Although not such an all-embracing a term as originally envisaged by Haeckel, the Protista represents a very diverse group of organisms, united by their possession of eucaryotic characteristics, and failure to fit satisfactorily into the animal, plant or fungal kingdoms. Some scientists limit use of the name to unicellular organisms, while others also include organisms such as the macroscopic algae, which are not accommodated conveniently elsewhere.
It has become clear from molecular studies that some members of the Protista bear only a very distant relationship to each other, making it an unsatisfactory grouping in many respects. In an introductory text such as this, however, it is convenient and, it is hoped, less confusing, to retain the name Protista as a chapter heading, as long as the student understands that it does not represent a coherent taxonomic grouping of phylogenetically related organisms. At the end of the chapter, we shall look at how members of the Protista are placed in modern, phylogenetic taxonomic schemes.
It has been found helpful in the past to think of Protists as being divided into those with characteristics that are plant-like (the Algae), animal-like (the Protozoa) and fungus-like (the water moulds and slime moulds). We shall discuss each of these groups in turn in the following pages. It should be borne in mind, however, that molecular evidence suggests such a division to be artificial; on the basis of molecular and cytological comparison, the 'animal-like' protozoan Trypanosoma, for example, is closely related to the photosynthetic (and therefore 'plant-like') Euglena.
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