ABSTRACT. The order Euplotida represents a monophyletic order of five families of hypotrich ciliates united by morphology, stomatogenesis, ultrastructure, cyst structure, and behavior. A review of variability of ciliation and nuclei among the 14 genera suggests that lines of evolution may have involved both the loss of cirri and nuclear simplification. We present a binary key to genera in the families Aspidiscidae (Aspidisca and Euplotaspis), Certesiidae n. fam. (Certesia), Gastrocirrhidae (Cytharoides, Euplotidium, and Gas‐trocirrhus), Uronychiidae (Diophryopsis, Diophrys, Paradiophrys, and Uronychia), and Euplotidae. The latter family contains species formerly in the genus Euplotes. Based primarily on cortical structure, endosymbionts, data from morphometric analysis, and ecology, we recognize four different groups. The first group of species remains in Euplotes with Euplotes charon as type. We place a second group of species into the genus Moneuplotes Jankowski 1979 with Moneuplotes vannus (Müller, 1786) as type. We erect two new genera: Euplotoides n. g. and Euplotopsis n. g. with Euplotoides patella (Müller, 1773) n. comb. and Euplotopsis affinis (Dujardin, 1841) n. comb. as type species respectively. We discuss possible phylogenetic relationships within the order.

ABSTRACT. Species belonging to the genus Diophrys Dujardin, 1841, are easily recognized due to possession of the usual complement of approximately seven frontoventral cirri, five transverse cirri, two left marginal cirri, and three large caudal cirri. Separation of these species has been based upon differences in cell length and width, the number and arrangement of cilia in dorsal kinetics, the configuration of the adoral zone of oral polykinetids, the number and distribution of cirri within cirral groups, and the number and arrangement of macronuclei. Jankowski used some of these characteristics to divide the genus into two genera, Diophrys and Paradiophrys, with several subgenera [Jankowski, A. W. 1978. Systematic revision of the class Polyhymenophora (Spirotricha), morphology, systematics and evolution. Tezisy Dokl. Zool. Inst. Akad. Nauk SSSR, 197839‐40. (in Russian); Jankowski, A. W. 1979. Systematics and phylogeny of the order Hypotrichida Stein, 1859 (Protozoa, Ciliophora). Trudy. Zool. Inst. Akad. Nauk SSSR, 86:48–85. (in Russian with English summary)]. Data obtained from light microscopic examination of stained (nigrosin‐butanol, Chatton‐Lwoff, and Protargol) cells in interphase or division supports and modifies the use of particular structural features of these ciliates for the purpose of taxonomic classification. The structural variability within and among populations of different species within the genera Diophrys (D. appendiculata, D. oligothrix, and D. scutum) and Paradiophrys (P. irmgard and P. multinucleata) is described. D. hystrix is redescribed as the type of the new genus Diophryopsis n. g. Comparative information on the cortical morphogenesis of division of selected species within each genus is reviewed. Two taxonomic classifications of these hypotrichs are discussed: 1) a listing of diagnoses and synonymies and 2) a binary key for identification of all species at the light microscope level. An alternative evolutionary explanation of variations among isolates is presented.

SYNOPSIS Free‐living marine ciliates occur in the interstitial spaces of a wide vareity of filamentous and particulate substrata, on the surfaces of planar substrata, and in the plankton. In addition, they are found in association with a wide variety of plant and animal hosts. In this paper I review the progress during the past decade in understanding the distribution of marine ciliates, with particular emphasis on the relationship between ciliate biogeography and the species problem. It is concluded that as a general rule among marine ciliates, genera and species complexes are cosmopolitan. Specific locales may support a confusing array of sibling species or subspecific morphologic variants. Because the distributional processes and breeding biology of marine ciliates are only beginning to be understood, conventional ideas that marine ciliate species are cosmopolitan may require modification.

