Scientists sort the millions of animal species into groups so they can study them. The first big division is whether an animal has a backbone. Animals with a backbone are vertebrates, and they are divided into five classes. Fish live in water, breathe with gills and have scales and fins. Amphibians, such as frogs and salamanders, begin life in water with gills and later breathe with lungs on land. Reptiles have dry scales and lay eggs on land. Birds have feathers, hollow bones and lay hard-shelled eggs. Mammals have hair, are warm-blooded, and feed their young with milk. Whales, bats and humans are all mammals.
Animals without a backbone are invertebrates, and they make up more than 95 percent of all animal species. Insects have six legs, three body parts and usually wings; there are more kinds of beetle alone than all vertebrates put together. Spiders and scorpions have eight legs. Crabs and prawns have hard outer shells and jointed legs. Snails and squids are molluscs with soft bodies. Earthworms have bodies made of rings. Jellyfish and corals are so simple that they have no brain at all. Many invertebrates have an exoskeleton, a hard covering on the outside, which they must shed, or moult, in order to grow bigger.
Example: Go on an invertebrate hunt in a garden or park for 20 minutes with a magnifying glass. Look under stones, on leaves and in soil. Draw each animal you find and count its legs: six means insect, eight means spider or its relatives, more than ten means centipede or millipede, none means worm or snail. Prediction: you will find at least four different invertebrate groups, and more insects than anything else.