Protozoan Protists

The protozoan protists are also known as animal-like protists. They are represented by unicellular animals which were earlier placed in the phylum Protozoa under Animal kingdom. They are now identified in four groups zooflagellates, sarcodines, ciliates and sporozoans.

Zooflagellates

Zooflagellates are unicellular organisms which may be free living or parasitic. Many of them occur as parasites in various hosts. Some of them occur as symbionts.

They have an elongated body containing a single, large nucleus. The body is surrounded by a transparent pellicle. One or two flagella are present which are used for locomotion and food capturing.

Nutrition may be holozoic or saprotrophic or parasitic. Reserve food is stored as glycogen.

The most common method of reproduction is by binary fission. Some forms exhibit cyst formation. Sexual reproduction is very rare.

The most familiar example is that of Trypanosoma which occurs as a parasite in human body. It causes the African sleeping sickness. Leishmania is another human parasite which causes a disease called Kala-azar. Giardia is another example of a human parasite. Trychonympha is symbiotic parasite in the body of wood termites.

fig. 8.14 - Trypanosoma

Sarcodines

Sarcodines are protistans which are known for not having a definite body shape. They are amoeboid forms which may be free living or parasitic.

The sarcodines are known for developing temporary projections of the protoplasm, called pseudopodia, which help in locomotion and food capturing.

The body usually does not have a pellicle. The cytoplasm may be uninucleate or binucleate or multinucleate.

Nutrition is holozoic or parasitic.

Asexual reproduction is most common. It occurs by binary fission or multiple fission or in some cases, by spore formation. Sexual reproduction may involve isogamy or anisogamy.

Sarcodines can be recognised under the following categories

Amoeboids

• with a totally irregular body shape.

Eg: Amoeba, Entamoeba

Radiolarians

• which have a siliceous skeleton. Eg: Acanthometra

Foraminiferans

• which have a perforated chambered shell.

Eg:Elphidium

Heliozoans

• with a glassy silica shell. Eg: Actinophryx

fig. 8.15 - Amoeba and Elphidium

Ciliates

These are a group of diverse protozoans characterized by the presence fine hair like cilia as locomotor structures. They are aquatic and free living or parasitic forms.

Ciliates show a high degree of morphological and physiological specialisation compared to other animal like protistans. They possess specialised organelles that perform specific functions.

The body is covered by a pellicle. Cilia may be found all over the body or restricted only to certain regions. Trichocysts are present all over the body as structures involved in offence and defence.

Ciliates are binucleate and exhibit a characteristic nuclear dimorphism. The small, round micronucleus controls reproduction while the large bean or horseshoe shaped meganucleus controls vegetative functions.

Ciliates reproduce asexually by binary fission. Sexual reproduction is common. It occurs by a process called conjugation which involves exchange of genetic material between two individuals.

Paramecium is the most familiar example of ciliates. Another common example is Vorticella a colonial form. Balantidium is a human parasite. Blepharosoma is large, free living and pink in colour.

fig. 8.16 - Paramecium and Vorticella

Sporozoans

These are protozoans which occur as endoparasites in different groups of animals. They lack locomotor structures. The body has a tough covering called cuticle and a single nucleus.

Asexual reproduction occurs by multiple fission. Sexual reproduction involves anisogamy. Life cycle may involve more than one host.

The most familiar example is Plasmodium, which causes malaria in human beings. It requires two hosts for completing its life cycle mosquito and man.

Monocystis is a parasite in earthworms.

fig. 8.17 - Life cycle of Plasmodium