Their chloroplasts could be photosynthetic prokaryotes that became endosymbionts. The kinetoplastids are symbiotic, and some are pathogenic to their hosts. For example, species of Trypanosoma cause African sleeping sickness, a human disease that is spread by the bite of the tsetse fly. Flagellates reproduce by binary fission. Some flagellates eat other protists.
Dinoflagellates are mostly unicellular but some are colonial.
Each dinoflagellate species has a characteristic shape reinforced by internal plates of cellulose. The beating of two flagella in perpendicular grooves in this armor produces a spinning movement for which these organisms are named. Toxins produced by some red-tide organisms have produced massive invertebrate and fish kills and can be deadly to humans as well. Some cause malaria. They lack contractile vacuoles because their rigid cell walls do not allow excess water in.