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Description of Colpoda cucullusColpodid ciliate, 40-120 microns, usually 60-80 x 40-60 microns. Broadly reniform, semicircularly excavated at oral opening, anterior and posterior halves approximately equal in size. Keel with 8-10 notches, usually distinctly convex, but may recede slightly; more rarely almost straight and more distinctly receding. Slightly flattened, postoral sack moderately distinct, almost disappearing in well- or overfed cells. Macronucleus globular to slightly ellipsoid, location highly variable. Chromatin laminar at periphery, surrounding star-like achromatic centre. Chromatin structure changes during life-cycle. The ramified karyosome is found only in the logarithmic growth phase; during the stationary phase and in cysts it dissolves into several, fairly large aggregates. Micronucleus lenticular about 3 x 2 microns in size, usually attached to macronucleus. Contractile vacuole and excretory pore at posterior end, pulsation frequency approximately 10-30 seconds, depends on size of cell, rate of movement and degree of nourishment. Most or all populations have fine canals associated with the contractile vacuole, similar to those found in C. flavicans. Cytopyge near contractile vacuole. Cortex flexible, indistinctly furrowed by somatic kineties. Extrusomes in vivo 1-2 x 0.5 microns, sometimes yellowish, in several rows between and within somatic kineties, usually moderately refractive. Extrusion often difficult to induce, but iodized Indian ink produces voluminous coat consisting of approximately 10 microns long, slightly thorn-shaped rods. Such thorn-shaped extrusomes must be regarded as characteristic for the species. Cytoplasm colourless to yellowish and without crystalline inclusions, in well-nourished specimens crammed with up to 200 compact, 5-8 microns sized food vacuoles; such specimens appear blackish under low magnification. Cyclosis distinct. Feeds mainly on bacteria, but zooflagellates and algae are also consumed occasionally. Movement left-spiralling, irregular and at a speed of 310-700 microns/s. Often fairly motionless for long periods during food intake. Cilia approximately 10 microns long, paired; single cilia occur postorally. Somatic kineties on right side slightly, on left distinctly spirally coursing, approaching keel at approximately same angle, markedly less densely ciliated in posterior quarter. Polymerization of dikinetids in postoral kineties (diagonal groove) recognizable up to median of left side. Number of somatic kineties highly variable both in respect of extremes (24-45) and averages (26-38). The majority of populations, however, have about 29-35 kineties. 8-12 postoral kineties. Oral aperture inclined at about 45 degrees to longitudinal body axis. Vestibulum moderately large with distal half of left oral polykinetid protruding. Left polykinetid slightly crescentic, forms an angle of 30 degrees-60 degrees (usually around 45 degrees) with longitudinal body axis, almost parallel to right polykinetid. Basal bodies of right polykinetid arranged in fairly regular rows, especially distally. 1-2 vestibular kineties.