You may think that all animals have bones. But this is not so. We are in the minority. Most animals – the insects, crabs and clams – have suits of armour. We call them shells.
The shell is for protection, and all of the muscles are inside, not outside like ours. But they still work in pairs in the same way.
Shelled animals don't grow steadily like us. They grow in steps, each time shedding a shell and growing a new, bigger one.
