ch pigment spot Genus _Onychodactylus_

c. Cilia on right edge only of Genus *_Dysteria_
greatly reduced ventral surface

* Presence at Woods Hole indicated by asterisk.

Genus NASSULA Ehr. '33

(Dujardin '41; Stein '67; Cienkowsky '55; Cohn '66; Clap. et Lach.
'58; Kent '81; Maupas '83; Entz '84; Fabre-Domergue '88; Buetschli
'88; Shevyakov '96.)

The body is ovoid or cylindrical, with well-rounded ends, and in
some cases slightly flattened. The mouth is ventral and placed some
distance from anterior end (1/4 to 1/3 total length). A slight
depression on the ventral surface marks the mouth region, which is
further indicated by larger and more powerful cilia. The rest of the
body is uniformly ciliated. The entire body is marked by clearly
defined spiral stripes. The mouth is circular and the oesophagus is
supported by a considerable armature, which usually extends dorsally
and to the left, rarely to the right. In some cases the structure of
this armature is indistinct; again it can be clearly seen to consist
of definite rods (Staebchen). The anus is probably always terminal.
Contractile vacuoles are variable in different species. In some cases
there is but one, which is placed at the posterior end or centrally
on the ventral side; in others there may be four--two dorsal and
two ventral. In many cases trichocysts are uniformly distributed.
Sometimes the body is colorless; again, and more often, it is
brightly colored with red, blue, brown, or black pigment. The
macronucleus is globular and central, occasionally band-form and
with numerous attached micronuclei. Food substance varied, usually
vegetable matter, see, however, below. Cysts are globular. Movement
is a steady progression, combined with rolling.

Nassula microstoma Cohn '66. Fig. 34.

Synonyms: _Paramoecium microstomum_ Cl. et Lach. '58, Gourret et
Roeser '88; _Isotricha microstomum_ Kent '81.

Body subcylindrical, rounded at each extremity, not quite twice as
long as broad. A slight depression on one surface marks the position
of the mouth, this depression being indicated by a row of longer
cilia. The mouth is extremely small and is surrounded by a curious
buccal armature. This is not made up of bars or rods, as in most
species of _Nassula_, but appears perfectly smooth and uniform
except for the considerable swelling at the inner end. The cuticle is
firm and unyielding and marked by longitudinal and somewhat spiral