Protozoa

Protozoa:

(Protos→ first/ primitive, Zoon→Animal)

Protozoa was first proposed by Gold fuss in the year 1817. However, protozoa were first studied by Antony van Leeuwenhoek in 1677. The scientific study of protozoa is called protozoology. Protozoans are called unicellular as well as a cellular organism.

On the structural basis, protozoan is called unicellular as it can be compared with a single cell of multicellular organisms but on the functional basis, it is called a cellular as it can be compared with the whole body of multicellular organisms i.e. all physiological processes are performed by a single cell.

Characters of phylum protozoa:

Habit and Habitat:

Most of them are free living. Some are parasitic or commensal on symbiotic. Free-living protozoans are mostly aquatic. Some are found in damp soil.

Body form:

They are unicellular, eukaryotic and microscopic. Their shape is variable. The body is covered with the cell membrane, pellicle, siliceous and calcareous shell.

Level of body organisation:

Their body organisation is molecular or protoplasmic.

Symmetry:

They have bilateral or radial symmetry. Some are asymmetrical.

Mode of nutrition:

Nutrition is mainly holozoic. Some are parasitic.

Digestion:

Digestion is intracellular i.e. occurs inside the cell.

Locomotion:

Locomotory organelles may be present or absent. If present they may be pseudopodia or cilia or flagella.

Respiration

It occurs in through general body surface.

Excretion:

It also occurs general body surface.

Osmoregulation:

It is carried out by one or more contractile vacuoles which are absent in parasitic and marine protozoans.

Reproduction:

It is carried out by the asexual method and sexual method. Asexual method of binary fission, budding and sexual method like conjugation, syngamy i.e fusion of gametes.

Encystment:

It is the formation of protective covering(cyst) around the body during unfavourable condition for survival.

Classification of phylum Protozoa:

On the basis of locomotory organelles, phylum protozoa is divided into five different classes:

Class: Rhizopoda (Gr. Rhizo-root Podos-foot)

• Pseudopodia are the chief organelles of locomotion and food capturing.
• The body is covered with the cell membrane and hard shell.
• Most of them are free living. Some are parasitic.
• One or more nuclei are present.
• Fresh water forms have one or more contractile vacuole.
• Reproduction mainly occurs by binary and multiple fission. Some reproduce by syngamy.
• Eg: Amoeba, Entamoeba, Arcella.

Class: Flagella/ Mastigophora:( Gr. Mastix- whip Phorrs- bearing)

• Flagella are chief organelles of locomotion and food capturing.
• The body is covered with the thin pellicle.
• They are free-living and parasitic.
• Only one nucleus is present.
• Fresh water forms have one or more contractile vacuole.
• Reproduction mainly occurs by longitudinal binary fission.
• Example: Giardia, Leishmania.

Class: Cilia( Gr. Cilium- hair like ata- bearing)

• Cilia are the chief organelles of locomotion and food capturing.
• The body is covered with the thick pellicle.
• Mostly free living some parasitic.
• Morphologically two different type of nucleus is present.
• Fresh water forms have one or more contractile vacuole.
• Reproduction mainly occurs by transverse binary fission and conjugation.
• Example: Paramecium, Varicella

Class: Sporozoa: (Gr. spora- spore zoon- animal)

• They do not have locomotory organelles.
• The body is covered with the pellicle.
• They are exclusively parasitic.
• Single nucleus is present.
• A multinucleated stage is found.
• Absent of contractile vacuole.
• Reproduction mainly by multiple fission and syngamy or formation of spores.
• Example: Plasmodium, Monocystis.

Class: Suctoria

• No locomotory organelles in adult stage but cilia are present in the young stage.
• The body is covered with cell membrane or gelatinous sheath.
• Mostly free living.
• One macronucleus and many micronuclei are present.
• Several contractile vacuoles are present.
• Reproduction occurs by budding.
• They are sessile i.e attached to the substratum by the stalk.
• They have suctorial tentacles for food capturing.