[Illustration: Fig. 41.--_Pleuronema setigera_.]
Genus LEMBUS Cohn '66.
(Cohn '66; Quennerstedt '69; Kent '81; Fabre-Domergue '85; Gourret &
Roeser '88; Bütschli '88; Shevyakov '96.)
Free-swimming animals of elongate form, more or less elastic, and
flexible, bending readily to avoid obstacles, etc. The anterior half
is usually drawn out into a slightly curved neck-like portion. The
peristome is a small groove leading from the anterior end to the
mouth about midway down the ventral side of the body. Bütschli,
following Quennerstedt, describes an undulating membrane on each side
of the peristome groove. Other observers, however, usually describe
but one, the left, which is clearly defined and stretches out some
distance from the body, while the right border is described as having
smaller but very active cilia. The general body surface is clothed
with fine, uniform cilia, and body striæ are usually absent. One
or more caudal bristles may be present. The contractile vacuole is
posterior and terminal, and may be multiple. The macronucleus is
spherical and perhaps double (Kent). Food is chiefly bacteria, and
the animals are frequently found with the anterior end embedded in
zoogloea masses. Salt water, usually in infusions.
Lembus infusionum, n. sp. Fig. 42.
The body is elongate, lancet-shaped, with a tapering anterior
extremity. The dorsal outline is concave through the bending of the
anterior end, while the ventral outline presents an even, convex
curve. The mouth lies slightly above the center of the body and marks
the posterior limit of the ventral peristomial groove, which curves
slightly from the anterior extremity. Each side of this groove bears
an undulating membrane, the left being much larger and conspicuously
striated. The general form of this left membrane is triangular,
the widest part is anterior, the narrowest at the mouth. The
right membrane is similar in form, but smaller and more active.
The endoplasm is colorless and finely granular, not regionally
differentiated. The ectoplasm consists of a relatively thick cortical
plasm specially noticeable in the posterior half of the body and
a delicate cuticle which bears almost imperceptible longitudinal
markings--the insertion points of the fine cilia. The body is covered
with uniform cilia except at the anterior extremity. Here they are
much larger and bristle-like. I was unable to find any cilia in the
peristome. One long caudal bristle, one-quarter of the length of the
body, trails out behind. The macronucleus is spheroidal and placed
near the center of the body; a conspicuous micronucleus lies near it.
A row of contractile vacuoles extends from the posterior end. I have
seen as many as six of nearly equal size and one or two smaller ones.
The intervals of contraction are quite long. Length 70 to 75µ;
greatest diameter 10 to 12µ.
_L. infusionum_ resembles _L. elongatus_ in its general form and in
its mode of life, for it excavates a retreat in zoogloea masses and
lies there for considerable periods perfectly quiet. It differs
from _L. elongatus_ and from _L. velifer_ (probably the same as _L.
elongatus_ of Claparède & Lachmann) in the presence of the caudal
bristle, in the absence of annular markings, number of contractile
vacuoles, and in the slightly smaller size. It resembles _Lembus
verminus_ (Müller) as described by Kent (_Proboscella vermina_), and
_L. intermedius_ as described by Gourret & Roeser (_Lembus verminus_
syn.)in the absence of annular markings and in the presence of a
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.
Although Quennerstedt's description of _L. pusillus_ makes no mention
of a caudal bristle, the size and other characters are so closely
similar that I hesitate to make a new species. The bristle is
extremely delicate, scarcely thicker than a cilium, and easily
overlooked, yet with proper focussing of the condenser I found it on
every specimen examined.
[Illustration: Fig. 43.--_Lembus pusillus_.]