SPECIAL CHARACTERS 29 



forms. Bacteria in their ordinary development are said to 

 be vegetating, and we must differentiate between the vege- 

 tative stage and the spore-forming stage. 



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

 mals of protean shape. They vary in size from that of 

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

 length. The cell wall varies considerably; it may be well 

 formed and 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 homo- 

 geneous, at other times full of granules, septa, or a dividing 



FIG. 9. Unstained spores in distended ends of bacilli. (Park.) 



meshwork. The nucleus is a complex 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 centrosome. 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 structure and producing locomotion by a thrash- 

 ing or whip- like motion. Perhaps the simplest and surely 

 the most primitive form of motion is produced by what are 



