12 GENERAL MORPHOLOGY AND BIOLOGY 



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. 137). 



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 /u, to 2 /x, in diameter, but most measure about 1 //. 

 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 strepto- 

 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 

 in packets of four (called tetrads) or sixteen may be found, 

 the former number being the more frequent. To all these forms 

 the word micrococcus is often generally applied. The individuals 

 in a growth of micrococci often show a tendency to remain 

 united in twos. These are spoken of as diplococci, but this is 

 not a distinctive character, since every coccus as a result of 

 division becomes a diplococcus, though in some species the 

 tendency to remain in pairs is well marked. The adhesion of 

 cocci to one another depends on the character of the capsule. 

 Often this has a well-marked outer limit (micrococcus tetrayenus), 

 sometimes it is of great extent, its diameter being many times 

 that of the coccus (streptococcus mcsenteriodes). It is especially 

 among the streptococci and staphylococci that the phenomenon 

 of the formation of arthrospores is said to occur. In none of 



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

 vide Fig. 1. 



