Protozoa,minute,aquatic creatures each of which consists of a single cell of protoplasm,consitute a classification of the most primitive forms of animal life. They are fantastically diverse,but three major groups may be identified on the basis of their motility. The mastigophora have one or more long tails, which they use to project themselves fordward. The Ciliata, which use the same basic means for locomotion as the Mastigophopra,have a larger number of short tails. The Sardocina, which include amoebae,float or row themselves about on their cursted bodies.
In addition to their form of movement, several other features discriminate among the three groups of protozoa. For example,at least two nuclei per cell have been identified in the Ciliata,usually larger nucleus that regulates growth but decomposes during reproduction,and a smaller one that contains the large nucleus.
Protozoan are considered animals because,unlike pigmented plants to which some protozoans are otherwise almost identical,they do not live on simple organic compounds. Their cell demonstrates all of the major characteristics of the cells of higher animals.
Many species of protozoans collect into colonies, physically connected to each other and responding uniformly to outside stimulate. Current research into this phenomenon along with investigation carried out with advanced microscopes may necessitate a redefinition of what constitues protozoans,even calling into question the basic premise that they have only one cell. Nevertheless, with the current data available,almost 4.000 species of protozoans have been identified. No doubt,as the technology improves our method of observation, better models of classification will be proposed.
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