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
45µ in length by 27µ in width. Claparède & Lachmann and Shevyakov
describe it as 70µ long.


[Illustration: Fig. 36.--_Dysteria lanceolata_.]


KEY TO MARINE GENERA OF CHILIFERIDÆ.

Diagnostic characters: Mouth never lies behind the middle of the
body; the oesophagus is but slightly developed. The undulating
membranes are placed either on the edge of the mouth or in the
oesophagus. A peristomial depression leading to the mouth is absent
or very slightly indicated.