- from Bones
You have fewer bones in your body now than you did when you were born. Everybody starts life with more than 300 bones. By age 20, we have only about 206 bones.
So what happens to almost 100 bones in those 20 years? Some bones fuse, or grow together, so that your skeleton can become stronger.
Skeletons are rare in the animal kingdom. In fact, just 3 percent of animals have backbones. Those animals are called vertebrates. There are way more invertebrates (animals with no backbone), but 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 (think crabs), or no skeleton at all (think jellyfish).