ON THE EVOLUTIONARY STABILITY OF THE 9 + 2 PATTERN

Abstract

A single variant cilium, found among a field of normal cilia of the lateral cells of the gill of Elliptio complanatus, possesses twelve peripheral filaments instead of the usual nine. The structure of these filaments and the other parts of the ciliary stalk are normal, but the filaments do not strictly form a ring around the central pair. This implies that the usual ciliary pattern, 9 + 2, is maintained by selection because it functions most efficiently in motility or development, and not only for geometrical or spatial reasons.