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Description of Gymnodinium arcuatumCiliate, 15-25 microns long. Shape highly variable, outline roughly semicircular to elliptical, ventral side flat to slightly concave, dorsal distinctly convex; some populations tapering posteriorly and thus easily confused with Pseudocyrtolophasis terricola. Moderately to distinctly (2-3:1) flattened laterally, right side slightly to distinctly concave, left convex. Macronucleus globular, with distinct membrane, usually in the centre of cell, rarely opposed to oral apparatus. Micronucleus spherical, attached to macronucleus, not in perinuclear space. Contractile vacuole at posterior end, single excretory pore in centre of pole. Cortex usually distinctly furrowed by somatic kineties. No extrusomes or particular cortical granules recognizable in living cells, but cells appear covered by thin, argyrophilic layer which obscures the infraciliature in protargol slides. Cytoplasm colourless, but made opaque by many tiny granules and some crystal-like, irregularly shaped inclusions. Food vacuoles small, contain bacteria. Movement moderately rapid, usually creeping or resting on soil particles, swims clumsily by rotation about longitudinal axis; slight contractions may occur. Somatic cilia about 7 microns long. On right side 4 slightly spirally coursing kineties composed of dikinetids. Both basal bodies ciliated in kinety 1 and in anterior third of kineties 2 and 3. Anterior basal bodies of kinery 4 and of postoral portion of kineties 2 and 3 bare and slightly smaller. Last dikinetid of kinety 1 separated by small gap from the one in front of it and bears prominent caudal cilium approximately as long as cell. Posterior dikinetids of kineties 2-4 slightly but recognizable closer spaced, producing more densely ciliated patch similar to that occurring in Hausmanniella discoidea. On anterior end of left side, there are 3 short kineties composed of 1-2 ciliated dikinetids. 2-4 ciliated dikinetids near proximal margin of oral aperture. Oral apparatus in anterior ventral third of cell. Vestibulum very shallow. Paroral membrane comparatively distant from adoral organelles, half as long as oral aperture, curved, extends from anterior end to mid of oral aperture; composed of basal bodies in zigzag arrangement with only the outer ones presumably bearing short cilia. There are 4 adoral organelles on the left vestibular slope, each presumably comprising 2 rows of basal bodies; organelles become longer proximally and form a vertical ribbon. The right margin of this ribbon is argyrophilic Pharyngeal fibres very thin and form short funnel.