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Comprehensive Description

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Description of Colpoda magna

Colpodid ciliate, 120-400 microns long, usually 150-200 x 100-150 microns. Narrowly to broadly reniform, markedly indented at vestibular entrance. Keel convex to distinctly receding, right margin (dorsal side) sharply notched in posterior quarter by postoral sack. Laterally slightly to almost 2:1 flattened, right side almost flat, left convex. Left side distinctly tripartite due to deep, spiralling diagonal groove extending around cell in slightly more than 1 revolution producing large anterior and slightly smaller posterior lobe, terminally on which there is a somewhat smaller sack, usually creased and indistinct in silver preparations, containing the contractile vacuole. Macronucleus distinctly ellipsoid, in vivo about 60 x 25 microns, hence remarkably large (about 1/3 of body length), near centre of cell between vestibulum and dorsal side; nucleolus reticulate. Usually several (2-16) slightly ellipsoid micronuclei, each surrounded by distinct membrane and attached to macronucleus. Average number of micronuclei in individual clones varies from 4.2 to 12.6. Amicronucleate strains presumably result from unequal distribution of micronuclei during cytokinesis. Contractile vacuole subterminal in second, smaller postoral sack. 6-15 tubular collecting canals with phial-shaped dilatations extend on left side to anterior end. Canals sometimes difficult to recognize - cells may only be squashed slightly! Number of canals highly variable, about 5-12 in individuals from a clone culture. Excretory pore circular to elliptical, subterminally near median of cell. Cortex firm but flexible, bright due to approximately 3 microns thick layer of extrusomes, mentioned in older descriptions as distinct seam of rods. The extrusomes, in vivo about 3 microns long and located in several rows between the somatic kineties, are released when iodine solution or methyl green-pyronin is added, whereby they elongate to differing degrees, sometimes extending to threads up to about 50 microns long which produce a reticulated coat around the cell. Cytoplasm colourless, but cells appear brown-black under low magnification due to numerous anisotropic structures concentrated in vicinity of contractile vacuole. These refractile rods are up to 6 microns long and are probably crystals covered by an organic layer containing fatty acid. Lipid droplets 1-2 microns in diameter accumulate on dorsal wall of vestibulum. Feeds predominantly on bacteria digested in about 20 microns sized, compact food vacuoles, often completely packing the cell. Occasionally coccoid green algae, flagellates or small ciliates are ingested. Movement moderately rapid by rotation about longitudinal axis, often settles on detritus or on bottom of culture dish. Cilia approximately 10 microns long, paired throughout, very densely spaced. Somatic kineties with spiral course. Dikinetids form highly ordered, rhomboid pattern in diagonal groove but are, like the silverline system, arranged rather irregularly on rear (smaller) postoral sack. Kineties of right side extend almost parallel to a straight preoral suture and longitudinal body axis respectively whereas those of left side abut at right angles. About 25 postoral and 8 vestibular kineties.

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