Animals live everywhere. They roam the land. They burrow in the ground. They swim in the sea. They fly through the air. They creep, they leap, they soar, and they dive. A very few—including corals and barnacles—stay in one place.
Animals come in all sizes. The biggest animals are whales, which can be 100 feet (30 meters) long. The smallest animals can only be seen through a microscope.
Zoologists (scientists who study animals) have found more than 2 million species (kinds) of animals. They think they have discovered only a small portion of all animals on Earth.
Several things make animals different from other living things. Unlike plants, animals cannot make their own food. Animals eat other living things—plants and other animals—to get energy. Animal bodies are made up of more than one cell, unlike bacteria and other life forms with only one cell. Cells are the building blocks of living things. Animals also have senses, such as eyes or ears, that tell them what is going on around them.
WHAT KINDS OF ANIMALS ARE THERE?
Zoologists divide animals into about 30 groups. First, they divide them by whether they have a backbone. Animals that have a backbone are called vertebrates. Animals that do not have a backbone are called invertebrates.
The biggest and best-known animals are vertebrates. Mammals, birds, reptiles, amphibians, and fish are vertebrates. You are a vertebrate. Your backbone is also called your spine. There are about 40,000 species of vertebrates.