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Description of Mesodinium pulexCiliate, 20-30 x 14-21 microns long. Body pyriform, divided by a constriction into a truncated anterior cone and a globular trunk. Two tiny macronuclear nodules near mid-body. Two tiny (possibly contractile) vacuoles in basal region of cone (difficult to recognize, and not necessary for identification). Often a sizeable, crescent-shaped terminal vacuole. No extrusomes have been described. Two distinct ciliary girdles, each consisting of 23-34 (mean about 29) short rows, in constricted body area. The anterior girdle is composed of ciliary tufts, which form, in individuals at rest, three circlets (an oblique-anteriorly, a laterally, and an oblique-posteriorly directed circlet), each circlet composed of about 8-11, about 20-23 microns long tufts, which are bifurcated distally for about half of their length and lack barbs. The middle girdle is composed of ciliary tufts, which, when not beating, attach to the trunk and thus difficult to recognize. Posterior girdle and dorsal brush lacking. Oral apparatus occupies truncated anterior end, unciliated, but with up to 10 pin-shaped processes, which can be extruded individually or as a circlet from around the periphery of the cytostome. Often remains motionless for several minutes with tufts of the anterior girdle extended, then quickly darts backward for some distance. Attaches to substrates by the tentacular processes. Mostly reported from saline habitats.