44 MAN, — THE ANIMAL 



single-celled condition, just as the ameba was but 

 a representative of numerous single-celled ani- 

 mals. 



Bacteria appear in three general shapes — the 

 straight rod, known as the bacillus (plural ba- 

 cilli) ; the bent rod, the spirillum; and the sphere, 

 the coccus or micrococcus. When we remember 

 that there are more than 1500 different kinds of 

 bacteria and that all of these different kinds must 

 come within these different shapes, we see how 

 utterly impossible it is to distinguish the different 

 kinds by shape. (Fig. 2.) 



Bacteria used to be spoken of as the smallest 

 living units, but science has revealed a group of 

 living things known as ultra-microscopic, so that 

 bacteria are considered as relatively large. How- 

 ever, they are so small that the high powers of the 

 microscope are required to reveal even their 

 presence. When one speaks of their size and 

 suggests that one hundred of the micrococci might 

 be placed on a single period found at the end of 

 one of the sentences on this page, no accurate form 

 of measurement is presented. When scientific 

 workers tried to measure these minute plants, the 

 millimeter was found to be too large, the divisions 

 of the inch having been found previously to be en- 

 tirely unsatisfactory. The next problem was to 

 divide the millimeter into thousandth parts, each 

 of which is called a micron or micromillimeter. 



