Introduction:

Usually, the protozoa are grouped altogether based on the general similarities: they lack cell walls, they are colorless and motile and they show a broad range of morphologies and inhabit numerous various types of habitat, and they play main roles in the human society and health. The various forms of protozoa have been grouped altogether not because they are all associated in an evolutionary manner, however simply for convenience. The old categorization schemes of protozoa were based mainly on organelles of locomotion. The main groups are termed as Phyla (that is, singular, phylum).

The Flagellates (Subphylum Mastigophorea):

These protozoa are conventionally categorized into two groups:  The plant-like forms (class Phytomastigophorea, the phytoflagellates) and the animal-like forms (class Zoomastigophorea, the zooflagellates). Plant-like protozoa generally have yellow or green chloroplasts and also flagella and are photosynthetic. The zooflagellates contain no chlorophyll and are heterotrophic. All members encompass one or more flagella. Some encompass pseudopodia. Asexual reproduction takes place by longitudinal binary fission. A form of multiple fission occurs in some organisms. Encystment is common however sexual reproduction is not.

These organisms are considered as algae through some biologists. As the zooflagellates encompass no chlorophyll, they should obtain nutrition heterotrophically. All the members of this group encompass one or more flagella; some members are capable of making pseudopodia.

The Zooflagellates (Class Zoomastigophorea):

The Choanoflagellates (order Choanoflagellida) are distinctive in that they are either stalked or embedded in the jelly and each cell consists of a thin transparent collar which encircles a single flagellum. The collar functions as a food catching device.

Organisms in the order Kinetoplastida are grouped altogether since of the presence of a kinetoplast (that is, an extra nuclear area of DNA related having the mitochondrion). The single mitochondrion itself is extensive, traversing the length of the body as single tube, hoop or network of the branching tubes. One or two flagella might be present; if there are two; one is either trailing free or joined to the body, with undulating membranes taking place in some cases.

The Amoebas (Subphylum Sarcodina):

Structure:

a) Amoebas attain their name from the Greek word 'amoibe', meaning 'change' as their shapes are constantly changing. A usual instance is Amoeba proteus.

b) Amoebas are comprised of protoplasm distinguished into a cell membrane, cytoplasm and a nucleus. The cytoplasm exhibits granules and also vacuoles having food, water, wastes and possibly gases. The outer membrane is selective and permits the passage of certain soluble nutrients to the cell and waste materials out of the cell. Solid food is ingested by the help of pseudopodia. The nucleus functions in metabolism, reproduction and the transmission of hereditary features.

c) Amoeba reacts to different chemical and physical stimuli in their surroundings. This is an irritability response that is at least superficially analogous to responses of higher organisms to their atmosphere.

Nutrition of Amoeba:

• Employs pseudopodia to capture its food.
• Reproduction - amoeba is asexually binary fission.

The Sporozoa (Phylum Apicomplexa):

They are in the constant motion. They move by sending out parts of their bodies in one direction which the whole body follows. They employ pseudopodia to capture the food. Reproduction is asexually through binary fission. Some encompass the capability of encysting in unfavorable condition. Most are free living; a few are saprophytic; though, one species Entamoeba histolytica causes amoebic dysentery in the man. All Sporozoa are parasitic for one or more animal species.  Adult forms contain no organs of motility however all are probably motile by gliding at one phase of their life-cycle. They can't engulf solid particles, however feed on the host's cells or body fluids.

Most of them encompass complicated life cycles, certain phases of which might take place in one host and other phases in a different host. They all produce spores at several times in their life history. Their life-cycles show an alternation of generations of sexual and asexual forms, in such a way that the intermediate host generally harbors the asexual forms and the final host, the sexual forms.

At times, humans serve as the hosts to both forms.

Toxoplasmosis and malaria are the main human diseases caused by the Sporozoa. Malaria is caused due to Plasmodium asporozoa that infect the liver and red blood cells.

The Ciliates (Phylum Ciliophora):

The ciliates are protozoa having cilia for locomotion. Common illustrations of the ciliated protozoa are comprised in the genus Paramecium, found in the fresh-water ponds and lakes where sufficient food supplies exist.

Structure/Morphology:

Paramecium:

a) Paramecium moves fast by rhythmic beating of the cilia.

b) Nutrition: Paramecium takes in food via a fixed cytostoma at the base of the gullet.

c) Excretion is via the contractile vacuole.

d) Reproduction - Reproduce asexually through binary fission conjugation might as well take place.

Paramecia are microscopic, some, though, are merely barely visible to the unaided eye. The outer layer of the cell is less flexible than the outer membrane of the amoeba and the interior is comprised of semi fluid, granular protoplasm having nuclei and vacuoles of some kinds.

Paramecia are simply differentiated by their feature shape that has been likened to that of a slipper. The anterior (front) end of the cell is rounded and the posterior (rear) end is slightly pointed. The whole cell is covered having hundreds of short hair-like projections termed as cilia that are the organs of locomotion and as well serve to direct food into the cytosome.

Other Ciliated Protozoa:

The ciliated protozoa are represented by numerous forms other than the paramecia. Colpoda is a common freshwater genus. The genus Didinium lives on a diet of paramecia that are captured by a special structure and swallowed whole.  The genuses Stentor includes big cone-shaped protozoa which move about freely however join to some object through a tapered lower end while feeding.

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