These animals are echinoderms, which means "spiny skinned".
Sea urchins are shaped like a ball.
Have you ever found a shell on the beach that's shaped like a ball with small holes in it?
This sort of thin, fragile shell, called a test, is probably the shell of a dead sea urchin.
The holes in the test show where the sea urchin's tube feet once stuck out.
Most sea urchins use their tube feet to crawl along the seabed, but some move along using the long spines that cover their body.
Sea urchins have sharp teeth to chew their food.
They are scavengers, which means that they feed on decaying matter.
But they also eat seaweed and small animals living on rocks or on the seabed.
The hard shell, or test, of each of these sea urchins is covered in long spines and tiny tube feet with suckers. Some sea urchins use these tube feet to crawl and also to collect food.
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