MICROBES AND THEIR CONTROL 309 



the water molecules up tight, so that there is no movement of water or 

 salts. High temperatures, on the other hand, bring about a change in 

 the proteins (compare boiling white of egg) that is not reversed when 

 the temperature is lowered again. You can unfreeze ice, but you cannot 

 uncoagulate the hardened white of egg or make it liquid again. 



237. Animal microbes. The protozoa, as we have seen, are 

 the simplest of animals. Like the bacteria, they consist of one- 

 celled organisms. The ameba, which swims about freely in 

 water and finds its food in smaller organisms and bits of dead 

 organisms (see page 59), has many relatives that are parasitic 

 on man or other animals. :\Ialaria, dysentery, African sleep- 

 ing sickness, tick fever in cattle, and other diseases in man and 

 the lower animals are caused by different species of protozoa. 

 Many of these simple animals go into a resting stage, and in 

 some there are several distinct stages in the complete life his- 

 tory. A very striking fact in the life history of these animals is 

 that one stage is passed in one host, while another stage is 

 passed in a totally different host. The same fact has been 

 observed in the life of other parasitic animals, as the tapeworm, 

 the liver fluke, and the trichina; and this is also 'true of many 

 parasitic plants, as the wheat rust, one variety of which spends 

 part of the cycle. on the wheat and part on the barberry plant. 

 This fact led to a great deal of confusion when scientists first at- 

 tempted to make a complete study of any of these species ; but 

 in the end it turned out to be of great help in our struggle to 

 overcome these parasites, as we shall see later. 



238. Useful microbes. Since bacteria bring about the decay 

 of dead plants and animals, they return to the soil materials that 

 can again be used by plants and eventually become animal 

 protoplasm. Some bacteria, because of their mutual relations 

 to plants of the bean family, may add to our supplies of pro- 

 tein food (p. 301). The bacteria themselves, feeding upon dead 

 organic remains, are in turn eaten by various protozoa and 

 other minute animals ; these are then eaten by larger animals, 

 and so on until we get to forms that are large enough to serve 

 as food for man, as shrimps, clams, fish, etc. Still further uses 



