Diagnostic characters: Attached or unattached forma of peritrichous ciliates in which the adoral zone seen from above forms a right-wound spiral. A secondary circlet of cilia around the posterior end may be present either permanently or periodically. 1. Posterior ciliated girdle 3 permanent around an attaching disk 2. Posterior ciliated girdle, 4 temporary during motile stage 3. Body cylindrical: (a) With ring of stiff bristles Genus _Cyclochæta_ above the ciliated girdle (b) Without accessory ring of Genus _Trichodina_ bristles; with velum Body conical; general Genus _Trichodinopsis_ surface ciliated 4. No test and no stalk Genus _Scyphidia_ 5. No test; with stalk containing 8 contractile thread 6. No test; with stalk but without Genus _Epistylis_ contractile thread 7. With a test; with or without Genus *_Cothurnia_ a stalk 8. Individuals solitary Genus *_Vorticella_ Individuals colonial; Genus *_Zoothamnium_ entire colony contractile Individuals colonial; parts Genus _Carchesium_ only of the colony contractile * Presence at Woods Hole indicated by asterisk. Genus VORTICELLA (Linnæus 1767) Ehr. '38 (Bell Animalcule Leeuwenhoek 1675; Ehrenberg '38; Dujardin '41; Stein '51; Cl. & Lach. '58; Greeff '70; Bütschli '88; Kent '81; Stokes '88; etc.) Medium-sized ciliates of general bell-like form. They may be colorless, or yellow and green through the presence of Zoochlorella. When not contracted, the peristome end is widespread, rarely narrowed. The adoral zone and peristome agree with the details given in the family characteristics. The chief character is the attachment of the posterior end by means of a single, longer or shorter, stalk, which contains a highly contractile thread easily distinguished in the living animal. Another character is the absence of colony formation. Contractile vacuole, single or double, usually connected with a sac-like reservoir. The macronucleus is invariably long and band-formed, with attached micronucleus. Fresh and salt water. So many species of _Vorticella_ have been described that the task of collecting data and of arranging the synonyms is extremely irksome and difficult. Stokes enumerates 66 species, inhabiting fresh and salt water, and several other new species have been added since his work. I am impressed with the fact that new species have been created without proper regard for the manifold variations which nearly all of the _Ciliata_ show, and I believe the 66 species might be safely reduced to 12 or 15. Vorticella patellina Müller. Fig. 59.