Tong, 1997. Cells are 1.2-4 microns (usually 2.5microns), arms 3.5-8microns, stalk 2-16.5 microns Cells are spherical with between 16 and 30 (20 seems usual) fine radiating arms spaced at regular intervals. The arms vary in length, but are all of the same length on all individual cell. Cells arc frequently seen attached to surfaces (diatoms, filamentous cyanobacteria or detrital flocs) by a stalk. The stalk is thicker than the arms at the proximal end, but is frequently extended by a fine cytoplasmic thread which seems identical to the arms except that it can greatly exceed them in length. When attached, cells are sometimes seen to vibrate, the purpose and mechanism of this activity is unknown. Cells in mono-culture only rarely have stalks. When cells are in contact with a surface (e.g. on a microscope slide) they can shift position slightly, with the aid of the arms, which bend slightly during the process.