Kingdom Protista

Kingdom Protista holds all the single-celled eukaryotes. Each cell has a proper nucleus and membrane-bound organelles. Most live in water. They reproduce both asexually and sexually (two cells fuse to form a zygote).

• Chrysophytes (diatoms and golden algae) float as plankton. A diatom's wall is two thin overlapping shells, like a soap box, made with silica, so it does not decay. Their walls pile up over ages into diatomaceous earth. Diatoms are the chief producers of food in the oceans.
• Dinoflagellates are mostly marine, with stiff cellulose plates and two flagella. Red ones like Gonyaulax can multiply so fast that the sea turns red: a red tide. Their toxins can kill fish.
• Euglenoids have no cell wall; instead a flexible protein layer called a pellicle, and two flagella. Euglena photosynthesises in sunlight, but in the dark it feeds on smaller organisms.

Slime moulds feed on dead and decaying matter. In good conditions many cells come together into a spreading mass called a plasmodium, which you may see on rotting wood. In bad conditions it forms fruiting bodies with tough-walled spores that can survive for years.

Protozoans are all heterotrophs, and are thought to be relatives of animals. Four groups:

• Amoeboid: move with false feet (Amoeba, and Entamoeba, a parasite).
• Flagellated: Trypanosoma, which causes sleeping sickness.
• Ciliated: covered in tiny hairs (cilia); Paramoecium has a gullet into which food is swept.
• Sporozoans: Plasmodium, which causes malaria.