Protozoans

Prokaryotes represent only a few thousand of the estimated 2 million species of living beings known on the earth today. The rest are mostly eukaryotes that form the subject matter of the rest of this course. You have also learnt that living organisms are now classified into 5 kingdoms. Monera, Protista, Fungi, Plantae and Animalia. Of these, Protista are the primitive unicellular eukaryotes.

Figure: The Five-Kingdom System of Classification and its relationship to the plant / animal dichotomy.

Some of these have given rise to Fungi, others have give rise to Plantae, yet others have given rise to Animalia, all the three groups being multicellular. However, some of the Protista, though unicellular, still so much resemble Animalia that earlier, this group used to be designated Protozoa and was treated as a phylum under Kingdom Animalia of the two kingdom classification. Now we know that protozoans are really a heterogenous group of unicellular organisms add the term has no formal biological status. Even then, because of their similarity to animals and because most zoologists are interested In this group of organisms too.