Sand dollars are shallow-domed echinoids which act as lifting bodies in water currents. Coefficients of lift (CL) were determined experimentally in air for a lunulate species, Mellita quinquiesperforata (0.040) and for a nonlunulate species, Echinarachnius parma
(0.077). The difference in CL, which was shown to be statistically significant (P = 0.001), can be attributed partly to differences in camber and partly to the presence of lunules. Observations of water flow at 10 cm·sec−1 confirmed that flow was
attached and of a lift-generating pattern. Flow around E. parma was hindered by a relatively large standing vortex at the anterior margin. For M. sexiesperforata, which is thinner edged and less steeply cambered, the flow was more perfectly attached; flow from oral to aboral
surfaces via the lunules was distinct. Similarly, Encope emarginata had a closely attached flow with some passage through the anal lunule and ambital notches. Calculations from weight-length relationships and coefficients of lift indicated that M. quinquiesperforata should
be able to maintain its position in more severe current regimes than could E. parma. Excess pressure on the oral surface of lunulate sand dollars is relieved by flow along pressure drainage channels which lead from the central region of the disc into the lunules and ambital notches.
Scanning electron micrographs show that these lack the podia characteristic of food grooves and that podia in adjacent food-gathering areas sweep parallel to or away from the pressure drainage channels. Several earlier theories of lunule function appear to be either insufficiently general
or untenable. That lunules shorten the food-path is not fully supported by observation; that they aid in righting movements is contrary to observations of Mellita species and that they might strengthen the test is shown by both observation and theoretical considerations to be incorrect.
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