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 AZefazoa, into a space between a 
number of cells. The same consideration applies to every 
other organ. This is sometimes emphasised by using the 
terms cel/-mouth, 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 /rofozoa rather than a metazoan 
individual. Ina 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 possibilities 
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 Aezazoa. 
M. | 8 
