- from Bones
Believe it or not, you have fewer bones in your body today than on the day you were born. All of us start off with more than 300 bones, yet by the age of 20, we have only about 206.
What happens in those two decades is nothing strange—some bones simply grow together to make the skeleton stronger.
Our skeletons make us part of an exclusive club. We’re among the 3 percent of animals known as vertebrates (those with a backbone). Though greatly outnumbered by invertebrates (animals with no backbone), vertebrates have greater size and mobility. A flexible system under the skin gives us our shape. It’s called an endoskeleton. Invertebrates have either a hard outer shell called an exoskeleton (like crabs), or no skeleton at all (like jellyfish).