Polar filament

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A polar filament is a structure found in the polar capsule of myxosporean organisms, and is analogous to the "penetrant" structure found in cnidocytes.

The polar filament is coiled along the inner wall of the polar capsule, and is capable of rapid extrusion, during which it everts "inside-out". When everted, it is sticky, and probably serves to hold the spore on to the intestinal wall of the prospective host, and to help separate the valves of the spore.

The polar filament is important in species classification. In some specied of Ceratomyxa, the polar filament forms a straight basal section, which the rest of the filament coils around, while in the genus Sphaeromyxa, the filament is folded in a zig-zag arrangement, rather than being coiled.