30 THE FUNDAMENTAL PRINCIPLES. 



THE MORPHOLOGICAL CLASSIFICATION OF BACTERIA. 



Bacteria are divided into three varieties : (1) the rounded 

 form or coccus (plural cocci) ; (2) the rod-shaped form or 

 bacillus (plural bacilli); and (3) the curved or spiral form, 

 spirillum (plural spirilla). 



I. The Coccus. 



Varieties. The cocci, which are not always round, but 

 very often oval in form, are further distinguished according 

 as they appear : singly and of large size, as megacocci; of 

 small size, as micrococci ; double that is, two of the cells 

 adhering together, as diplococci ; in chains that is, a number 

 of cells adhering together in single file as streptococci; in 

 groups very like a bunch of grapes, as staphylococci ; in 

 groups of four, as tetrads or merismopedia ; in groups of eight 

 arranged in cubes, as sarcinas ; in irregular masses united by 

 an intercellular substance and imbedded in a tough gelati- 

 nous matrix, as ascococci. 



II. The Bacillus. 



Morphology. The bacilli or rod-shaped (desmo-) bacteria 

 are distinguished by the fact that their two longest sides are 

 parallel to each other ; the two short sides being at times 

 straight, at others concave, and at others again convex. 



Varieties. They are said to be (1) slender when their 

 breadth is to their length as 1 to 4 or more, and (2) thick 

 when it is as 1 to 2. They develop singly or in pairs or in 

 long threads or filaments, being attached together always by 

 their narrow ends. 



III. The Spirillum. 



The spirilla or curved or spiral bacteria develop either 

 singly or in pairs or in long twisted or corkscrew filaments. 



