Sub-Kingdom I. 



Protozoa. 



It has already been noticed in the general part that the animals 

 included in the group Protozoa are simple cells, each individual 

 consisting of a single cell only. In certain cases, however, several 

 individuals may be united to form colonies, approaching the 

 metazoan condition, where each individual consists of groups of cells. 

 There is, however, an essential difference between a protozoan colony 

 and a metazoan individual, since the former consists of cells which 

 are identical in the more important respects, whilst the cells of the 

 latter vary in structure and in function, division of labour having 

 occurred. 



In all the Protozoa, the body consists of protoplasm with a 

 nucleus; in many there are numerous nuclei. The nucleus is 

 usually spherical or oval, but sometimes more elongate. The protoplasm 

 frequently contains vacuoles, small cavities filled with a fluid ; 

 some of these are contractile, i.e., they contract and enlarge 

 alternately as the fluid is expelled or fresh supplies are obtained from 

 the cell substance. Their contraction is, of course, dependent upon pro- 

 toplasmic movements, since they have no definite walls. Apparently 

 their function is essentially excretory, and in various forms uric 

 acid has been found in their contents. Not infrequently different 

 substances are secreted by the protoplasm, such as oil globules, 

 pigment granules, and so forth. Very often, too, it secretes 

 skeletal structures usually consisting of lime or silica ; these 

 will be considered in greater detail under the Rhizopods and Radio- 

 larians. 



Many Protozoa, like the Amoeba, can protrude pseudopodia 

 from any part of the body, so that its external form is constantly 

 changing. In others this power of amoeboid movement is entirely 

 wanting, the outer layer of protoplasm being firmer than the inner 

 softer portion, from which, however, it is often not, sharply marked 



