FEEDING STRUCTURES, BEHAVIOR, AND MICROHABITAT OF ECHINOCYAMUS PUSILLUS (ECHINOIDEA: CLYPEASTEROIDA).

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.