BACTERIOLOGY. 145 



CHAPTER XXIII. 



MORPHOLOGY, KINDS, AND PRODUCTS. 

 MORPHOLOGY. 



The present classification of bacteria depends largely upon their 

 forms. When a species assumes but one form, it is said to be mono- 

 morphous; but if it possesses several forms, it is pleomorphous. 



Monomorphous bacteria are represented by three important di- 

 visions : Cocci, spherical cells ; Bacilli, or rod-shaped bacteria ; and 

 Spirilla, or curved bacteria. Cocci are represented by several gen- 

 era Micrococcus, in which the cells occur singly; Diplococcus, in 

 which the cells occur in pairs, the result of binary division; Tetra- 

 coccus, in which the cells occur as tetrads; Sarcina, in which the 

 cells occur in cubes, or packets ; Streptococcus, where Jhe cells occur 

 in chains, or filaments, the result of fission in one direction ; StapJiy- 

 lococcus, in which the cells occur in masses, like a cluster of grapes : 

 Ascococcus, in which the cocci are in globular masses. 



Spirilla include the genera Spirillum, Vibrio, and Spirochoete. 

 The elements are curved, sometimes comma-shaped, and again in 

 long spirals. 



Bacilli are represented by the genera Bacillus, Proteus, and Bac- 

 terium. The species of these genera are illustrated by rod-shaped 

 cells. The rods may be pointed at the ends, as in the clostridium ; 

 truncate, as with anthracis; round, as with subtilis. Some form 

 filaments, as anthracis; others are always single, as pyocyanus. 

 Some are slender, as tuberculosis; others, large and thick, as sub- 

 tilis. Involution forms occur where the rod deviates from the char- 

 acteristic form, due to external conditions or the death of the cell. 



Pleomorphus bacteria are represented by Spirulina, Leptothrix, 

 and Cladothrix. Among these the individuals of each species as- 

 sume more than one form. 



KINDS OF BACTERIA. 



The following are the important kinds of bacteria : 



1. Parasitic, those depending for subsistence upon a living host. 

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