PROTOZOA. 97 



THE PROTOZOA. 



We have seen in Chapter III. that the animal kingdom 

 can be naturally divided into two sub-kingdoms, 



1. PROTOZOA. 



2. METAZOA. 



All the Protozoa are homologous with single cells. The 

 body of a Protozoan is a single cell, and all differentiations 

 take place within the cell, or are intra-cellular. For 

 example, the mouth of a Protozoan leads into the interior of 

 a cell and not, as in the Metazoa, into a space between a 

 number of cells.' The same consideration applies to every 

 other organ. This is sometimes emphasised by using the 

 terms cell-mouthy cell-anus, &c. 



In a number of sedentary Protozoa {cf, Vorticella) the 

 products of binary fission remain in organic continuity, and 

 form a *' colony " of many individuals. The colony is 

 evidently a multicellular aggregate, but in the majority of 

 cases each cell retains all its vital functions of alimentation, 

 locomotion, sensation, and excretion. Hence there is little 

 or no united individuality of the aggregate, and it is 

 regarded as a colony of Protozoa rather than a metazoan 

 individual. In a few colonial Protozoa^ such as Zootham- 

 nium, there is a physiological division of labour not affecting 

 the primary vital functions, but only between these and the 

 secondary reproductive function. Some of the individuals 

 of the colony have no mouth nor cilia, and are themselves 

 solely concerned with the production of reproductive 

 elements, depending for the exercise of vital functions upon 

 the other individuals. This is the nearest approach in 

 colonial Protozoa to the complete physiological dependence 

 of the constituent units of a metazoan. 



The Protozoa must be regarded as the representation in 

 miniature of the metazoan type, showing us the possibihties 

 of adaptation with the single cell as a unit ; hence, although 

 the sub-kingdom only includes very small and apparently 

 unimportant animals, it must be regarded as having the 

 same morphological value as the Metazoa. 



M. 8 



