30 SPECIAL CHARACTERS 



they resist the agencies quickly fatal to the adult 

 forms. Bacteria in their ordinary development are 

 said to be vegetating, and we must differentiate 

 between the vegetative stage and the spore-forming 

 stage. 



Protozoa (sing., Protozoan). Protozoa are single-cell 

 animals of protean shape. They vary in size from that 

 of the smallest bacterium to nearly one-quarter of an inch 

 in length. They are made up of a fairly well-formed wall 

 which may have an appreciable thickness or be merely 

 an immeasurable line. Their cytoplasm, unlike that of 

 bacteria, is usually far in excess of the nucleus. It is 

 sometimes homogeneous, at other times full of granules, 

 septa, or a dividing mesh work. The nucleus is a com- 

 plex body varying from a simple bladder-like mass to a 

 dense and intricately wound skein. The vital activity 

 of the protozoan cell seems to lie in a small body 

 usually in the protoplasm, but originating from the 

 nucleus, called the centresome. Protozoa move by 

 several methods. Some possess short, delicate, hair- 

 like projections from the wall which exhibit a slow, 

 wavy motion. These are cilia. Others have one, two 

 or three long coarser threads, the flagella (sing., 

 flagellum) arising from various parts of the struc- 

 ture and producing locomotion by a thrashing or 

 whip-like motion. Perhaps the simplest and surely 

 the most primitive form of motion is to be seen 

 in what are called pseudopods or false feet, a phe- 

 nomenon characteristic of the amebse. This is a 

 pushing out or budding of a portion of the cell wall 

 into which the cytoplasm of the protozoon flows, 

 enlarging the false foot until it embraces all the con- 



