al aspects of _Chilodon
cucullulus_.]

Genus DYSTERIA Huxley '57.

(Cl. et Lach. '58; Entz '84; Moebius '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.

Outline of the flattened body ovoid; body consists of two valve-like
portions; the edge of the right valve is nearly straight, that of the
left valve more or less sinuous; anteriorly it is cut away, obliquely
and posteriorly it has a deep indentation in which the seizing spine
rests. The cilia are confined to the ventral surface, here reaching,
however, from the anterior dorsal extremity to below the posterior
indentation. Posteriorly the cilia become larger, corresponding to
the larger cirri of _D. armata_, which are posterior to the spine.
The mouth lies between the two valves and is surrounded by a long
and smooth buccal armature which passes downward and backward to
the left a distance equal to about half the entire body length. The
macronucleus is situated in the dorsal region in the central part of
the body. There are two contractile vacuoles, one behind the center
of the buccal armature, the other near the inner end of this organ.
Movement is in circles, the animal moving around quite rapidly when
not attached by its posterior process. It is colorless and measures
45 mu in length by 27 mu in width. Claparede & Lachmann and Shevyakov
describe it as 70 mu long.

[Illustration: Fig. 36.--_Dysteria lan