Protista

The Kingdom Protista includes all the Eukaryotes except for the Plantae, Fungi, and Animalia. A few forms are multicellular, ie the Phaeophyta (brown algae) and Rhodophyta (red algae). The vast majority, though, are single-celled and are typically only 0.01-0.5 mm in size, too small to be seen without a microscope. Protists are ubiquitous throughout aqueous environments and the soil, commonly surviving dry periods in the form of cysts; a few are important parasites.

Traditionally protists have been divided into plant-like forms that have chloroplasts, the Algae, and animal-like forms, the protozoa. The last are generally divided on the basis of morphology and locomotion into flagellates, amoeboids, sporozoans, and ciliates. Except for the last these are all polyphyletic and frequently overlap.

More recently protists have been divided into more genuine groups on the basis of ultrastructural and chemical features. Aside from basal forms, these can be assorted into a few broad categories on the basis of the the form of the mitochondria and in particular the cristae within them.