PROTOPHYTA. 109 
frequently appear in tightly closed vessels whose contents 
have been boiled. Some people have been led to explain 
their appearance under such circumstances by “spontane- 
ous generation;” but thus far the facts are capable of other 
explanation. 
225. On account of their minuteness, bacteria may be 
Fic. 46.—Forms of Bacteria. a, Micrococcus prodigiosus; b, Bacterium termo 
Ssivooteste blcatliies ¢, Spiriium volutans. Maguined ad tunes SF 
picked up by currents of air and borne long distances, and 
in this way they are doubtless often carried from place to 
place. When a pool of putrid water dries up, the bacteria 
with which it swarmed are blown away with the dust and 
dirt, dropping everywhere into pools, upon plants and ani- 
mals living and dead, and even entering our lungs with 
the air we breathe. 
