Metacystis Cohn, 1866 (ref. ID; 2013)
Class Kinetofragminophora: Subclass Gymnostomata: Order Prostomatida: Suborder Prostomatina: Family Metacystidae (ref. ID; 2013)
Order Prostomatida: Metacystidae Small & Lynn, 1985 (ref. ID; 7485)

[ref. ID; 2013]
Ovoid to cylindrical body, strongly striated transversely, usually tapering anteriorly. Apical circular oral aperture with about 4 rings of peribuccal cilia surrounding it. Posterior end broadly rounded with a peculiar protruding terminal clear vacuole. Somatic cilia uniform, usually with single (rarely more) caudal cilium which does not arise terminally. Macronucleus ovoid, centrally located. Contractile vacuole terminal with smaller lateral one. Constructs and lives in a membranous lorica which is usually open at one end only but occasionally at both ends. Found in anaerobic situations feeding on sulfur bacteria.
Quote; Colin R. Curds "British and other freshwater ciliated protozoa Part I Ciliophora: Kinetofragminophora" Cambridge University Press, 1982 (ref. ID; 2013)

Metacystis truncata Cohn, 1866 (ref. ID; 1619, 3540, 7485) reported year? (ref. ID; 1618)
Description; Elongate, not much difference in body width at different levels; with about twelve furrow rings; salt water. (ref. ID; 1618)
[Lorica]: The membranous hyaline lorica is cylindrical (64.6-115 um length) with a well-developed, slightly curved anterior neck, and a sac with a round or narrowly-round posterior end, although some sacs had a taperd posterior end. The neck, which opens to the outside, has a circular aperture. Under the optical microscope, the neck appears hyaline and smooth. Under SEM, approximately 11 longitudinal folds were observed, an artifact probably due to shrinkage of the fragile neck during preparation for SEM. The sac is annulated with 15-26 rings, the anterior 12-14th are more spaced (distance between rings 3.3 um) than the posterior ones (distance between rings 1.5 um); sac height 24.6 um. The sac is attached along the length of one of its sides to the seagrass leaf and the narrow polyethylene strips while the neck rises up free at an angle of about 45 degrees. Rod-bacteria are attached to the sac. (ref. ID; 7485)
[Lorica development]: Structures in the shape of hemispherical prominences with a hole in the center were interpreted to be developing sacs. The hole represents the lorica aperture. The neck develops first, upward, followed by the anterior part of the sac reinforced with the rings and, finally, the posterior part of the sac with the more tightly spaced, less developed rings (five-six). (ref. ID; 7485)
[Cell body]: The body of this lorica-dwelling form is highly variable, ovoid to elongate and cylindrical (in vivo 19-91 um, stained 17.5-60 um length). The cell body varies from colorless to grey-brownish, possibly depending on the consumed food. It is not attached to the lorica, and can swim out of it. The anterior end is truncated with well-developed peribuccal cilia, subapical cytostome, and a small, distinct, precytostomal depression 5-10.5 um (average = 7.5) is present in some cells. Below the anterior end, as light constriction is especially obvious in ovoid forms. The posterior end is round. The somatic ciliature is uniform without terminal cilium. The body is contractile and transversely striated by 10-18 striae. A large terminal vacuole protrudes in the small and medium-sized individuals, but is inconspicuous in the elongate ones. A contractile vacuole is located in the mid-anterior body and other vacuoles (one-three) are present in the anterior to mid-posterior end. Food vacuoles, 3.5-7 um (average = 5.5) diam, and 2-9 (average = 4.6) in number as well as other small vacuoles with granular content and fine granulations are also present in the cytoplasm. The globular macronucleus is terminal in the elongate forms or near the mid-body in the ovoid form, with one micronucleus (2 um) adjacent to the macronucleus. A light zone without granulations is frequently seen around the nuclear appratus. We considered that M. truncata has two forms with the same lorica morphology: The small-ovoid form results from asexual division of the large-elongate form; the medium sized individuals are contracted elongate forms. (ref. ID; 7485)
Remarks; The lorica of M. truncata has never been described, and only data on the cell body are avaiable. Kahl (1930) gave a cell body size of 30 um, and mentioned that Cohn made several drawing of M. truncata but he was not sure if this species has different forms. Kahl (1930) chose the small ovoid form with 12 striae as the type for the species and included two drawings, an ovoid and a slightly elongate form, both with a protruding terminal vacuole. The most recent description of this species is Carey's (1992): size variable with some individuals 100 um in length and other attaining 300 um; elongate and cylindrical, the body is transversely striated and contractile; well-developed peribuccal cilia are present the cytopharynx is indistinct; a large terminal vacuole is present, but not a terminal caudal cilium; macronucleus large and terminal; no lorica has been described. The size (300 um) mentioned by Carely (1992) is a mistake and his drawing shows an ovoid form with a central macronucleus, a variation not mentioned in his description. The cell body traits of the populations correspond in our study with those from Kahl's (1930) description. As well our populations correspond with drawings and the habitat of M. truncata in Cohn's original description, especially the small ovoid and slightly elongate form with truncated anterior end, conspicuous protruding terminal vacuole, absence of caudal cilium, number of striae (12), and the central macronucleus. On the other hand, the elongate form described herein, with the macronucleus and the micronucleus located in the posterior end, without a clear protruding terminal vacuole, the presence of one or more vacuoles in other parts of the cell body, and a contractile vacuole in the mid-posterior part, is similar to M. annulata, although the position of the macronucleus is not mentioned. Metacystis elongata, M. hyperhalina, and M. striata also show similarity with the elongate form observed in the present study; however, M. elongata is more slender, the macronucleus and micronucleus are located in the mid-body, and a large contractile vacuole is present in the posterior end above the protruding terminal vacuole. Metacystis hyperhalina (with a lorica similar to that of M. annulata) and M. striata have many features in common and also have been described as having the macronucleus in the mid-body. These five species have been observed in marine or brackish habitats. Both forms (ovoid and elongate) were found to have the small lorica morphology, characterized by an annulated sac with a round or narrowly round posterior end, a hyaline neck, and a circular aperture. Of the loricated species, M. annulata seems to most closely resemble the one described in our study, the differences being the longer lorica (140 um) of M. annulata and its more tapered posterior end. However, Kahl (1930) mentioned that the posterior end could be round. In conclusion, the individuals of the populations attached to the leaves of T. testudinum and to the artificial substrate have different shape and sizes and are similar to M. truncata in cell body and M. annulata in cell body and lorica. The variability is due to the contractility of the cell body especially of the elongate forms. Taking into account the present results and the incompleteness of previous specific descriptions, M. truncata and M. annulata are probably conspecific, M. annulata being a form of M. truncata with the latter name having priority of publication. A detailed revision of Metacystis is required to define the number of species of this genus, as well as the features that distinguish the species. (ref. ID; 7485)
Ecology; The temperature had a range of 24-31 degrees C, and the salinity of 26-36 o/oo. (ref. ID; 7485)
Sampling site; Metacystis truncata was found as an epibiont of the seagrass Thalassia testudinum, a dominant marine phanerogam (Lot-Helgueras 1971) of the coral reef lagoon of Isla Verde, Veracruz, Mexico (96 degrees 04" N, 19 degrees 12" W). (ref. ID; 7485)
Deposited material; The microscopic slides were deposited in the collection of the Laboratorio de Protozoologia, Facultad de Ciencias, Universidad Nacional Autonoma de Mexico. (ref. ID; 7485)
Measurements; Body length up to 30 um. (ref. ID; 1618)