The movement of Bacillaria occurs because the cells slide against each other, but can only move to the end of their valves, then they slide back. This species is unique in this movement. All of these pennate diatoms move, but the centric ones cannot.
Centric diatoms are radially symmetrical, pennate diatoms are bilaterally symmetrical.
You should be able to distinguish diatoms from dinoflagellates and from reproductive structures on seaweeds. This is a tiny scrap of a tube-dwelling diatom that was lurking among the filaments of an ectocarpoid brown seaweed in the Marine Lab tanks. There are lots more pictures of tube-dwelling diatoms in this part of the web, where you can see how they can get big enough to be mistaken for seaweeds. The only one that gets so big here is Nitzschia martiana . This specimen is probably Parlibellus delognei , but there's not enough to isolate the cells are acid clean the valves.