Molecular phylogeny of a new gonostomatid ciliate revealing a discrepancy between interphasic and cell divisional patterns (Ciliophora, Hypotricha).

The morphology, ontogenesis, and molecular phylogeny of a new ciliate, Gonostomum koreanum n. sp., discovered in a terrestrial moss sample from South Korea, were investigated. Morphologically, it is characterized by a gonostomatid oral apparatus, two macronuclear nodules, six frontoventral rows, the two rightmost of which (frontoventral rows V and VI) extend posteriorly to near pretransverse and transverse cirri, and three dorsal kineties each with a single caudal cirrus posteriorly. The new species is easily confused with members of the genus Metagonostomum because of the long frontoterminal cirral row but differs mainly in the de novo (vs. intrakinetal) origin of anlage VI, a character found only in Gonostomum and Paragonostomum. To solve the discrepancy between the interphasic and ontogenetic patterns, we additionally performed morphological and multigene analyses on three gonostomatid species, namely Gonostomum koreanum n. sp. and its morphologically (M. gonostomoida) and ontogenetically (G. kuehnelti) most similar species. The multigene analyses show that the new species is closely related to G. kuehnelti and the core gonostomatids consists of five groups based on the origin of the frontoventral rows.

Morphology, ontogenesis and molecular phylogeny of a new saline soil ciliate, Uroleptoides salina nov. spec. (Ciliophora, Hypotrichia).

The morphology, morphogenesis and molecular phylogeny of a new saline soil hypotrich ciliate, Uroleptoides salina nov. spec., discovered from China, was investigated. The new species is characterized as follows: body 150-215 × 40-50 µm in vivo, slender and highly flexible; usually four ellipsoidal macronuclear nodules; contractile vacuole absent; cortical granules absent; endosymbiotic algae present; amphisiellid median cirral row consists of 14-25 cirri and terminates about 47% down length of body; usually three buccal cirri and 3-13 cirri left of anterior portion of amphisiellid median cirral row; 3-5 transverse cirri. Morphogenesis during binary fission is characterized by: (1) the parental adoral zone of membranelles is retained completely, parental paroral contributes to the formation of the undulating membranes anlage for the proter; (2) the oral primordium of the opisthe is formed apokinetally; and (3) the amphisiellid median cirral row is formed from two anlagen. Phylogenetic analyses based on SSU rDNA sequence data show that Uroleptoides salina nov. spec. has a close relationship with its morphologically similar species, U. longiseries, U. magnigranulosus, Orthamphisiella breviseries, and Parabistichella variabilis.

Morphology, Morphogenesis and Phylogenetic Position of the Soil Hypotrichous Ciliate, Parabistichella dieckmanni (Foissner, 1998) Foissner, 2016 (Ciliophora, Hypotrichia), with Notes on the Phylogeny of Parabistichella.

The morphology, morphogenesis and molecular phylogeny of Parabistichella dieckmanni (Foissner, 1998) Foissner, 2016, isolated from north China, were investigated. The Chinese population was characterized as having five to seven frontal cirri in corona, four to seven buccal cirri arranged in a row, two to four short frontal rows and two long frontoventral rows, three to seven transverse cirri, four macronuclear nodules, three dorsal kineties following a Gonostomum-pattern, and caudal cirri absent. Morphogenetic research reveals that the main characteristics during binary fission are as follows: (1) the long left frontoventral row is formed by two or three anlagen; (2) the posterior part of the parental adoral zone of membranelles is renewed, and the oral primordium of the opisthe is formed intrakinetally; (3) FVT-anlagen I to VI (or V, VII) produce each a frontal cirrus to form the frontal corona; (4) development of dorsal kineties follows the Gonostomum-pattern. Phylogenetic analyses showed that P. dieckmanni does not group with other Parabistichella species. Therefore, the genus Parabistichella is polyphyletic. Additionally, Parabistichella variabilis n. comb. (basionym: Bistichella variabilis He & Xu, 2011) and Parabistichella cheni n. nom. (basionym: Parabistichella variabilis Jiang et al., 2013) were suggested.

Morphology and molecular phylogeny of two new soil ciliates, Hemiurosomoida warreni nov. spec. and Hemiurosoma clampi nov. spec. (Ciliophora, Hypotrichia) from Tibet.

