ABSTRACT
A typical clypeasteroid non-respiratory podium is composed of three basic systems: 1) a specialized external epidermis, 2) a collagenous connective tissue sheath enclosing a hydrostatic skeleton, 3) a smooth muscle system which operates the tip and stem. The functional anatomy of tissues of each system has now been described with histology and EM. Most podia have both large and small secretory cells in their tip epithelia. Circular fibres form the connective tissue sheath, on the outside of which there are three bundles of longitudinal fibres running the length of the podium. There are five types of non-respiratory podia in the Clypeasteroida. Accessory podia are sensory and manipulatory, barrel-tipped podia collect food and pass it towards the food grooves, where food groove podia aggregate the particles with mucus and move them to the peristome. Large food groove and buccal podia then move the food into the mouth. The diversity of podia accounts for differences in feeding mechanisms of clypeasterine, laganine, rotuline and scutelline forms. Podia of very young sand dollars are different in size and distribution from those of adults. The division of labour amongst podia of the oral surface can be intimately related to the evolution of clypeasteroid feeding mechanisms. Contrary to previous opinion podia, not spines, are the primary feeding organs. In particular, miliary spines cannot now be seriously regarded as the source of feeding mucus. Recent work on clypeasteroid behaviour and podial activity seriously undermines the so-called “aboral rocking sieve” feeding hypothesis and necessitates a reappraisal of outdated accounts of sand dollar feeding mechanisms.
