A polar filament is a structure found in the polar capsule

Polar capsule

Polar capsules are structures found in the valves of Myxosporean parasites, which contain the polar filament. The polar capsule is constructed of a proteinaceous and a polysaccharide layer, both layers of which continue into the polar filament....

 of myxosporea

Myxosporea

The Myxosporea are a class of microscopic parasites, belonging to the Myxozoa. They have a complex life cycle which comprises vegetative forms in two hosts, an aquatic invertebrate and an ectothermic vertebrate, usually a fish. Each host releases a different type of spore...

n organisms, and is analogous to the "penetrant" structure found in cnidocyte

Cnidocyte

A cnidocyte, cnidoblast, or nematocyte is a type of venomous cell unique to the phylum Cnidaria . The cnidocyte cell provides a means for them to catch prey and defend themselves from predators. Despite being morphologically simple, lacking a skeleton and usually being sessile, cnidarians prey on...

s.

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.