Two novel hypotrichous ciliates, Hemiurosomoida warreni nov. spec. and Hemiurosoma clampi nov. spec., isolated from soil in the Lhalu Wetland and Motuo Virgin Forest in Tibet, respectively, were investigated using live observation and protargol staining. Hemiurosomoida warreni nov. spec. strongly resembles the type species H. longa but can be distinguished by its body size in vivo (110-145 × 30-40 µm vs. 50-100 × 18-40 µm), number of adoral membranelles (25-38 vs. 15-22), and numbers of right (29-39 vs. 14-23) and left (26-35 vs. 13-23) marginal cirri, transverse cirri (3 vs. 4 or 5) and macronuclear nodules (4-8 vs. 2). Hemiurosoma clampi nov. spec. is characterized by its vermiform body shape, colourless cortical granules distributed in irregular rows, two macronuclear nodules, three frontal cirri, one buccal cirrus, four frontoventral cirri ranged in a line, two transverse cirri, lacking postoral ventral and pretransverse ventral cirri, and marginal rows that are not posteriorly confluent. Phylogenetic analyses based on SSU rDNA gene sequences suggest that Hemiurosomoida is not monophyletic. A close relationship is revealed between Hemiurosomoida warreni nov. spec., Parakahilella macrostoma, Hemiurosoma clampi nov. spec., and the type species Hemiurosoma terricola. As expected, all these species are classified within the "non-oxytrichid Dorsomarginalia".

Morphology, morphogenesis and molecular phylogeny of a novel soil ciliate, Afrokahliella paramacrostoma n. sp. (Ciliophora, Hypotrichia).

A soil hypotrich ciliate, Afrokahliella paramacrostoma n. sp., was discovered in China. Its morphology, morphogenesis and molecular phylogeny were investigated using standard methods. The new species is characterized as follows: body about 140-180 × 60-70 µm in vivo, cortical granules absent, contractile vacuole positioned about 40% down length of body, 5-9 macronuclear nodules, 34-49 adoral membranelles, 3-5 buccal and 3-6 parabuccal cirri, usually two frontoventral rows, three or four left and two or three right marginal rows, three dorsal kineties and one dorsomarginal kinety; 1-3 and one or two caudal cirri located at the ends of dorsal kineties 1 and 2, respectively. The ontogenetic process is characterized by: (1) the marginal anlagen on each side develop in the outer right and the inner left marginal rows, respectively; (2) five frontoventral-transverse cirral anlagen, anlagen II-IV develop in secondary mode; (3) dorsal morphogenesis follows a typical Urosomoida-pattern, no parental dorsal kineties are retained; (4) caudal cirri are generated at the ends of dorsal kineties 1 and 2. Phylogenetic analyses based on SSU rDNA sequence data reveals that Afrokahliella paramacrostoma n. sp. is closely related to Parakahliella macrostoma and Hemiurosomoida longa.

Morphology and taxonomy of Pseudokeronopsis rubra (Ehrenberg, 1836) and Pseudokeronopsis parasongi sp. nov. (Ciliophora, Hypotrichia, Urostylida) from the Yellow Sea.

Pseudokeronopsis rubra (Ehrenberg, 1836), type species of Pseudokeronopsis, is a widely reported but taxonomically confused species with most available sequences either misidentified or dubious. Based on the Yellow Sea population, of which the molecular data have been analyzed in a previous work, we redescribe P. rubra and try to clarify the identifications of the populations designated as P. rubra or those allocated to this species. Nonetheless, there is still a need of integrated investigation of the species combing both morphological and molecular data based on specimens from its type locality. Recent molecular analyses indicate a Yellow Sea population of Pseudokeronopsis (marked as P. cf. songi) likely represents a new species. Here we describe it as P. parasongi sp. nov. based on the specimens from the same population. The new species possesses a yellow-brownish body with both pigmented cortical granules and colorless blood cell-shaped structures underneath the body surface, invariably one contractile vacuole at 66% of body length and occasionally a second one at 25%, and the midventral complex terminating 2-23 µm ahead of 2-4 transverse cirri. It differs from closely related species by its yellow-brownish body color and separates from them in the ITS1-5.8S-ITS2 and COI genetic distances.

A new species of Uronychia (Spirotrichea: Euplotida) from Argentina.

Uronychia clapsae sp. n. was discovered in an artificial channel that drains an endorheic area from the "sandy Pampa" into the upper basin of Salado River, Buenos Aires, Argentina. This euplotid measures 56-112 µm × 42-70 µm in vivo, is oval in shape and the buccal field is enormous, occupying ca. 80% of body length. It is characterized by having two macronuclear nodules and one micronucleus; usually 10 anterior and invariably three posterior membranelles; right end of paroral hook-like; buccal cirrus base about 3.5-6.0 µm long; invariably four frontal, two ventral, three left marginal, four transverse, and three caudal cirri; six dorsal kineties, kinety 1 with 15-22 dikinetids. Most Uronychia species were recorded in marine habitats, while this new isolate was found in a slightly saline, inland water body. Taxonomic and nomenclatorial concerns on some species assigned to Uronychia are also discussed.

