5. a. Caudal spine with posterior 6 bristle-like cilia b. Caudal spine without posterior Genus _Trochilia_ bristle-like cilia; ventral cilia reduced 6. a. With pigment spot on anterior Genus _Ægyria_ angle b. Without such 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; Bütschli '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 (Stäbchen). 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.