Subregnum of Animalia
The Protozoa are named after the Greek term for first (proto) and animal (zoon). Their size varies greatly, ranging from a few micrometers (e.g., Cryptosporidium or Plasmodium) up to several millimeters (e.g., gregarines or opalanids). They are all organized according to the basic pattern of the eukaryotic cell, the same type being found in all metazoan cells. They are heterotrophic, lacking the ability to use light and inorganic materials to obtain energy and to synthesize structural components. Therefore, they must obtain pre-formed organic compounds and on this basis may be considered to be animals. Apart from a few sedentary species, most protozoa are motile. Because they have difficulty in retaining water, due in part to their small size, most live in aquatic (or at least moist) environments. Although the majority of Protozoa are free living, many species are mutualists, commensals or true parasites. Some are highly pathogenic to their plant or vertebrate hosts and hence are relevant to veterinary and human medicine and agriculture.
Classification of the Protozoa remains in flux, as new data continue to be obtained. According to the systematics of Levine et al. which, however, are not generally accepted, the Protozoa are considered as a subkingdom divided into seven phyla of which only the first five include parasitic stages to be considered in this book (cf. Classification, Phylogeny):
Phylum: Sarcomastigophora (25 000 recent species)
Phylum: Apicomplexa (Sporozoa) (4800)
Phylum: Microspora (800)
Phylum: Myxozoa (875)
Phylum: Ciliophora (7500)
Phylum: Ascetospora (30)
Phylum: Labyrinthomorpha (35)