An Integrative Investigation of Parabistichella variabilis (Protista, Ciliophora, Hypotrichia) Including Its General Morphology, Ultrastructure, Ontogenesis, and Molecular Phylogeny.

Hypotrichs are a highly differentiated and very diverse group of ciliated protists. Their systematics and taxonomy are challenging and call for detailed investigations on their general morphology, ultrastructure, ontogenesis, and molecular phylogeny. Here, a comprehensive study is conducted on a brackish water population of Parabistichella variabilis using light and electron microscopy and phylogenetic analyses based on small subunit ribosomal DNA sequence data. Its morphology, including the infraciliature, pellicle, nuclei, buccal seal, and extrusomes, is documented. The present findings indicate that in P. variabilis: (i) the cortical granules are extrusomes, which differ from those of other hypotrichs; (ii) the buccal seal is bounded by the plasma membrane and contains a single layer of longitudinal microtubules; (iii) two contractile vacuoles might be present rather than one; and (iv) the pharyngeal disks are bounded by a single membrane. Early-to-middle stages of ontogenesis are described for the first time, enabling the complete characterization of this process. Phylogenetic analyses indicate that Parabistichella variabilis is closely related to several species from different genera, such as Orthoamphisiella breviseries, Uroleptoides magnigranulosus, and Tachysoma pellionellum. However, ultrastructural and gene sequence data for more taxa are needed in order to resolve the systematics of Parabistichella.

Reevaluation of the 'well-known' Paraurostyla weissei complex, with notes on the ontogenesis of a new Paraurostyla species (Ciliophora, Hypotrichia).

Two hypotrichous ciliates, Paraurostyla wuhanensis nov. spec. from Wuhan (China) and a new North American population of the Paraurostyla weissei complex, were studied based on live observations and protargol impregnation. Paraurostyla wuhanensis nov. spec. differs from the congeners by the combined features of having six or seven frontal cirri, 2-4 frontoventral cirri, 5-7 ventral rows, and yellow-greenish cortical granules. Ontogenesis proceeds as in the type species, except that fewer frontoventral cirri are formed in the new species. The morphology of the new population of the Paraurostyla weissei complex corresponds well with other American populations. In the phylogenetic trees based on the 18S rRNA gene, Paraurostyla sequences nest in a large clade together with Apoamphisiella and Notohymena. Monophyly of Paraurostyla is rejected by the results of the approximately unbiased test analyses. Morphological, morphogenetic, and phylogenetic analyses show a close relationship between the North American populations of the Paraurostyla weissei complex and Apoamphisiella, indicating that the taxonomic position of the former needs to be reassigned. A new combination, viz. Apoamphisiella polymicronucleata (Merriman, 1937) comb. nov., a reevaluation of the Paraurostyla weissei complex, and emended diagnoses of Paraurostyla and Apoamphisiella, are provided.

Morphology, morphogenesis, and molecular phylogeny of a novel saline soil ciliate, Heterourosomoida sinica n. sp. (Ciliophora, Hypotrichia).

The living morphology, infraciliature, morphogenesis and phylogenetic position of a new hypotrichous ciliate, Heterourosomoida sinica n. sp., discovered in saline soil in Northwest of China, were investigated. Heterourosomoida sinica n. sp. is characterized as follows: body size about 70-95 × 20-30 µm in vivo, ellipsoid or elongate; cortical granules dark green, 0.5 µm across, arranged around dorsal cilia and in short irregular rows on ventral side; consistently 18 frontoventral-transverse cirri; left and right marginal rows composed of 17-24 and 13-21 cirri, respectively; two macronuclear nodules and one or two micronuclei. The detailed morphogenesis of a species of Heterourosomoida is reported for the first time. The ontogenesis on dorsal side is basically similar to that in Urosomoida-species and the process on ventral side is characterized by: 18 frontoventral-transverse cirri develop from six anlagen with the anlagen V and VI formed in the primary mode. Phylogenetic analyses based on SSU rDNA sequence data reveal that Heterourosomoida sinica n. sp. clusters with Kleinstyla dorsicirrata and forms a clade with the type species H. lanceolata.

