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Description of Gonostomum affineThis is a colourless, ellipsoidal, and laterally more or less distinctly flattened, oxytrichid ciliate, usually about 90 x 30 microns but with a length ranging from 50 to 160 microns. As it is a hypotrich, the cilia are grouped into compound organelles, the cirri, which form a distinctive pattern. Usually, there are two ellipsoidal macronuclear nodules, each with a single micronucleus attached. The oral apparatus extends along the left margin of the anterior body. It is composed of a conspicuous adoral zone of membranelles to the left of the oral field and two short, inconspicuous undulating membranes at the right. One of the undulating membranes, the paroral, is composed of few, comparatively widely spaced cilia, a diagnostic feature of gonostomoid ciliates. The buccal cavity is small and inconspicuous. Only the ventral side bears cirri, the dorsal surface is bare, except for three rows of minute bristles associated with three caudal cirri at the posterior body end. The cirri form one row each at the right and left margin of the cell and several groups on the ventral surface. There are a highly variable number of fronto-ventral cirri (6-15) in the area between the anterior body end and the oral region, and an also highly variable number of transverse cirri (0-7) near the posterior body end. The fronto-ventral cirri are classified as frontal cirri (usually three at anterior body margin), buccal cirri (usually one near paroral membrane), fronto-terminal cirri (usually two at right anterior body margin), and ventral cirri (all other cirri m the area mentioned above) forming a more or less distinct row never surpassing the oral vertex. This organism usually feeds on bacteria.