the same species or as a way to warn them away. Male birds sing to attract female birds as mates.

Amphibians are animals that can live on both land and in water. The word amphibian means "living a double life". Amphibians are divided into three groups - frogs and toads, newts and salamanders, and caecilians. Caecilians are the only amphibians without legs. They also have no eyes. Caecilians live under the soil or in streams. Most amphibians spend the first part of their life in water and the rest on land. Newts and salamanders have long bodies with long tails. Amphibians lay their eggs in water or in damp areas. Herpetologists are scientists who study amphibians and reptiles.

Reptiles are different from amphibians because they have dry, scaly skin while the amphibians have soft, wet skins. Some examples of Reptiles are turtles, crocodiles, alligators, snakes and lizards. Reptiles are cold blooded and get their heat from their surroundings. Reptiles lay their eggs on or under dry ground.

There are about 21,000 different types of fish. Fish can be divided into three groups which are: jawless fish, sharks and rays and bony fish. The fastest fish is the blue-finned tuna.

The largest animal is the Blue Whale. A Blue Whale can reach up to 90 feet long and weigh 150 tons.

The largest land animal is the African Bush Elephant.

The simplest type of animal is a protozoan which is made up of just one cell.

Bald Eagles are the national symbol of the United States.

There are about 250 types of sharks.

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