Echinoids

Oligopygus Haldemani, An echinoid.

The Echinodermata phylum of organisms include sand dollars, sea stars, sea cucumbers, brittle stars, and crinoids, are a small but important part of the sea. Many fish, otters and other sea-dwelling organisms feed upon them. At first glance, they seem unable to move, but in all reality they are fully able to move. They have an outside shell called a "test" that is usually round (though not always, as you'll find in the fossils) and filled with spines, which are usually rather "spiky" and sharp.