The only characteristic all protozoa have in common is that they are single celled.
The phyla into which protozoa fall vary with the taxonomic scheme. Four phyla that are commonly recognized are Sarcodina (those moving by cytoplasmic projection, or pseudopodia; the amoebas); Ciliophora (those moving by cilia); Zoomastigina or Mastigophora (those characterized by one or more flagella); and Sporozoa (immobile and parasitic protozoans).
Below are some characteristics found in each phylum (from BiologyJunction.com):
Sarcodina
Found in freshwater, marine, & moist soil habitats
Usually reproduce asexually
Their cytoplasm consists of clear, outer ectoplasm and granular, inner endoplasm
Cytoplasm extensions are called “false foot” or pseudopods
React to stimuli such as light
Ciliophora
Called ciliates because they move by short, hairlike cilia lining the cell membrane
Cilia may be modified into teeth, paddles, or feet
Most found in freshwater, but some are marine
Form protective cysts to survive unfavorable conditions
Can reproduce sexually by conjugation (two paramecia join together & exchange DNA)
Zoomastigina
Called Zooflagellates because have one or more whiplike flagella to move
Flagella made of bundles of microtubules
May be freshwater or marine
Some are parasites such as Trypanosoma that destroy red blood cells & causes fatal African sleeping sickness
Trichonympha lives symbiotically inside termites & digests cellulose
Sporozoa
Adult sporozoans have no structures for movement
Form spores
Most are parasitic using one or more hosts
Immature sporozoans are called sporozoites & live in body fluids of hosts
Plasmodium is transmitted by mosquitoes & causes malaria
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