caudal bristle. It differs from the former, however, in the absence
of a tentacle-like process, and from both in the absence of a double
nucleus and in the presence of many vacuoles. These features are so
characteristic of all the specimens examined that I have concluded,
somewhat reluctantly, to give it a specific name. It is common in old
infusions of algæ, especially after decomposition is well advanced.
Its food consists of bacteria.


[Illustration: Fig. 42.--_Lembus infusionum_.]


Lembus pusillus Quennerstedt 1869. Fig. 43.

Synonym: _L. subulatus_ Kent 81.

This species is much smaller than the preceding, and might easily
be mistaken for _Uronema marina_. It is subcylindrical in form, the
anterior end bluntly pointed, the posterior end rounded. The oral
apparatus is quite different from _Uronema_. The mouth, as in the
preceding species, is at the end of a long peristomial groove
extending from the anterior end to the middle of the body. The edges
of the peristome bear undulating membranes as in _L. infusionum_.
Like the latter, there is one caudal bristle, but unlike it there is
only one posterior contractile vacuole, while the endoplasm is filled
with large granules or food balls. The cuticle is distinctly striated
with longitudinal markings, and the cilia are uniform in length.

Habitat similar to that of _L. infusionum_, in zoogloea masses.
Length 26 to 30µ; diameter 7 to 8µ.