This study confirms and extends previous work on the lateral cilia of the fresh-water mussel, Elliptio complanatus, in support of a "sliding filament" mechanism of ciliary motility wherein peripheral filaments (microtubules) do not change length during beat (see Satir, 1967). Short sequences of serial sections of tips are examined in control (nonbeating) and activated (metachronal wave) preparations. Several different tip types, functional rather than morphogenetic variants, are demonstrated, but similarly bent cilia have similar tips. The peripheral filaments are composed of two subfibers: a and b. The bent regions of cilia are in the form of circular arcs, and apparent differences in subfiber-b length at the tip are those predicted solely by geometry of the stroke without the necessity of assuming filament contraction. Various subfibers b apparently move with respect to one another during beat, since small systematic variations in relative position can be detected from cilium to cilium. While subfiber-b lengths are uniform throughout, subfiber-a lengths are morphologically different for each filament: 8 and 3 are about 0.8 µ longer than 1, 4 and 5, but each unique length is independent of stroke position or tip type. Subfiber-a does not contract, nor does it move, e.g. slide, with respect to subfiber-b of the same doublet. The central pair of filaments extends to the tip of the cilium where its members fuse. Subunit assembly in ciliary microtubules is evidently precise. This may be of importance in establishing the relationships needed for mechanochemical interactions that produce sliding and beat.