Sand dollars :)

This is so random.

Remember the amount of sand dollars we found on the beaches of Darwin? (Casuarina beach)... Well, I used to wonder what they were and why they were called sand dollars... Did a google search after getting very sian of studying.


"Sand-dollar is a name used for many species of flattened, burrowing sea urchins belonging to the order Clypeasteroida. All members of Clypeasteroida have a rigid skeleton known as a test. This is the typical fossil found washed up on beaches.

The living animals have a skin of motile spines covering the test. Movement is accomplished by the coordinated action of the spines. Like other sea urchins, sand dollars have five paired rows of pores. The pores are arranged in a petal-like pattern. These pores are perforations in the endoskeleton through which the podia, used in gas exchange, project from the body.

The name sand dollar comes from the shape and color of the test after it washes up on the beach. At that point the test is usually missing its velvety covering of minute spines and is often bleached white by the sunlight. In many species the beachworn test is quite similar in shape and size to a large coin, and the whiteness makes it resemble a large silver coin, for example the old Spanish Dollar coin, or the American Dollar coin that are between 38 and 40mm in diameter."


Here's a video of a sand dollar when it is alive...




Credits: Wikipedia