Algae, dinoflagellates. Color enhanced scanning electron micrograph (SEM) of Protoperidinium leonis. The name "dinoflagellate" refers to their forward- spiraling swimming motion. They are unicellular eukaryotic microorganisms with a cell covering structure (theca) that differenciates them from all other algae. They share features common to both plants and animals: they swim, and many have cell walls.