POSITION— RELATIONSHIPS 37 



are characteristically storers of energy while Animals are 

 liberators of it. Some bacteria which have the power of 

 swimming in a liquid certainly liberate relatively large 

 amounts of energy, and in the changes which bacteria bring 

 about in the material which they use as food considerable 

 heat is evolved ("heating" of manure, etc.). Nevertheless 

 the evidence is good that the bacteria as a class store much 

 more of the energy contained in the substances actually 

 taken into the body cell as food than is liberated in any 

 form. 



Bacteria do show some resemblance to the protozoa, or 

 single-celled animal forms, in that the individuals of each 

 group consist of one cell only and some bacteria have the 

 power of independent motion from place to place in a liquid 

 as most "infusoria" do, but here the resemblance ceases. 



Bacteria are among the smallest of organisms, so small 

 that it requires the highest powers of the microscope for 

 their successful study, and the use of a special unit for their 

 measurement. This unit is the one-thousandth part of a 

 millimeter and is called the micro-millimeter or micron. 

 Its symbol is the Greek letter mu (ix) . 



The size varies widely among different kinds but is fairly 

 constant in the same kind. The smallest described form is 

 said to be only O.lSju long by 0.06m thick and is just visible 

 with the highest power of the microscope, though it is pos- 

 sible and even probable that there are forms still smaller 

 which cannot be seen. Some large rare forms may measure 

 40/i in length, but the vast majority are from Ifi to 4n or 5^ 

 long, and from one-third to one-half as wide. 



From the above description a bacterium might be said to 

 be a microscopic, unicellular plant, without chlorophyl, which 

 reproduces by dividing transversely. 



