A backbone is part of a skeleton of bone which grows inside many animals, such as fish, birds and mammals. Most animals do not have a skeleton of bone inside them so they do not have a backbone. These animals are called invertebrates.

A skeleton supports the body and allows it to move. It may even protect some parts of the body. Invertebrates have skeletons but they are not made of bone. Invertebrates such as worms and snails have a skeleton of water inside their body. Invertebrates such as spiders and insects have a skeleton of hard materials on the outside of their body. This kind of skeleton is called an exoskeleton.

The invertebrates are divided into groups according to the structure of their bodies. The main groups of invertebrates are the jellyfish, worms, insects, spiders, crustaceans, molluscs and starfish.