sand dollar

any of various flat, disklike sea urchins, as Mellita testudinata or Echinarachnius parma, that live on sandy bottoms off the coasts of the U.S.

[1880-85, Amer.]

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Any echinoderm (order Clypeastroida, class Echinoidea) that has a coinlike, thin-edged body.

Five "petals" spread out from the center of the upper body. It burrows in sand, feeding on organic particles wafted to the mouth, located in the center of the body's underside. Small spines covering the body are used for digging and crawling. Tests (external skeletons) of the common sand dollar (Echinarachnius parma), which often wash up on beaches in North America and Japan, are 2–4 in. (5–10 cm) in diameter.

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▪ echinoderm

      any of the invertebrate marine animals of the order Clypeastroida (class Echinoidea, phylum Echinodermata) that has a flat, disk-shaped body. They are close relatives of sea urchins and heart urchins. The sand dollar is particularly well adapted for burrowing in sandy substrates. Very small spines used for digging and crawling cover the entire surface of its body and are appressed backwards toward the posterior anus. The mouth is located in the centre of the body's underside. The upper surface exhibits pentaradiate symmetry, with a pattern of five “petals” spreading out from the centre. Some species found stranded on the shores of North America have five or six slots, or lunules, through the test (external skeleton). Most sand dollars measure from 5 to 10 cm (2 to 4 inches) in diameter. Species of comparable size occur in shallow coastal waters throughout the rest of the world, except in Europe and Antarctica.

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