Beachcombers wandering along shorelines sometimes find sand dollars. When bleached white by the sun, sand dollars may even vaguely resemble the silver coins for which they are named. They aren't money, however. They are skeletons.
Some people think of sand dollars as shells, but this perception is technically incorrect. Shells grow on the outside of some marine organisms (like clams, oysters, and conchs) where they surround and protect delicate, interior bodies. Sand dollars are an animal's endoskeleton, a part that forms on the inside.
Sand dollars are a type of flat sea urchin. When sand dollars are alive, their endoskeletons are covered by skin and very short spines that seem like a velvety coat. They live primarily in areas where the water is relatively shallow and the bottom is sandy. You can read more about sand dollars in this article from the Monterey Bay Aquarium.
Among Christians, a popular poem relates Easter and Christmas symbols to the markings on sand dollars. In the life of a sand dollar, those markings represent anatomical structures. For example, the five flower-petal shapes are regions where the animal extended sets of tube feet that served to help it eat, breathe, move, and burrow. The groves on the undersurface were part of the system that carried food particles to the mouth. The "doves" that rattle about inside a sand dollar are pieces of its chewing apparatus. In this entry of The Echinoblog, Dr. Chris Mah, a researcher with the National Museum of Natural History (part of the Smithsonian Institute), explains the biological functions of the pieces that play a role in the sand dollar legend along with more information about what sand dollars are and how they differ from other related animals (such as starfish and sea cucumbers).
This article, including a list of links for more information, originally appeared in the July 2015 issue of Observations: A Pier Press® Newsletter. You can access the archived issue here, and if you're not already a subscriber, click here to sign up. It's free.