If you've ever been to the beach, you may have found flat, white discs, with a little star/flower on them. These are known as "sand dollars," and they are the remnants of a once-living organism belonging to the sea urchin family. Sand dollars are relatives of starfish, sea urchins, and sea cucumbers. They get their name from the fact that their sun-bleached remains in the sand resembled large, silver coins to early beachgoers.
Because they're most usually seen long after they're already dead, it's easy to think of sand dollars as being an inanimate object rather than the remains of a once-living creature. In this clip from the Sea Something YouTube channel, we get a closer look at a live sand dollar, as well as what it looks like compared to an already dead one. The dead sand dollar has visible grooves and a clear opening on the underside. The living sand dollar, however, has a blackish skin on its exoskeleton, and the underside is covered in a writhing mass of tiny bristles, known as cilia.
These cilia can vary in color drastically from one species to another, but they all serve the same purpose. By coordinating their movement, the sand dollar is able to move along the ocean floor. These microfine bristles also help trap the microscopic organisms the sand dollar feeds on, and moves them towards its mouth at the center.
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H/T: Sea Something