THE LOWER BACTERIA. 15 



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 repre- 

 sented in every cell. They present three distinct type 

 forms, the coccus, the bacillus, and the spirillum, and endo- 

 genous sporulation may occur. They may also be motile. 



i. The Cocci. In this group the cells range in different 

 species from .9-8 //, in diameter, but mostly measure 

 1-1.5 p. 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 streptococcus. If division 

 takes place irregularly the resultant mass may be compared 

 to a bunch of grapes, and the species is often called a staphy- 

 lococcus. 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 frequent unit. 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, as 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 tetragenus), sometimes 

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

 the coccus (streptococcus mesenteriodes). It is especially 

 among the streptococci and staphylococci that the pheno- 

 menon of the formation of arthrospores is said to occur. 

 In none of the cocci have endogenous spores been certainly 

 observed. The number of species of the streptococci and 

 staphylococci probably exceeds 150. Besides those men- 



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

 vide Fig. i. 



