Small forms, greatly flattened dorso-ventrally and almost egg-form in outline.  The anterior end is bent distinctly to the left and forms a characteristic process, which, together with the entire margin of the body, is soft and flexible.  The posterior end is, as a rule, broadly rounded.  The ventral surface is finely striate, and this surface alone is ciliated.  The lines of cilia converge at the mouth, and at this region the cilia are somewhat larger and more distinct, thus forming a functional adoral zone.  The mouth is median and is situated in the anterior half of the body.  It is surrounded by a well-defined armature, composed usually of from 10 to 16 rods.  The contractile vacuoles are quite varied and from one to many in number, the number increasing with the size of the individual.  The macronucleus is usually single, elliptical in form, and centrally placed; one micronucleus.  Reddish granular pigment and trichocysts are occasionally present.

Chilodon cucullulus Muell., sp.  Fig. 35.

Synonyms; Colpoda cucullus O. F. Mueller; Loxodes cucullulus; Chilodon uncinatus Ehr. ’58, Perty ’52, Dujardin ’41; L. dentatus Duj., etc.

This extremely variable form has received so many different names that it hardly pays to enumerate them.  It is one of the commonest and most widely spread ciliates known, although at Woods Hole I was surprised to see it so rarely.  It is the type species of the genus and needs no further description.  The specimens observed at Woods Hole had numerous contractile vacuoles and were 42 to 45 mu long and from 28 to 32 mu wide.

[Illustration:  Fig. 35.—­Ventral and dorsal 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.