the depressed portion. The body is covered with fine cilia
in longitudinal lines, except on the peristome. The mouth is small
and situated in the hollow of the peristome near the left border. On
the left peristome edge is a large undulating membrane. It begins
near the anterior end of the body and increases in height posteriorly
following the peristome edge around on the right side. This posterior
bend of the membrane causes the appearance of a full sail, so
often seen. It can be entirely withdrawn and folded together in
the peristome. On the right edge of the peristome are large,
powerful cilia. The contractile vacuole is central and dorsal; the
macronucleus is in the anterior half of the body, with one attached
micronucleus. Food consists of bacteria. Not very common. Fresh
and salt water. It often remains quiet, with membrane and cilia
outstretched, as though dead, but suddenly gives a spring and is
gone.

[Illustration: Fig. 40.--_Pleuronema chrysalis_.]

Pleuronema setigera, n. sp. Fig. 41.

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 setae, 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
45 mu to 50 mu; greatest diameter 17 mu