3o8 



BIOLOGY AND HUMAN LIFE 



the case in the ameba. In the presence of water, however, they 

 are able to make use of solid food by first secreting a digestive 

 ferment, or enzym, which liquefies the organic matter, and then 

 absorbing the digested material (see Fig. 73). Air is absorbed 

 through the moist membrane, and carbon dioxid and other 

 wastes resulting from metabolism are excreted by diffusion 

 through the cell wall. Material taken in is assimilated and the 



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Fig. 146. Types of bacteria 



There are three main groups of bacteria, classified according to the general form or 

 shape of the cell: a, round-celled, or coccus, type (plural, cocci, pronounced kok's't); 

 in some species the cells scatter; in some they cling together in clumps; in others 

 they cling together in chains, b, rod-shaped t\T)e, or bacilli; in some the length is 

 barely greater than the thickness; in other species the cell is several times as long 

 as it is thick, r, the spiral type, or spirilla; in some this is hardly more than a rod 

 with two or three bends; in other species it has a rather definite corkscrew appearance 



cell grows. After reaching a certain size the cell divides into 

 two by a rather simple process of splitting through the middle 

 crosswise. Bacteria of one kind or another will grow wherever 

 there is organic matter, moisture, and not too low or too high a 

 temperature. They are destroyed by sunshine, by various chemi- 

 cals, and by the temperature of boiling water ; in many species 

 a much lower temperature is fatal. When the temperature gets 

 too low, the metabolism of bacteria will be suspended, but as a 

 rule they cannot be destroyed by freezing. 



Like many other organisms, bacteria that are frozen can resume their 

 activities on being thawed out again, provided the change is not too 

 rapid. Apparently freezing has the same effect as drying, since it locks 



