Backbone or No Backbone? Sorting the Animal Kingdom

Sit up straight and reach one hand behind your neck. Run your fingers slowly down the middle of your back. Can you feel a bumpy ridge of bones? That ridge is called your spine, or backbone. Now think about a worm wriggling through soil — does it have any hard bones at all? In this lesson you are going to discover that this one difference — backbone or no backbone — is one of the most important ways scientists sort ALL animals on Earth.

Scientists sort living things into groups. This is called classification. One of the first big splits they make is between animals that have a backbone and animals that do not. Animals WITH a backbone are called vertebrates. The backbone is made of small, linked bones called vertebrae (say: VER-teh-bray). These bones protect a tube of nerves called the spinal cord, which carries messages between the brain and the rest of the body. Fish, frogs, snakes, birds, and mammals (including you!) are all vertebrates. Animals WITHOUT a backbone are called invertebrates. Because they have no vertebrae to protect them, many invertebrates have other ways to stay safe — some have a hard shell on the outside of their body, called an exoskeleton. Insects, spiders, worms, snails, and jellyfish are all invertebrates. Here is an important fact: about 97 out of every 100 animal species on Earth are invertebrates. Vertebrates are actually the smaller group, even though they are the ones we notice most often! Worked Example — Sorting a Robin and an Earthworm: A robin is a bird. Birds have a spine running through their body, so a robin is a VERTEBRATE. An earthworm has a soft, stretchy body with no bones at all, so an earthworm is an INVERTEBRATE.