oligotrich, any spherical to pear-shaped protozoan of the ciliate order Oligotrichida, found in fresh, salt, and brackish water. Body cilia (minute, hairlike projections), when present, are often fused into groups of bristles, or cirri. The oligotrichs have conspicuous adoral (on margin of groove leading to the mouth) ciliature. For many species an anterior spiralling band of membranelles (cilia fused into a flat plate) serves as an efficient, and in some cases the only, means of locomotion. The species Halteria grandinella is a common freshwater representative of the order. Small and spherical, it has seven groups of three cirri set in small grooves along the middle of the cell. The action of the cirri causes the cell to bounce through the water.