Morphology and molecular phylogeny of a new hypotrich ciliate, Anteholosticha songi nov. spec., and an American population of Holosticha pullaster (Müller, 1773) Foissner et al., 1991 (Ciliophora, Hypotrichia).

A new urostylid ciliate, Anteholosticha songi nov. spec., isolated from forest soil in Tibet, and an American population of Holosticha pullaster (Müller, 1773) Foissner et al., 1991, isolated from a freshwater pond in the USA, are investigated in terms of their morphology, ontogenesis, and molecular biology. Anteholosticha songi nov. spec. is characterized by a slender to ellipsoidal body measuring 160-205 × 40-55 µm in vivo; rod-shaped yellowish cortical granules arranged in irregular short rows; four dorsal kineties; adoral zone consisting of 35-40 membranelles; three frontal, one buccal, one parabuccal, two frontoterminal, two pretransverse, and four to six transverse cirri and 14-25 midventral pairs; 12-22 ellipsoidal macronuclear nodules longitudinally arranged in pairs left of cell mid-line. Supplemental information on morphogenesis in Holosticha pullaster is also presented. The phylogenetic relationship of Anteholosticha and Holosticha inferred from SSU rDNA sequence data are concordant with previous studies and showing that Holosticha is monophyletic whereas Anteholosticha is polyphyletic and should be split into two or more genera.

Morphological and molecular characterization of Neogastrostyla aqua nov. gen., nov. spec. (Ciliophora, Hypotrichia) from River Yamuna, Delhi; comparison with Gastrostyla-like genera.

A novel hypotrichous ciliate, Neogastrostyla aqua nov. gen., nov. spec. isolated from River Yamuna, Delhi, India, is described using morphological, morphometric and morphogenetic characters from live and stained cells supplemented with derivation of phylogenetic relationships using SSU rRNA gene. Neogastrostyla aqua nov. gen., nov. spec., is characterized by light yellow cortical granules present singly or in clusters of two to five, distributed randomly; parental adoral zone of membranelles retained unchanged for proter; Oxytricha-pattern of paroral and endoral; 25-27 frontal-ventral-transverse cirri including a distinct bipartite frontoventral row of composite origin; three dorsal kineties with no fragmentation of the third kinety, two dorsomarginal kineties, three caudal cirri, two macronuclear nodules and two or three micronuclei. Though Neogastrostyla aqua nov. gen., nov. spec. has a frontoventral row like Gastrostyla species, it differs distinctly in the combination of characters from Gastrostyla and other genera with Gastrostyla-like ventral ciliature (Protogastrostyla, Hemigastrostyla, Apogastrostyla and Pseudogastrostyla) particularly in the colour and distribution of cortical granules as well as arrangement and formation of ciliature. According to the phylogenetic analyses, Neogastrostyla aqua nov. gen., nov. spec. clustered consistently with Gastrostyla sp. Y2 (no description available), Oxytricha granulifera and Architricha indica as close neighbours.

Morphogenesis and Molecular Phylogeny of a Soil Ciliate Uroleptoides longiseries (Foissner, Agatha and Berger, 2002) Berger 2008 (Ciliophora, Hypotrichia).

Morphogenesis of the hypotrich ciliate Uroleptoides longiseries, isolated from sandy soil beside the Yellow River, Suide, Yulin, Shaanxi Province, China, was investigated using protargol staining. The main events during binary fission are as follows: (1) the long frontoventral row is formed by a single anlage; (2) five frontoventral-transverse cirral anlagen are formed in primary mode; (3) only the posterior part of the parental adoral zone of the membranelles is renewed; and (4) the oral primordium of the opisthe is formed intrakinetally. This is the first detailed record of all stages of morphogenesis for Uroleptoides. We also provide the first record of the small subunit ribosomal DNA (SSU rDNA) sequences for U. longiseries. Phylogenetic analyses based on the SSU rDNA sequences data show that Uroleptoides longiseries clusters with U. magnigranulosa with moderate to high support which together form a clade with Orthoamphisiella breviseries. These three species share the morphogenetic feature of the long frontoventral row being formed by a single anlage.

Morphology and morphogenesis of a new soil urostylid ciliate, with the establishment of a new genus Caudiurostyla gen. nov. (Ciliophora, Hypotrichia).

