22 INVERTEBRATE ZOOLOGY 



If histological differentiation of the somatic cells occurs, the 

 organism is recognized as a metazoan. 



Definition. — -An inclusive definition of the Protozoa might be 

 given as follows: The Protozoa constitute that phylum of the 

 animal kingdom which includes all single-celled animals and cell 

 aggregates in which there is no histological differentiation of the 

 somatic cells. 



Organization. — Because of their small size, Protozoa^ were 

 entirely unknown to the early scientists. Their study dates 

 from the introduction of the microscope. Most of the early 

 observers maintained that Protozoa are made up of complete 

 systems of organs such as are found in the higher animals. 

 Dujardin denied the presence of organ systems. Definite com- 

 parisons with single cells of the Metazoa were first made by 

 de Bary as early as 1843 but it was left for von Siebold (1848) 

 to describe them as unicellular. 



In lax usage, the term organ is still used in referring to those 

 parts of the protozoan cell which have become adapted to special 

 functions. More correct usage restricts this term to cell groups. 

 Accepting this limitation, it becomes necessary to designate 

 differentiated structures in Protozoa as cell organs or organ- 

 ellae. This distinction has led to a rather common usage of 

 terms such as cytostome (cell mouth), cytopyge (cell anus), 

 cytopharynx (cell pharynx), in referring to the organellae of 

 Protozoa. 



The five classes of this phylum differ so widely in structure and 

 degree of specialization of parts that little may be said that would 

 apply equally to all animals included here. While most Protozoa 

 are small in size, the plasmodia of the Mycetozoa may cover a 

 surface several inches in diameter. At the opposite extreme 

 stand the Sporozoa, many of which pass through a spore stage 

 in which the individual is less than 1 micron in diameter. These 

 represent about the smallest cells known in the animal kingdom. 



The cytoplasm of a protozoan cell is usually divided into a 

 covering ectoplasm and a more distinctly granular internal mass, 

 the endoplasm. Within the ectoplasm, there are extreme differ- 

 ences in organization, for in some instances it is reduced to an 

 extremely thin layer while in others it is stratified into several 

 distinctly separable regions. In the Microsporidia, and some 

 other forms, no differentiation of the cytoplasm into layers has 

 been demonstrated. 



