Algae are protists that have a cell wall. They are also possess a box-like shape and a nucleus. The cells walls comprise of cellulose. Protozoans have a variety of shapes, in contrast, and they are usually more circular than box-like in structure. Volvox, for ex., have a very distinct circular shape. Protozoans are also unicellular, but algae can be either multicellular or unicellular. Protozoans also lack the concise, rigid shapes that algae possess because protozoans lack a cell wall. Spirostomum, for ex., are "worm-like" in structure as they lack a cell wall.