WEIGHT BELTS, DIVERTICULA, AND THE PHYLOGENY OF THE SAND DOLLARS

The intestinal diverticulum found in some juvenile clypeasteroids cons ists of a series of tubes and pouches in the peripheral part of the te st. When distended with sand grains, the diverticulum can form a weigh t belt. We conducted a survey of the occurrence of the weight belt in the three major clades: the suborders Clypeasterina, Laganina, and Scu tellina. Dissections and radiographs of juvenile representatives of al l extant clypeasteroid families, the majority of genera, and some foss il species, indicate that the weight belt is found only in the Scutell ina. The diverticulum and weight belt are absent throughout the ontoge ny of clypeasterines and laganines. The weight belt is therefore a syn apomorphy for the extant scutelline sand dollars. Some implications of this discovery for the phylogeny of clypeasteroids and the adaptive s ignificance of the weight belt are discussed. Primary among these are the observations that: 1) the presence of a weight belt in miniaturize d tars such as Sinaechinocyamus and Marginoproctus supports their assi gnment to the Scutellina, and 2) the distribution of the weight belt c haracter in the Clypeasteroida is best interpreted as a result of phyl ogenetic history, and not of adaptive factors.