The body is ovoid to globular and usually much flattened dorso-ventrally. The anterior portion is very much reduced and is somewhat head-like or cap-like. The longitudinal furrow extends through the entire posterior body length and is apparently capable of widening and narrowing. It is probably naked (see here Klebs, Pouchet, Bütschli), although Stein maintained that there is a delicate cuticle-like shell. Chromatophores of brown or green colors present and usually grouped radially about a central amylum granule. The nucleus is posterior. Fresh and salt water. Amphidinium operculatum Clap. & Lach. Fig. 27. The body is oval and flattened. The transverse furrow is at the extremity (posterior) of the body and the small portion, which is thus apparently cut off, is the cap-like or operculum-like structure which gives the name to the species. Klebs maintains that the two furrows are not connected, but in this he is certainly mistaken, provided we have the same species under consideration. Very common about Woods Hole. Length from 40 to 50µ; width 30µ; thickness 15µ. [Illustration: Fig. 27.--_Amphidinium operculatum_.] KEY TO INFUSORIA. 1. With cilia Subclass _Ciliata_. 3 2. Without cilia (in adult state) Subclass _Suctoria_ tentacles 3. a. Without a specialized fringe of Order _Holotrichida_ large cilia (ad. zone) b. With general covering of cilia Order _Heterotrichida_ + adoral zone c. With cilia on ventral side Order _Hypotrichida_ + adoral zone d. With cilia in region of adoral Order _Peritrichida_ zone, and about mouth only KEY TO FAMILIES OF THE HOLOTRICHIDA. A. Mouth closed except during food 1 ingestion; no undulating membrane Mouth always open; with undulating 2 membrane 1. _Gymnostomina_. a. Mouth terminal or subterminal. Family _Enchelinidæ_ Food is swallowed and not introduced by currents b. Mouth terminal or subterminal; Family _Trachelinidæ_ body frequently drawn out into long process; mouth may have specialized framework. c. Mouth central or posterior; Family _Chlamydodontidæ_ pharynx with supporting framework 2. _Trichostomina_. a. Mouth anterior or central; Family _Chiliferidæ_ pharynx short or absent; peristomial depression faint or absent b. Mouth central; pharynx long, Family _Urocentridæ_ tubular; cilia in two broad zones c. Mouth posterior; form Family _Microthoracidæ_ asymmetrical; cilia dispersed or limited to oral region d. Mouth anterior or central. _Paramoecidæ_ Peristomial depression (One genus, _Paramoecium_) clearly marked.