extends outwards from the posterior end. The contractile vacuole is
terminal or subterminal and near the anal opening. The macronucleus
is spherical, centrally placed, and with one micronucleus closely
applied. Movement is rapid and usual forwards in a straight line,
often found resting, however, with outstretched cilia in contact with
some foreign body. Food mainly bacteria. Fresh and saltwater, usually
in decomposing vegetable substances.

Uronema marina Duj. Fig. 39.

Synonyms: _Enchelys triquetra_ Dujardin; _E. corrugata_ Duj.;
_Cryptochilum griseolum_ Maupas '83; _Philaster digitiformis_
Fabre-Domergue '85.

Small animals with ellipsoidal form and about twice as long as
broad. The mouth lies in the upper half of the body and bears a
well-developed undulating membrane upon its left edge. The membrane
is longitudinally striped and covered with long and vibratile cilia.
The right edge of the mouth bears cilia which are about the same in
size as the body cilia, but are more closely inserted (Schewiakoff).
The most characteristic feature is the long caudal bristle, which
is extremely delicate and about two-thirds the length of the body.
Schewiakoff thinks this bristle has a sensory function. I could not
make this out, for although other protozoa ran against this bristle,
often bending it well over to one side, the animal showed no sign
of irritability but lay quiescent. A spherical macronucleus with
attached micronucleus lies in the center of the body. The contractile
vacuole is posterior in front of the bristle. The macronucleus was
found to be double, as though just divided, in a large percentage of
cases. This may be a precocious division of the nucleus long before
signs of the body division are evident. Such a phenomenon, however,
is rare, the macronucleus usually dividing at a late stage of cell
division. Length 30 to 50 mu; width 15 to 20 mu. Common in decomposing
algae.

[Illustration: FIG. 39.--_Uronema marina_.]

KEY TO MARINE GENERA OF PLEURONEMIDAE.

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-develo