Myxozoans are obligate parasites that have complex life cycles requiring alternate vertebrate and invertebrate hosts, with transmission via microscopic waterborne spores. Unusually for parasites, they belong to the phylum Cnidaria, alongside thousands of free-living corals, sea anemones, jellyfish and hydrozoans. Their cnidarian affinity is affirmed by genetic relatedness and the presence of nematocysts, historically called “polar capsules” in myxozoan research. Free-living cnidarians utilise this cellular weaponry for defence, predation and adhesion, whereas myxozoans use it to anchor to their hosts as the first step in infection. Despite the ~650 million years of divergence between free-living cnidarians and myxozoans, their nematocysts retain many shared morphological and molecular characters. Both are intra-cellular capsules with a single opening, and contain a coiled, evertable tubule. They are composed of unique nematocyst proteins, nematogalectin and minicollagen, and both likely contain an internal matrix of metal cations covalently bound to the anionic polymer poly-gamma glutamate. The rapid dissociation of this matrix and the resulting increase in internal osmotic potential is the driving force behind tubule elongation during discharge. In this review, we compare the structure and function of nematocysts in Myxozoa and free-living Cnidaria, incorporating recent molecular characterizations. We propose that terminology for homologous myxozoan structures be synonymized with those from other Cnidaria, hence, “polar capsule” as a taxon-specific nematocyst morphotype and “polar filament” as “tubule.” Despite taxonomic divergence, genome reduction and an evolution to parasitism, myxozoans maintain nematocysts that are structurally and functionally homologous to those of their free-living cnidarian relatives.

粘虫是专性寄生虫，具有复杂的生命周期，需要交替的脊椎动物和无脊椎动物宿主，通过微小的水生孢子进行传播。与寄生虫不同的是，它们与数以千计的自由生活的珊瑚、海葵、水母和水生动物一起属于刺胞动物门。它们的刺胞动物亲和力通过遗传相关性和线虫囊的存在得到证实，线虫囊在粘液动物研究中历来被称为“极地囊”。自由生活的刺胞动物利用这种细胞武器进行防御、捕食和粘附，而粘液动物则利用它锚定宿主作为感染的第一步。尽管自由生活的刺胞动物和粘液动物之间存在约 6.5 亿年的分歧，但它们的刺丝囊保留了许多共同的形态和分子特征。两者都是具有单个开口的细胞内胶囊，并包含盘绕的、可外翻的小管。它们由独特的线虫囊蛋白、线虫半乳糖凝集素和微胶原组成，两者都可能含有与阴离子聚合物聚γ谷氨酸共价结合的金属阳离子内部基质。这种基质的快速解离和由此产生的内部渗透势的增加是放电过程中小管伸长的驱动力。在这篇综述中，我们比较了粘虫和自由生活的刺胞动物中刺丝囊的结构和功能，并结合了最近的分子特征。我们建议同源粘液动物结构的术语与来自其他刺胞动物的术语同义，因此，“极囊”作为分类群特定的刺细胞形态型，“极丝”作为“小管”。尽管存在分类差异，