Diagnostic characters: The mouth is at the end of a long peristome
running along the ventral side; the body is dorso-ventrally or
laterally compressed. The entire left edge of the peristome is
provided with an undulating membrane which occasionally runs around
the posterior end of the peristome to form a "pocket" leading to
the mouth. The right edge of the peristome is provided with a
less-developed membrane. There may or may not be a well-developed
pharynx.
Body small; not produced into Genus *_Pleuronema_
neck-like elongation
Body medium-sized; anterior end Genus *_Lembus_
produced into neck-like elongation
* Presence at Woods Hole indicated by asterisk.
Genus PLEURONEMA Dujardin '41.
(Perty '52; Clap. & Lach. '58; Stein '59, vol. I; Quennerstedt '67;
Kent '81; Bütschli '88; Schewiakoff '89; Shevyakov '96.)
Small to medium-sized ciliates, with an unchanging form. They are
somewhat lens-shape and laterally compressed, the two surfaces
about equally arched. The ventral surface is nearly straight or
but slightly arched; the dorsal is quite convex. The anterior and
posterior extremities are equally rounded. The peristome begins as
a small depression, but becomes larger until it takes in nearly all
of the ventral surface. The depression becomes much deeper at about
the center of the body, and is especially marked on the left side of
the peristome. In this deeper portion is the mouth, with an almost
imperceptible oesophagus. Upon the left edge of the peristome is a
high, undulating membrane, sail-like in appearance when extended.
This may stretch around the posterior edge of the peristome and upon
the right aide, thus forming a pocket by means of which the food
particles are directed into the mouth. The rest of the right edge
of the peristome is occupied by closely approximated powerful cilia
(Bütschli) or a second undulating membrane (Stein). The body cilia
are relatively long. Trichocysts and caudal bristles may be present.
The contractile vacuole is subterminal and dorsal; it is questionable
whether there are canals leading to it. A round macronucleus with
one micronucleus is in the anterior half of the body. The anus is
terminal. Food is chiefly bacteria. Movement combines springing with
swimming and rotation. Fresh and salt water.
Pleuronema chrysalis Ehr., sp. Fig. 40.
Synonyms: _Pleuronema crassa_ Dujardin '41; _P. marina_ Duj. '41;
Fabre-Domergue '85; _P. coronata_ Kent '81; _Paramoecium chrysalis_
Ehr. '38; _Lembadion orale_ G. & R. '88; _Histiobalantium agile_
Stokes '85, '88.
The body is ovoid, slightly flattened, rounded at both ends, the
anterior end sharper than the posterior. The ventral surface is
almost entirely taken up by a peristome which extends from the
anterior end posteriorly three-quarters of the body length. The
posterior end of the peristome is straight, the left curved,
following 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 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.