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.