• W. C. Corning
  • R. von Burg


Although all animals experience an ontogenetic stage when they are unicellular (the fertilized egg), the protozoans remain unicellular throughout life and have most successfully used this condition in the diverse environments in which they are found. While protozoans are usually thought of as being single-celled, they are not a single version of a metazoan cell nor are they generally as simple. It is probably more accurate to designate them as acellular organisms, as animals not divided into cells (Barnes, 1966; Hyman, 1940). Some of the organelles in this phylum (cilia, for example) perform functions taken over by highly specialized cells in metazoans. The term unicellular is also misleading because it tends to obscure the fact that some protozoans are colonial—they exist in multicellular aggregates. Accordingly, the following definition seems appropriate:

“The Protozoa are acellular animals without tissues or organs, existing singly or in colonies of a few to many individuals; such colonies differ from a metazoan in that their components are all alike except when engaged in reproductive activities.” (Hyman, 1940, p. 45)

The characteristic of “acellularity” is the only one applicable to all protozoans, as the diversity and range within the phylum are wide, perhaps due to their multiple origins.


Mechanical Stimulation Ciliated Protozoan Multicellular Aggregate Response Decrement Locomotor Apparatus 
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Copyright information

© Plenum Press, New York 1973

Authors and Affiliations

  • W. C. Corning
    • 1
  • R. von Burg
    • 2
  1. 1.Division of Biopsychology, Department of PsychologyUniversity of WaterlooWaterlooCanada
  2. 2.Division of BiologyRamapo State CollegeMahwahUSA

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