The Protozoans constitute a
classification of the most primitive forms of animal life. Each one of those minute, aquatic
creatures consists of a single cell of protoplasm. Although they are amazingly divers, three major groups can be recognized according to their motility. The first group is the Mastigophora, which have one long tail or more used to push themselves forward. The second is the Ciliata, which use the same basic means for locomotion as the Mastigophora and have much more short tails. The third one is the Sarcodina, a well known example is amoebae, float or row themselves about on their crusted bodies. Besides movement features, several other features discriminate among the three groups. For instance, you can find at least two nuclei in one cell of the Ciliata, one of them is a large nucleus that regulates growth but decomposes during reproduction. The smaller nucleus contains the genetic code which is needed for the generation of the larger one.
Why Protozoans are considered as animals? They do not live on simple organic compounds, different from pigmented plants to which some Protozoans almost identical, additionally, their cells demonstrate the major characteristics of the cells of higher animals.
Many species of Protozoans collect into colonies. They are physically accumulated and responding to the outside as one unit. Current researches and investigations on this kind of
creatures still can redefine the Protozoans that they have only one cell. Nevertheless, and according to the available data almost 40,000 Protozoans can be defined. As more technology employed in the investigation of Protozoans, better models of
classification will be proposed.