dinoflagellate, originally uploaded by sconartist.
I showed this image to introductory biology students, and some were able to identify the single-celled algae right away. The give away is the spiral-like flagellum resting in the central groove. (If you don't know what a flagellum is you can think of it as a tail that propels a single cell.)
The spiral-like tail causes the cell to spin as it moves through the water - thus it's name. "Dino" means spinning, and "flagellate" refers to the cell's tail.