The means by which cilia remove particles from suspension are of ecological interest because the clearance mechanism determines how much and what kind of food can be obtained by a suspension feeder in a given environment. Clearance is of physiological interest because cilia which must remove particles from a current of water may require greater strength or more rapid alteration of beat than cilia moving in water alone. When both these aspects are considered, the organiza-

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SYNOPSIS. Mastigonemes occur on flagella of many species of Protozoa, but the only function so far proposed for the mastigonemes is to increase the effective surface of the flagellum. Locomotion of Ochromonas malhamensis, O. danica, and an unidentified marine chrysomonad was examined by means of high speed microcinematography up to 600 pictures per second. In addition, analyses were performed on the movement of the organism in a suspension of polystyrene spheres (1.17 μ in diameter). The films clearly show that the organisms move by means of a planar sine wave in the long anteriorly‐directed flagellum. The wave starts at the basal end of the flagellum and progresses distally. It is apparent that the organisms are pulled forward by the action of the long flagellum, and that the flagellum does not push backward as would ordinarily be expected with a forwardly‐directed sine wave. It is well known that flagellar sine waves usually start at the base, move distally, and produce a push toward the point of attachment. It also is known (2) that in Mastigamoeba a helical wave may start at the tip, progress proximally toward the base, and pull the organism. However, in the present organisms the wave begins at the base, moves distally (i.e., forward), and pulls the organism. The flagellum is a true tractellum. The theoretical calculations of Taylor (1952, Proc. Roy. Soc., 214A, 158) show that if a thin cylinder, undergoing sinuous movements produced by a traveling sine wave of active bending, has a roughness of surface which is sufficiently great and has a certain directional character, propulsion may be achieved in the direction opposite to that expected of a smooth cylinder bending in exactly the same manner. It is known that the long flagellum of the chrysomonads is pantonematic, i.e., has numerous stiff lateral mastigonemes on at least two and probably all sides. It is proposed that one function of these mastigonemes is to provide a roughness of surface which brings about this type of reversal.