The Protozoa

The name Protozoa comes from the Greek, meaning 'first animal', and was originally applied to single-celled organisms regarded as having animal-like characteristics (multicellular animals were termed Metazoa). Protozoans as a group have evolved an amazing range of variations on the single-celled form, particularly with respect to the different means of achieving movement. They are a morphologically diverse group of well over 50 000 species; although the majority are free-living, the group also includes commensal forms and some extremely important parasites of animals and humans.

A commensal lives in or on another organism, deriving some benefit from the association but not harming the other party.

Most protozoans are found in freshwater or marine habitats, where they form a significant component of plankton, and represent an important link in the food chain. Although water is essential for the survival of protozoans, many are terrestrial, living saprobically in moist soil.

Remember that a protozoan needs to pack all the functions of an entire eucaryotic organism into a single cell; consequently a protozoan cell may be much more complex than a single animal cell, which is dedicated to a single function. Thus, protozoans display most of the typical features of a eucaryotic cell discussed in Chapter 3, but they may also have evolved certain specialised features. The single cell is bounded by the typical bilayer membrane discussed earlier, but depending on the type in question, this may in turn be covered by a variety of organic or inorganic substances to form an envelope or shell.

One of the most characteristic structural features of protozoans is the contractile vacuole, whose role is to pump out excess amounts of water that enter the cell by osmosis. The activity of the contractile vacuole is directly related to the osmotic potential differential between the cell and its surroundings. This is vitally important for freshwater protozoans, since the hypotonic nature of their environment means that water is continually entering the cell. The contractile vacuole often has a star-shaped appearance, the radiating arms being canals that drain water from the cytoplasm into the vacuole.

Most protozoans have a heterotrophic mode of nutrition, typically ingesting par-ticulate food such as bacteria, and digesting them in phagocytic vacuoles. Since they actively 'hunt' their food rather than simply absorbing it across the cell surface, it is not surprising that the majority of protozoans are capable of movement. The structural features used to achieve locomotion (e.g. cilia, flagella) are among the characteristics used to classify the protozoans.

We shall now examine the characteristics of the principal groupings into which the protozoans have traditionally been divided. It should be repeated, however, that the Protozoa do not represent a coherent taxonomic grouping with a common ancestor, but rather a phylogenetically diverse collection of species with certain features in common. Indeed, each of the four groups is now regarded as having a closer evolutionary relationship with certain 'algal' groups than with each other. See Figure 9.18 for a modern view of how the various taxonomic groupings of protozoans are related.

Plankton are the floating microscopic organisms of aquatic systems.

The contractile vacuole is a fluid-filled vacuole involved in the osmoregu-lation of certain protists.

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