The term polar filament may refer to either of two analogous structures used for host invasion by different groups of parasites: Myxozoa (Metazoa)[1] and Microsporidia (Fungi),[2][3] respectively.
It is homologous to the "penetrant" structure found in cnidocytes.
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.
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