The living morphology, infraciliature and morphogenesis of a new urostylid ciliate, Caudiurostyla sinensis gen. nov., spec. nov., isolated from a soil habitat in China, was investigated. The new, monotypic genus is defined by having frontal cirri arranged in a bicorona, distinct transverse cirri, midventral pairs only, buccal cirrus, frontoterminal cirri, multiple marginal cirral rows that replicate independently via within row anlagen formation, more than three dorsal kineties, and caudal cirri. The main morphogenetic features of the new species are: (1) the posterior part of the parental adoral zone is renewed in situ; (2) the marginal cirral rows and dorsal kineties develop intrakinetally; (3) a caudal cirrus is formed at the posterior end of each dorsal kinety; and (4) macronuclear nodules fuse into a single mass during the middle stage. Based on the morphology and morphogenesis, the new genus differs from Pseudourostyla and Hemicycliostyla by presence of caudal cirri.

Morphology, morphogenesis and molecular phylogeny of a new brackish water subspecies, Neourostylopsis flava paraflava nov. subsp. (Ciliophora, Hypotrichia, Urostylidae), with redefinition of the genus Neourostylopsis.

The morphology, morphogenesis and small subunit (SSU) rRNA gene-based phylogeny of Neourostylopsis flava paraflava nov. subsp. were investigated. Neourostylopsis flava paraflava nov. subsp. was separated from N. flava flava nov. stat. by habitat (brackish water vs. fresh water), pretransverse cirri (absent in all specimens vs. present in 15% of specimens), the numbers of frontal cirri (8-15 vs. 6-8) and left marginal cirral rows (6-9 vs. 4-5). The main morphogenetic features of N. flava paraflava nov. subsp. are as follows: (1) streaks I-VI (or I-VIII, deduced from morphological data) produce the bicorona; (2) the oral primordium and frontoventral-transverse cirral anlagen in the opisthe are formed de novo on the cell surface; (3) the numerous macronuclear nodules fuse into a branch-like mass; (4) two pretransverse ventral cirri are formed initially but disappear in the later stages; and (5) some of the frontoventral-transverse cirral anlagen develop in a primary mode. Based on SSU rDNA sequence data, phylogenetic analyses show a close relationship between N. flava paraflava nov. subsp., N. flava flava nov. stat. and other Neourostylopsis species. An improved diagnosis for Neourostylopsis is provided: Urostylidae with five or more frontal cirri which form an indistinct or distinct bicorona; pretransverse cirri present or absent; transverse cirri present; buccal cirri present; two frontoterminal cirri; midventral complex composed of midventral pairs only; more than one row of marginal cirri on each side which derive from individual anlagen within each parental row; caudal cirri lacking; three dorsal kineties; endoral and paroral rather long, endoral straight, paroral distinctly curved anteriorly.

Morphology and morphogenesis of a new soil urostylid ciliate, Australothrix xianiensis nov. spec. (Ciliophora, Hypotrichia).

The living morphology, infraciliature and morphogenetic events of a new soil urostylid ciliate, Australothrix xianiensis nov. spec., collected from Chanba National Wetland Park in Xi'an, China, were studied in vivo and after protargol preparation. Australothrix xianiensis nov. spec. is characterized as follows: about 190-240 × 40-60 µm in life; body pisciform and dark; cortical granules arranged in longitudinal rows on both sides, colourless, rod-shaped and about 2 × 1 µm in size; single contractile vacuole slightly ahead of mid-body with two long collecting canals; macronuclear nodules scattered throughout cytoplasm; adoral zone occupies about 20% of body length, composed of about 32 membranelles; three frontal cirri and one buccal cirrus; two to four midventral pairs and four or five midventral rows, one left and two right marginal rows; four or five dorsal kineties; four or five caudal cirri. The main features of divisional morphogenesis are: (1) each posterior streak generates a midventral row together with the midventral pair; (2) the old adoral zone of membranelles is retained with the exception of the posterior part, which is renewed in situ; the undulating membranes are completely renewed; (3) parental cirri do not contribute to the construction of the oral primordium in the opisthe in very early dividers; (4) two sets of frontoventral cirral anlagen are formed.

Morphology and Molecular Phylogeny of a New Hypotrich Ciliate, Pseudourostyla guizhouensis sp. nov. from Southern China, with Notes on a Chinese Population of Hemicycliostyla franzi (Foissner, 1987) Paiva et al., 2012 (Ciliophora, Hypotricha).

