2 GENERAL MORPHOLOGY AND BIOLOGY 



filaments may be more or less septate, may be provided with a 

 sheath, and may show branching either true or false. The 

 minute structure of the elements comprising these filaments is 

 analogous to that of the lower forms. Their size, however, is 

 often somewhat greater. The lower forms sometimes occur in 

 filaments, but here every member of the filament is independent, 

 while in the higher forms there seems to be a certain inter- 

 dependence among the individual elements. For instance, 

 growth may occur only at one end of a filament, the other 

 forming an attachment to some fixed object. (2) The higher 

 forms, moreover, present this further development that in certain 

 cases some of the elements may be set apart for the reproduction 

 of new individuals. 



Terminology. The term bacterium of course in strictness 

 only refers to the rod-shaped varieties of the group, but as it 

 has given the name bacteriology to the science which deals with 

 the whole group, it is convenient to apply it to all the members 

 of the latter, and to reserve the term bacillus for the rod-shaped 

 varieties. Other general words, such as germ, microbe, micro- 

 organism, are often used as synonymous with bacterium, though, 

 strictly, they include the smallest organisms of the animal 

 kingdom. 



While no living organisms lower than the bacteria are known_ 

 (though the occurrence of such is now suspected), the upper 

 limits of the group are difficult to define, and it is further 

 impossible in the present state of our knowledge to give other 

 than a provisional classification of the forms which all recognise 

 to be bacteria. The division into lower and higher forms, 

 however, is fairly well marked, and we shall therefore refer to 

 the former as the lower bacteria, and to the latter as the higher 

 bacteria. 



Morphological Relations. The relations of the bacteria to the animal 

 kingdom on the one hand and to the vegetable on the other constitute a 

 somewhat difficult question. It is best to think of there being a group 

 of small, unicellular organisms, which may represent the most primitive 

 forms of life before differentiation into animal and vegetable types had 

 occurred. This would include the flagellata and infusoria, the myxomy- 

 cetes, the lower algae, and the bacteria. To the lower algae the bacteria 

 possess many similarities. These algae are unicellular masses of proto- 

 plasm, having generally the same shapes as the bacteria, and largely 

 multiply by fission. Endogenous sporulation, however, does not occur, 

 nor is motility associated with the possession of flagella. Also their 

 protoplasm differs from that of the bacteria in containing chlorophyll and 

 another blue-green pigment called phycocyan. From the morphological 

 resemblances, however, between these algae and the bacteria, and from 

 the fact that fission plays a predominant part in the multiplication ojf 



