Comparing the skeletons of different animals
Biology · Key Stage 2 · The School
A skeleton inside
Humans have a skeleton INSIDE their body. So do cats, birds and fish. Bones sit under the skin and the muscles. This kind of skeleton is called an ENDOSKELETON. Endo means inside. It grows as the animal grows, a little bigger each year. It also gives the body its shape from within, like a frame inside a tent.
A skeleton outside
A crab does not have bones inside it at all. Instead, it has a hard shell on the outside. This is called an EXOSKELETON. Exo means outside. Insects have one too. A ladybird has a hard case instead of bones under the skin. That hard case protects the soft body parts underneath it. A snail carries a shell for much the same reason.
No skeleton at all
Some animals have no hard skeleton at all. A worm has none. A slug has none either. Their soft bodies are held in shape by the fluid inside them, instead of by any bones or shell. This is called a hydrostatic skeleton. It works a little like a water balloon, held firm by what is inside it.
A cat has bones under its fur. That is an endoskeleton, growing bigger as the cat grows. A beetle has a hard shell outside its soft body. That is an exoskeleton, and it does not grow, so the beetle must shed it and grow a new one. A worm has neither. It has no hard parts at all, and its body is held in shape by fluid pressure instead. Three animals. Three completely different ways to be supported. Yet all three manage to move, feed and grow, each in its own way.