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6 THE MICRO-ORGANISMS OF THE HUMAN MOUTH.

g. Beggiatoa : long, thick, rigid threads, which deposit grains
of sulphur in their protoplasm,
h. Crenothrix : thread-forming water-fungi, whose threads
(which may be either unicellular or composed of chains of cocci
and rods) are inclosed by a sheath, and increase in diameter
from base to apex.
i. Cladothrix: thread-forming bacteria with false branchings
(pseudo-branchings), (Fig. 1, o).
Some writers question the correctness of classing Beggiatoa,
Crenothrix, and Cladothrix under bacteria.
To the third group (screw-forms) belong :
j. Yibriones: rods or threads with a slight undulating curve
or twist (Fig. 1, A).
k. Spirilla: rigid rods and threads with pronounced screw-
like windings (Fig. 1, z, k).
I. Spirochetes : flexible threads with narrow, unequal wind-
The
ings, sometimes as many as sixty in one thread (Fig. 1, 1).
genera Vibrio, Spirillum, Spirochaete, are not separated from
each other by any sharp line oF division; an organism which
one w^ould describe as a vibrio might by another be. desig-
nated as a spirillum; what one terms spirillum another calls
spiroch?ete, etc. Furthermore, micro-organisms belonging to
the second group not unfrequently show a slight curve or twist
in some of their cells, without thereby being entitled to be placed
among the screw-forms.
Threads, whose ends are wound about each other like a braid
of hair, have been termed Spirulina (Fig. l,"ti^ v^
Each of the three fundamental forms (spheres, rods, and
screws) is subject to extensive modifications, according to the
circumstances under which it is cultivated, and it is not always
easy to determine whether the particular organism under exam-
ination should be classed as a coccus or a bacillus, as a bacillus
or a vibrio, etc. "Whether these modifications can be carried so
far by any condition or conditions of cultivation that a mlcr.>-
organism normally occurring in any one of the three funda-
mental forms is transformed into another (for example a coccus
into a bacillus), or that a monomorphous organism becomes
pleomorph, is a question upon which much has been said pro