Microarchitectural and experimental studies of the common Atlantic sand dollar, Mellita quinquiesperforata, show the significance of spines in the dual process of feeding and burrowing. Scanning electron micrographs reveal seven structurally distinct spines. These include club-shaped,
lunule, miliary, test-margin, locomotive, geniculate, and aperture-margin spines. Spine serrations act as valuable sites for the sorting and probing of food particles and fine sand. Pores at the distal ends of miliary spines are the sites of mucus secretion. Sand dollars burrow most efficiently
in 3ø sand grains. When placed in artificially sorted fractions of 0ø and 4ø sediment, the organism is unable to burrow, acquiring aboral accumulation of 4ø sediment which obstructs spine movement and eventually causes death. Feeding
and burrowing by the echinoid take place simultaneously through spine manipulation. Mellita quinquiesperforata and M. sexiesperforata are similar in spine morphology and function, and these two species of echinoids may be regarded as selective deposit feeders.
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