14 THE GENERAL CHARACTERS OF MICROORGANISMS 
bacteria they grow out into long threads and often branch. 
Their chief similarity to true bacteria lies in their size and 
staining properties. 
Many types of Actinomyces occur in soil and appear as round, 
white opaque colonies often with an extensive brown halo 
upon the plates described in Experiment No. 24. 
Thus it will be seen that the term bacteria applies to the whole 
group of organisms that multiply by division, the study of which 
constitutes the study of bacteriology, while 
the term Bacterium refers to a single division 
of the group. The names Bacillus and Bac- 
lertum are sometimes confused; for example, 
the tubercle bacillus, according to the above 
classification, is a Bacterium, since it is non- 
motile and does not produce spores; and 
indeed recent study indicates that it belongs 
to the group of higher bacteria; but the name 
bacillus was given it years before the above 
distinctions were recognized, and we will 
still use the common name. Some other 
bacterla, named twenty years ago, retain 
their earlier names in some books, but they 
Fic. 10.—Actinomy- ate slowly having their names brought into 
ces a, a small colony; harmony with the above distinctions. 
b, single rods (Bostrom). . . . 
The term Coccus is applied to any spheri- 
cal organism of the group bacteria. 
This classification gives only what are recognized as the genera 
of bacteria. A further classification of the group into species is at 
the present time in a condition of the greatest confusion. Many 
hundred varieties have been described by different bacteriologists, 
but there is great difficulty in giving any distinctive description 
of such minute organisms, which have so few characters; and it is 
quite uncertain whether these many hundred described species 
represent distinct forms or whether they should be reduced to a 
