MORPHOLOGICAL CLASSIFICATION OF BACTERIA. 29 



erably with the staining of the protoplasm of the bacteria 

 cells, so that special methods of staining have to be adopted 

 for these. Again, those bacteria which are generally found 

 surrounded by a capsule, when grown in artificial media seem 

 to lose the power of developing capsules. 



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 sarcinae ; 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. 



