Sea urchin
Depth: 25 m · Sunlight Zone
The sea urchin is a living ball of spines. On its spines and tiny tube feet it slowly pushes itself across the bottom and rasps algae off the rock with a mouth on its underside. The long spines protect it from enemies.
For scientists
Sea urchins (Echinoidea) carry a rigid chalky shell (the test) of fused plates and move about on spines and tube feet. On the underside sits a complex chewing apparatus of five teeth, the Aristotle's lantern, with which they rasp algae off rock. As relatives of the sea stars, they are echinoderms with a five-rayed body plan and a water vascular system.
Source: Wikipedia
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