38 ANIMAL BIOLOGY 



the Bacteria. For reasons that will appear later, it is not practical 

 to focus attention on one particular species of Bacteria, as we have 

 just done in considering green plants and animals. Instead we 

 shall discuss in very general terms the group as a whole, referring 

 now and then to special kinds of Bacteria to illustrate particular 

 points. 



1. The Bacteria 



The wide distribution of the Protozoa is exceeded by the Bac- 

 teria. Representatives are literally found everywhere: floating 

 with dust particles in the air, in salt and fresh water, in the water 

 of hot springs , frozen in ice , in the upper layers of the soil , and in 

 the bodies of plants and animals. Bacteria have received a con- 

 siderable notoriety under the names of 'microbes' and 'germs,' 

 owing to the fact that certain types subsist within the human 

 body as parasites and bring about disturbances, chiefly chemical, 

 which we interpret as disease. But aside from these forms which, 

 though all too many, are relatively few in number, human life 

 and life in general on the Earth could not long continue without 

 their services. Indeed, the Bacteria have practically ruled the 

 world since they first secured a foothold when the Earth was young. 

 It is this aspect of the Bacteria which concerns us at present. 



Among the Bacteria are the smallest organisms known. Some 

 species are less than one fifty-thousandth of an inch in length and 

 much less in breadth. None of the typical forms comes within 

 the range of unaided vision, while some are not revealed by the 

 highest powers of the microscope — indeed there is room and to 

 spare for thousands of millions of Bacteria to live in a thimblefull 

 of sour milk. The small size and similarity of structure of many 

 of the Bacteria render their study particularly difficult, and accord- 

 ingly they are grouped and classified largely on the basis of chem- 

 ical changes which they produce in their surroundings, rather 

 than on structural characteristics. However, there are three chief 

 morphological types: the rod-like forms or bacilli: the spherical 

 forms or cocci; and the spiral forms or spirilla. Either bacilli 

 or cocci may be associated in linear, branching, or plate-like series, 

 or grouped together in colonies. (Fig. 14.) 



The individual bacterium is regarded as a single cell, though in 

 most species there is no definite nuclear body; the chromatin 

 material being distributed in the form of granules throughout the 



