26 GENERAL MORPHOLOGY 



condition or stained for examination, only the nucleus 

 and the wall can be seen, the intervening layer being 

 exceedingly thin. 



In shape, bacteria are either spherical, called cocci 

 (sing., coccus), or straight rods, called bacilli (sing., 

 bacillus}, or curved rods, called spirilla (sing., spiril- 



o 



-yv- ^ 



d e / 



FIG. 3. a, bacilli in pairs; b, single bacilli; c and d, bacilli in threads; 



e and/, bacilli of variable morphology. (Abbott.) 



//'v 



,'\r- -^ 



r * i I* / ^ ' V . T ^( 



a 5 c d 



FIG. 4. a and d, spirilla in short segments and longer threads 

 the so-called comma forms and spirals; b, the forms known as spiro- 

 chetse, c, the thick spirals sometimes known as vibrios. (Abbott.) 



lum). Each shape has slight variations, such as the 

 flattening of the sides when two organisms are apposed. 

 The spirilla are, perhaps, subject to more variations 

 than the others, extending from a simple comma shape 

 to that of a long, wavy spiral when looked at from the 

 side. These last are in reality corkscrews, as they twist 



