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The feeding mechanism of Mellita quinquiesperforata (Leske) has been examined in detail. This sand dollar is a deposit feeder, ingesting particles mostly in the range of 100-250 µm. The particles are picked out of the substrate individually by specialized long barrel-tipped podia, which form a narrow palisade surrounding the geniculate spine fields on the oral surface. Selected food items are passed to short barrel-tipped podia, thence from podium to podium until they reach the food grooves where they are finally aggregated into mucus cords. The cords are passed to the mouth by the activity of food groove podia. At the peristome, the cord is passed between the circumoral spines by large food groove podia and steered into the mouth by five pairs of buccal podia. The lantern is powerfully muscled and has hardened teeth which crush diatoms and fracture many sand grains. For this reason, there is an apparent accumulation of fine particles (<50 µm) in the gut. Analysis of size frequencies of the material in the mucus cords and substrate indicates that no selection of fine particles occurs and, in fact, that they are virtually absent from the native sediment. An account of spine and podial morphology and distribution is included with descriptions and measurements of surface ciliary currents. It is shown that the formerly accepted sieve hypothesis of feeding cannot be entirely rejected on theoretical grounds. However, during feeding there was no evidence of the operation of any of the elements of the supposed sieve mechanism. Furthermore, the ciliary currents are not fast enough to account for the movement of most ingested material. Patterns of ciliary flow on the oral surface are not simply centripetal, but are much more complex than previously supposed.

The sand dollars, Leodia sexiesperforata (Leske) and Encope michelini L. Agassiz, have overlapping geographical ranges and may co-occur in mixed flocks. Leodia is restricted entirely to biogenic carbonate sediments. Mellita quinquiesperforata (Leske), which has a similar geographical range to Leodia, occurs only on siliceous terrigenous substrates and the two species never co-exist. Encope michelini L. Agassiz occurs on both types of substrate. All three species are podial particle pickers, and use barreltipped podia, especially the long type surrounding the geniculate spine fields of the oral surface, for food collection. A typical mellitid of 100 mm diameter can have up to one million barrel-tipped podia. These podia have the same mean diameters in Leodia (71.6 5.62 ^m) and Mellita (71.8 3.59 urn). The diversity of sizes is significantly greater in Leodia. The barrel-tipped podia of E. michelini are very much larger (104.4 11.1 ^m). The substrates inhabited by the three species have approximately 90% of their particles in the 100-400 urn range. Whereas Mellita is nonselective in collecting food particles, Leodia clearly selects small particles (50-200 fj.m) and shuns those above 200 nm. Encope michelini includes 26% of particles over 200 ^m in its food grooves, but does not take those below 100 nm. Differences in feeding behavior thus provide a basis for resource partitioning between these sympatric species. They are discussed in relation to podial dimensions and spination, and compared with feeding behavior in Mellita quinquiesperforata.

In the Firth of Lorne, Scotland, Echinocyamus pusillus was found most abundantly in highly variable, poorly sorted substrates at depths of 10-20 m. It was common in areas exposed to extensive wave and tidal current activity, but absent in fine sediments in sheltered areas. In size, feeding mechanism, and behavior, the species is highly adapted for nestling in the interstices between relatively large pebbles. The feeding mechanism is atypical for clypeasteroids: substrate particles with attached organisms are selected and transported by the suckered podia. At the mouth, particles are held in place and slowly rotated by the free margin of the peristomial membrane, while the teeth strip away diatoms and organic debris. The peristomial membrane and ciliation of spines and podia are shown in scanning electron micrographs of critical point dried material. The histology of these structures is described with special reference to mucus secretion. High resolution SEM micrographs show mucus secreting pores among the epithelial microvilli of suckered and buccal podia but not in the epithelium of miliary spines. The suggestion that E. pusillus might represent a sand dollar ancestor is discussed. The evidence presented supports the view that it is specialized rather than primitive.

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