FORM TYPES OF BACTERIA 139 



caused by different specific living organisms which are 

 invisible and which will pass through the meshes of a 

 porcelain filter that excludes any visible bacterium. It 

 is at least probable that many different kinds of such 

 ultramicroscopic organisms exist, for there is room 

 between the lower limit of microscopic visibility and 

 the size of a complex protein molecule for a collection 

 of such molecules to be associated into a system which 

 would form an ultramicroscopic protoplasmic unit with 

 specific characters. 



The visible bacteria may be classed in two groups 

 a group of simpler forms which are the more numerous, 

 and a more highly developed group with which we need 

 not concern ourselves here. The simpler forms may 

 be roughly classed according to their shape into three 

 types : (i) the coccus, which is spherical, on the average 

 I ju, in diameter ; (2) the bacillus, a straight cylindrical 

 rod, i p. or less in diameter and up to about 10 /* long ; 

 and (3) the spirillum, a curved rod which may be spirally 

 coiled like a corkscrew. Some bacteria, however, are 

 not of these shapes, but may be oval in outline, etc. 

 (see Fig. 15). When bacteria were first beginning to 

 be systematically studied, sixty or seventy years ago, 

 these " form types" were largely used as generic names, 

 the different species being distinguished according 

 to the habitat or activity, and some of these species 

 are still recognised, e.g. Bacillus coli (which lives in 

 the human intestine), Bacillus anthracis (which causes 

 the disease anthrax), and so on. It is not, however, the 

 form of the bacterial cell but its specific activity 

 that is its most important feature, and many new 

 genera have been established. 



Each bacterial cell consists of a minute mass of 

 protoplasm in which a separate nucleus cannot be 



