Sand dollar
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 Closeup of a Sand Dollar
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Sand dollars are in the Echinoid (Echinoderms) class of marine animals. When alive, they are covered with a suit of moveable spines that encompass the entire shell. Like its close relative the sea urchin, the sand dollar has five sets of pores arranged in a petal pattern. The pores are used to move sea water into its internal water-vascular system, which allows for movement.
Sand dollars live beyond mean low water on top of or just beneath the surface of sandy or muddy areas. The spines on the somewhat flattened underside of the animal allow it to burrow or to slowly creep through the sand. Fine, hair-like cilia cover the tiny spines. Tubefeet or podia that line the food grooves, move food to the mouth opening which is in the center of the star shaped grooves on the underside of the animal called the oral surface. The anus is also located on the bottom, near the posterior edge. Its food consists of plankton and organic particles that end up in the sandy bottom.
On the ocean bottom, sand dollars are frequently found together. This is due in part to their preference of soft bottom areas, which are convenient for their reproduction. The sexes are separate and, as with most echinoids, gametes are released into the water column. The free-swimming larvae metamorphose through several stages before the test begins to form, and they become bottom dwellers.
The name "sand dollar" is a reference to their round flat shape, which is similar to a large coin. Sand dollars usually eat tiny particles of food that float in the water. They hide by burying themselves under the sand.
The term "sand dollar" can also refer to the skeleton or test left when a sand dollar dies. By the time the test washes up on the beach, it is usually missing its velvety covering of minute spines and has a somewhat bleached appearance due to its exposure to the sun.
