Araphid diatoms are non-circular symmetrical diatoms that don't have a raphe, the system that other species of diatoms use to move around on plants and through mud.  Because they have no raphe, araphids do not have fine control over their mobility.  Most are free-floating in the water column and drift with the currents, although some species can still control their up/down movement in the water column by producing gasses that make them float more or less.