2 GENERAL MORPHOLOGY AND BIOLOGY. 



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 interde- 

 pendence 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, more- 

 over, 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 king- 

 dom. 



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 impos- 

 sible 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 some- 

 what 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 myxomycetes, the lower algae, 

 and the bacteria. To the lower algs the bacteria possess many similarities. 

 These algae are unicellular masses of protoplasm, having generally the same 

 shapes as the bacteria, and largely multiply by fission. Endogenous sporiila- 

 tion, however, does not occur, nor is motility associated with the possession 

 of flagella. Also their protoplasm differs from that of the bacteria in contain- 

 ing 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 



