The term polar filament may refer to either of two

analogous structures Analogy is a comparison or correspondence between two things (or two groups of things) because of a third element that they are considered to share. In logic, it is an inference or an argument from one particular to another particular, as oppose ...

used for host invasion by different groups of parasites:

Myxozoa Myxozoa (etymology: Greek: μύξα ''myxa'' "slime" or "mucus" + thematic vowel o + ζῷον ''zoon'' "animal") is a subphylum of aquatic cnidarian animals – all obligate parasites. It contains the smallest animals ever known to have lived. ...

(Metazoa) and

Microsporidia Microsporidia are a group of spore-forming unicellular parasites. These spores contain an extrusion apparatus that has a coiled polar tube ending in an anchoring disc at the apical part of the spore.Franzen, C. (2005). How do Microsporidia inva ...

(Fungi), respectively.

In Myxozoa

The 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. T ...

myxosporea Myxosporea is a class of microscopic animals, all of whom are parasites. They belong to the Myxozoa clade within Cnidaria. They have a complex life cycle that comprises vegetative forms in two hosts—one an aquatic invertebrate (generally an ann ...

n organisms. It is homologous to the "penetrant" structure found in

cnidocyte A cnidocyte (also known as a cnidoblast) is a type of cell containing a large secretory organelle called a ''cnidocyst'', that can deliver a sting to other organisms as a way to capture prey and defend against predators. A cnidocyte explosively ...

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