Sand dollars or sea biscuits (phylum Echinodermata, class Echinoidea) are closely related to heart urchins and sea urchins, although they lack the visible long, protective spines of the latter. The body is flattened and almost circular in appearance—an adaptation for burrowing in soft sediment. It is protected by a toughened exterior known as the test, and is covered with short spines. The most striking feature of a sand dollar, however, is the distinctive five-arm body pattern on the upper surface. The mouth is located at the center of this pattern. Unlike sea urchins and most other echinoderms, sand dollars are bilaterally symmetrical. Ranging in colors from black to purple, these animals live below the low tide mark in all oceans of the world.
Sand dollars are active burrowing animals and do so with assistance from their moveable spines, which clear a path through the sediment. They are only capable of movement in a forward direction. Some species cover themselves with sediment while others leave their posterior end exposed. When submerged, the animal raises its hind end into the water column, its posterior end remaining buried in the sediment. By aligning itself at right angles to the water current, it is guaranteed a constant source of food.
Sand dollars feed on tiny food particles that are obtained from the sediment while burrowing or from the water current. In contrast to the majority of other burrowing invertebrates, sand dollars do not ingest vast quantities of sediment and sift through the materials. Instead, as the materials pass over the animal's body, particles are sorted between the spines; fine food items fall to the body where they are trapped in a layer of mucus secreted by the spines. Tiny cilia between the spines move this mucus to and along a series of special grooves on the animal's body towards the mouth. Some species, such as Dendraster exocentricus, feed on diatoms and other suspended matter.
Adult sand dollars are either male or female. During the breeding season, large quantities of eggs and sperm are released into the sea, where fertilization takes place. The resulting larvae are free-living and, after some time in the water column, settle to the sea bed and undergo a process known as metamorphosis, which results in a minute replica of the adult sand dollar.