The morphology and molecular phylogeny of a soil hypotrich ciliate, Pseudourostyla guizhouensis sp. nov., collected from southern China, were investigated. Pseudourostyla guizhouensis sp. nov. has an elongate elliptical body measuring 180-310 × 65-85 µm in vivo; invariably two right and three or four left marginal rows; six or seven dorsal kineties; adoral zone consisting of 57-70 membranelles; 12-16 frontal cirri, one buccal cirrus, 13-20 midventral pairs, two frontoterminal cirri, two pretransverse cirri, and five to seven transverse cirri. Morphogenesis during physiological regeneration indicates that the marginal rows of each side originate from a common anlage that differentiates into several rows. Molecular phylogenetic analysis based on SSU rDNA sequence data reveals that P. guizhouensis sp. nov. clusters with the type species P. cristata (Jerka-Dziadosz, 1964) Borror, 1972 and that the genus Pseudourostyla is monophyletic. The morphological characters of another soil hypotrich ciliate, Hemicycliostyla franzi (Foissner, 1987) Paiva et al., 2012, are also described based on a Chinese (Guizhou) population.

Morphology, cell-division, and phylogeny of Schmidingerothrix elongata spec. nov. (Ciliophora, Hypotricha), and brief guide to hypotrichs with Gonostomum-like oral apparatus.

The hypotrich Schmidingerothrix elongata spec. nov., discovered in saline (20‰) soil of the Longfeng Wetland, Daqing, northern China, was studied using live observation and protargol impregnation. It is characterized, inter alia, by colorless cortical granules arranged in short rows, three frontoventral cirral rows with the rightmost extending far posteriorly, and 4-8, usually six macronuclear nodules. Cell division proceeds as in congeners and confirms the lack of dorsal ciliature. In phylogenetic analyses based on SSU rDNA, S. elongata is sister of S. salinarum+Paracladotricha salina. A re-investigation of the type slides of P. salina, type of Paracladotricha, revealed a misobservation in the original description. Since P. salina lacks, like Schmidingerothrix spp., a dorsal ciliature, Paracladotricha becomes a junior, subjective synonym of Schmidingerothrix with S. salina comb. nov. as fourth species. A review of the phylogenetic analyses dealing with Schmidingerothrix shows that its position is variable. However, together with the gonostomatid oral apparatus it can be hypothesized that Schmidingerothrix is a member of the Gonostomatidae or a close relative. A list of genera (14) and species (58) which have - like Schmidingerothrix - a gonostomatid oral apparatus, as well as a key to these genera are provided.

Morphogenesis of a saline soil ciliate Urosoma salmastra (Dragesco and Dragesco-Kernéis, 1986) Berger, 1999 with notes on the phylogeny of Urosoma (Ciliophora, Hypotrichia).

Morphogenesis in a population of Urosoma salmastra collected from saline soil in Weinan, China, was investigated using protargol staining. The main morphogenetic event that characterizes U. salmastra is the inclusion of cirrus III/2 in the formation of the frontoventra-transverse cirral anlagen. We also provide small subunit ribosomal DNA gene sequences for a population of U. salmastra and for two populations of U. emarginata. The molecular phylogeny indicates that Urosoma is not monophyletic, viz, U. emarginata branches separately from its congeners and the closest relative of U. salmastra is Oxytricha granulifera.

Morphology, morphogenesis and molecular phylogeny of a new soil ciliate Paragonostomoides xianicum n. sp. (Ciliophora, Hypotrichia, Gonostomatidae).

A hypotrichous ciliate, Paragonostomoides xianicum n. sp., was discovered in soil on the surface of the Xi'an Circumvallation, Xi'an, China. Its morphology and some major ontogenetic stages were studied and the phylogenetic position was estimated using standard methods Paragonostomoides xianicum n. sp. is characterized as follows: body size about 75-90×15-25µm in vivo, elliptical and flexible; colourless mitochondrion-like cortical granules scattered throughout cell surface, about 0.8×0.6µm in size; contractile vacuole positioned at mid-body; frontoventral row IV extends beyond buccal vertex and frontoventral row VI extends to about body end; frontoventral cirri not in pairs; transverse cirri absent; three caudal cirri; two macronuclear nodules and one to four micronuclei. The ontogenetic process in P. xianicum n. sp. is basically similar to that in Metagonostomum gonostomoidum and Gonostomum-species and featured by: frontoventral-transverse cirri develop in five-anlage pattern, the rearmost cirri generated in the two rightmost FV anlagen are not set off posteriorly (like an ordinary TC), as well as FVT-anlagen n and n-1 form in secondary mode in P. xianicum. Phylogenetic analyses based on small subunit ribosomal DNA sequence data reveal a close relationship of P. xianicum n. sp. with Cotterillia bromelicola and two Gonostomum-species.