8 THE BIOLOGY OF BACTERIA 



cells united to each other at their sides. The fourth form is consti- 

 tuted by the micrococci being arranged in masses like grapes, the 

 staphylococcus ((rTa<j>v\i?, a bunch of grapes). The elements are 

 often smaller than in the streptococcus, and the name itself describes 

 the arrangement. There is no matrix and no capsule. This is the 

 commonest organism found in abscesses, etc. The sarciiia is best 



classified amongst the cocci, for it is 

 composed of them, in packets of four 

 or multiples of four, produced by divi- 

 sion vertically in two planes. If the 

 division occurs in one plane, we have 

 as a result small squares of round cells 

 known as merismopedia. In both these 

 conditions it frequently happens that 

 the contiguous sides of the elements of 

 packets become faceted or straightened 

 against each other. It may happen, 

 too, particularly in the sarcincc, that 

 FIG. 2,-Diagram of sarcina. segmentation is not complete, and that 



the elements are larger than in any 



other class of cocci. They stain very readily. Nearly all the cocci 

 are non-motile, though Brownian movement (see p. 11) may readily 

 be observed. 



2. The Bacillus. This group consists of rods, having parallel 

 sides and being longer than they are broad. They differ in every 

 other respect according to species, but these two characteristics 

 remain to distinguish them. Many of them are motile, others not. 

 The ends or poles of a bacillus may be pointed, round, or almost 

 exactly square and blocked. They all, or nearly all, possess a 

 capsule. Individuals of the same species may differ greatly, 

 according to whether they have been naturally or artificially grown, 

 and pleomorphic forms are abundant. 



3. The Spirillum. This wavy-thread group is divisible into a 

 number of different forms, to which authorities have given special 

 names. It is sufficient, however, to state that the two common 

 forms are the non-septate spiral thread (e.g. the Spirillum Obermeier 

 of relapsing fever), which takes no other form but a lengthened 

 spirillum ; and the spirillum which breaks up into elements or units, 

 each of which appears comma-shaped (e.g. the cholera bacillus). The 

 degree of curvature in the spirilla, of course, varies. They are the 

 least important of the lower bacteria. 



The Higher Bacteria group includes more highly organised 

 members of the Schizomycetes. They possess filaments, which may 

 be branched, and almost always have septa and a sheath. Perhaps 

 the most marked difference from the lower bacteria is in their 