Synopsis.The cortical anatomy and morphogenesis of Cladotricha koltzowii Gajewskaja and Cladotricha variabilis Rumen from the Great Salt Lake, Utah, are described. When compared with 2 marine hypotrichs, Uroleptoides kihni Wenzel and Stichotricha secunda Kahl, from the east coast of the United States, their structure and development suggest that Cladotricha may have descended from an intermediate species within the line of evolution of the Spirofilidae from primitive Kahliella‐like hypotrichs. It is suggested that the Kahliella‐Cladotricha‐Uroleptoides series is separated at the familial level from both the tubicolous and planktonic spirofilids. Further, the structural and morphogenetic differences between Cladotricha and the holostichid Uroleptus appear to be at a subordinal level. These findings are significant in revising definitions of the Hypotrichida, its suborders, and the Urostylidae.

Discotricha papillifera Tuffrau, a marine interstitial ciliate, is redescribed with the aid of light, scanning, and transmission electron microscopy from cells collected at a New Hampshire beach. Presence of primitive membranelles as well as an advanced stomatogenesis is demonstrated. The ultrastructure, including a unique membrane-bounded septate structure, is described. The cell is tentatively placed in the nassulid suborder Microthoracina, but affinities with other groups are discussed.

SYNOPSISTracheloraphis haloetes sp. n. occurred in a water sample from a Bahama industrial salt works (S = 82 ppt). Its measurement, shape, ciliation (pericytostomal, locomotory, and nonmotile), internal structure, and behavior are described. A simple binary key is used to indicate its similarities to and differences from its congeners. The key is based on characteristics of nuclear configuration, glabrous zone morphology, body shape, and patterns of ciliation.

SYNOPSIS Euplotes raikovi, an interstitial hypotrich ciliate described once from the Caspian Sea, was isolated from intertidal sand at Rye Harbor, New Hampshire. Specimens were observed in life, and also stained by Corliss’ modification of the Chatton-Lwoff wet-silver technic and by 2 nigrosin methods. Living individuals and those fixed with Parducz's fluid are 43 × 30 (37-50 × 25–35) μm. The AZM has an average length of 27 μm and contains 24–32 membranelles. The anterior part of AZM lies on the ventral face of an apical channel, much as in E. bisulcatus. There are 7 fronto-ventral, 4 transverse, 1 left marginal, and 2 right caudal cirri. An additional small, rounded argentophilic area resembling a cirrus base is evident in silver-stained preparations, but it is barren in virtually 100% of the population. There are 7–8 (usually 7) dorsal ciliary rows with E. patella-type argyrome. The modal number of cilia in rows I-VII are 3-7-9-9-9-10-10. The unique fronto-ventral cirrus pattern is stable and predictable at the time of streak phase. Morphogenetic development indicates that the barren cirrus base is 2/V (Wallengren system), and that it apparently buds from 1/V. The left marginal cirrus and right caudal cirri have different origins.

SYNOPSIS. Prior to the general use of methods revealing the silverline system, members of the genus Euplotes were separated on the basis of body shape, sculpturing and arrangement of cirri. Since 1954, species have been separated on the basis of number of dorsolateral cilium rows, dorsal silverline system, number of fronto‐ventral cirri, and form of the macronucleus. This paper describes E. charon Müller, 1773, E. quinquecarinatus Gelei, 1950, E. alatus Kahl, 1932, and E. bisulcatus Kahl, 1932, 4 species that are difficult to separate solely on the basis of the latter set of characters. By discussion of characters elucidated by a nigrosin‐HgCl2‐formalin method, it is suggested that the following can form a basis for identification of members of this genus: 1) cortical sculpturing, 2) arrangement of all ciliary organelles, including dorsal and endoral cilia, and 3) details of the silverline system. Existing methods are indicated, some far more simple than silver methods, with which these characters can be elucidated.

SYNOPSIS. This member of the Holostichidae differs from its five congenors in the possession of transverse cirri, the way in which its frontal cirri are arranged, and its estuarine habitat. Like other valid members of the genus Paraholosticha, this species has some of the frontal cirri arranged in a corona parallel with the anterior part of the zone of membranelles. This unique character, despite the possession of transverse cirri, places this ciliate in the genus Paraholosticha. Diagnostic characters of all valid members of this genus are presented, and the need for careful descriptions and taxonomic revisions within the suborder Stichotrichina is discussed.