[Illustration: Fig. 35.--Ventral and dorsal aspects of _Chilodon
cucullulus_.]


Genus DYSTERIA Huxley '57.

(Cl. et Lach. '58; Entz '84; Möbius '88; Shevyakov '96.)

Small forms, firm in outline, and colorless or slightly colored. The
body is somewhat clam-shaped, flattened, slightly curved or straight
on the right side, the other more convex. The true ventral side is
only a narrow strip along the right and anterior edge of the body,
the apparent ventral side being a fold of the very large dorsal
surface which comes around ventrally, forming a valved structure
somewhat analogous to a clam shell. Cilia are limited to the outer
edge of the small ventral surface, which also bears a peculiar spine
at the posterior end. Behind this spine are larger cilia. The mouth
opening lies in the anterior widened portion of the ventral surface
and is connected with a smooth tubular pharynx. The right half of the
dorsal side, _i.e._, the apparent dorsal side, is arched and bears
longitudinal ridges. Two to four contractile vacuoles are placed on
the ventral side. The macronucleus is usually dorsal, elliptical, and
cleft, with one micronucleus attached. Fresh and salt water.


Dysteria lanceolata Cl. et Lach. Fig. 36.

Synonym: _Cypridium lanceolatum_ Kent '81.