Double Spirals, Twice the Fun

Abstract

A logarithmic spiral shell is gnomonic—the snail or cephalopod inside it can add gnomon increments to the edge and thus grow without changing shape. But if one logarithmic shell is gnomonic, then surely the combination of two such shells is gnomonic too! This opens up interesting possibilities. You can connect the two shells with a hinge, making an enclosed space which you can open for feeding and close for protection (Fig. 35.1). The whole structure can be shaped like a wedge, which can dig down into the mud. The scallops, Pectinidae, can even flap their shells and swim away with quirky movements.