A polar filament is a structure found in the
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 is a class of microscopic parasites, belonging to the Myxozoa clade within Cnidaria. They have a complex life cycle which comprises vegetative forms in two hosts, an aquatic invertebrate (generally an annelid but sometimes a bryozoa ...
n organisms.
It is analogous to the "penetrant" structure found in
cnidocyte
A cnidocyte (also known as a cnidoblast or nematocyte) is an explosive cell containing one large secretory organelle called a cnidocyst (also known as a cnida () or nematocyst) that can deliver a sting to other organisms. The presence of this c ...
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 likely serves to hold the spore onto 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 species 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.
References
Animal anatomy
Myxozoa
{{Parasitic animal-stub