2 GENEKAL MORPHOLOGY AND BIOLOGY 



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 used as synonymous with bacterium, though these 

 are often made to include minute organisms belonging to 

 other groups. 



While no formed living organisms lower than the bacteria are 

 known (though, as will be seen later, the existence of life 

 associated with matter in an ultra-microscopic state is probable), 

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

 impossible at present to give other than a provisional classifica- 

 tion 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 

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

 unicellular organisms, which may be survivals of the most primitive 

 forms of life before differentiation into animal and vegetable types had 

 occurred and before in an individual cell nucleus had been differentiated 

 from cytoplasm. This would include the flagellata an d infusoria, the myxo- 

 myoetes, the lower algse, and the bacteria. To the lower algse the bacteria 

 show many similarities. These alga; are unicellular masses of protoplasm, 

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

 by fission. Endogenous sporulation, however, does not occur, nor is motility 

 necessarily associated with the possession of flagella. Also their proto- 

 plasm differs from that of the bacteria in containing chlorophyll and 

 another blue-green pigment called phycocyan. From the morphological 

 resemblances between these alga; and the bacteria, and from the fact 

 that fission plays a predominant part in the multiplication of both they 



