Protozoa
They are a group of single-celled eukaryotes, either free-living or parasitic, that feed on
organic matter such as other microorganisms or organic tissues and debris. Historically,
protozoans were regarded as "one-celled animals", because they often possess animal-like
behaviours, such as motility and predation and lack a cell wall, as found in plants and many
algae.
When first introduced by Georg Goldfuss in 1818, the taxon Protozoa was erected as a class
within the Animalia, with the word 'protozoa' meaning "first animals". In later classification
schemes it was elevated to a variety of higher ranks, including phylum, subkingdom and
kingdom and sometimes included within Protoctista or Protista.
Size and Shape
The size and shape of Protozoa vary greatly, from microbial (1µm) to large enough and can
be seen by the naked eye. The shell of unicellular foraminifera can have a diameter of 20 cm.
HABITAT
Free-living protozoa are common and often abundant in fresh, brackish and salt water, as well
as other moist environments, such as soils and mosses. Some species thrive in extreme
environments such as hot spring and hypersaline lakes and lagoon. All protozoa require a
moist habitat; however, some can survive for long periods of time in dry environments, by
forming resting cysts that enable them to remain dormant until conditions improve.
Motility
Organisms traditionally classified as protozoa are abundant in environments and soil,
occupying a range of trophic levels. The group includes flagellates (which move with the
help of undulating and beating flagella), ciliates (which move by using hair-like structures
called cilia) and amoeba (which move by the use of temporary extensions of cytoplasm called
pseudopodia. Many protozoa, such as the agents of amoebic meningitis, use both
pseudopodia and flagella. Some protozoa attach to the substrate or form cysts so they do not
move around sessile. Most sessile protozoa are able to move around at some stage in the life
cycle, such as after cell division.
Protozoa Classification and Examples
Protozoa is a phylum having unicellular heterotrophs. It comes under Kingdom Protista.
Protozoa are divided into four major groups based on the structure and the part involved in
the locomotion:
1. Mastigophora or Flagellated protozoans:
They are parasites or free-living.
, They have flagella for locomotion
Their body is covered by a cuticle or pellicle
Freshwater forms have a contractile vacuole
Reproduction is by binary fission (longitudinal division)
Examples: Trypanosoma, Trichomonas, Giardia, Leishmania, etc.
Trypanosoma
Figure 1: Trypanosoma (Source: https://byjus.com)
2. Sarcodina or Amoeboids:
They live in the freshwater, sea or moist soil.
The movement is by pseudopodia. They capture their prey by pseudopodia
There is no definite shape and pellicle is absent
The contractile vacuole is present in the amoeboids living in freshwater
Reproduction is by binary fission and cyst formation
Examples: Amoeba, Entamoeba, etc
Figure 2: Amoeba (Source: https://byjus.com)
3. Sporozoa or Sporozoans:
They are endoparasitic.
They don’t have any specialised organ for locomotion
The pellicle is present, which has subpellicular microtubules, that help in movement
