Body colorless, elongate, and with the general form of a cucumber,
the posterior end being somewhat pointed. The mouth and relatively
small peristome are situated in the lower third of the body. The
peristome begins as a shallow furrow at the center of the ventral
surface and dips sharply into the buccal depression, which is deep
and turned toward the posterior end. The left edge of the peristome
bears a high undulating membrane, which extends anteriorly only
as far as the center of the body; posteriorly it passes around to
the right edge of the peristome, thus forming the characteristic
membranous pocket. Inside the oral depression is a second undulating
membrane, running down to the mouth. This is small and without an
oesophagus. The body is clothed with long setose cilia which are
frequently fully outstretched when the animal is resting, a slight
tremor of the large membrane alone indicating vitality. Posteriorly
these appendages are drawn out into long filiform setæ, the number
varying in different individuals from three to nine or ten. These
are extremely fine and difficult to see without a high power (_e.g._
1/12 oil) and careful focussing of the substage condensor. Like _P.
chrysalis_, the resting periods are terminated by sudden springs,
otherwise the movements are steady and forward. The macronucleus is
central, and the contractile vacuole posterior and terminal. Length
45µ to 50µ; greatest diameter 17µ. In decaying algæ.

It was this form, I believe, that Peck '95 described as a "ciliate."


[Illustration: Fig. 41.--_Pleuronema setigera_.]