12 ' GENERAL MORPHOLOGY AND BIOLOGY 



fundamental principle running through all of which has been 

 the recognition of the two sub-groups and the type forms 

 mentioned in the opening paragraph above. In the attempts to 

 subdivide the group still further, scarcely two systematists are 

 agreed as to the characters on which sub-classes are to be based. 

 Our present knowledge of the essential morphology and relations 

 of bacteria is as yet too limited for a really natural classifica- 

 tion to be attempted. To prepare for the elaboration of the 

 latter, there should be studied in every species the habitat, best 

 food supply, condition as to gaseous environment, range of 

 growth temperature, morphology, micro-chemical reactions, life- 

 history, special properties, and pathogenicity (p. 136). 



We must thus be content with a provisional and incomplete 

 classification. We have said that the division into lower and 

 higher bacteria is recognised by all, though, as in every other 

 classification, transitional forms have to be accounted for. In 

 subdividing the bacteria further, the forms they assume con- 

 stitute at present the only practicable basis of classification. 

 The lower bacteria thus naturally fall into the three groups 

 mentioned, the cocci, bacilli, and spirilla, though the higher 

 are more difficult to deal with. Subsidiary, though important, 

 points in still further subdivision are the planes in which fission 

 takes place and the presence or absence of spores. The recogni- 

 tion of actual species is often a matter of great difficulty. The 

 points to be observed in this will be discussed later (p. 135). 



I. The Lower Bacteria. 1 — These, as we have seen, are. 

 minute unicellular masses of protoplasm surrounded by an 

 envelope, the total vital capacities of a species being represented 

 in every cell. They present three distinct type forms, the 

 coccus, the bacillus, and the spirillum ; endogenous sporulation 

 may occur. They may also be motile. 



1. The Cocci. — In this group the cells range in different 

 species from - 5 /x to 2 //, in diameter, but most measure about 1 jj& K 

 Before division they may increase in size in all directions. The 

 species are usually classified according to the method of division. 

 If the cells divide only in one axis, and through the consistency 

 of their envelopes remain attached, then a chain of cocci will be 

 formed. A species in which this occurs is known as a strephh[ 

 coccus. If division takes place irregularly, the resultant mass may 

 be compared to a bunch of grapes, and the species is often called 

 a staphylococcus. Division may take place in two axes at right 

 angles- to one another, in which case cocci adherent to each other 



1 For the illustration of this and the succeeding systafaatic paragraphs, 

 vide Fig. 1. f 



