are not typically present in somatic cells. Centrioles do not participate in the nuclear division of myxozoans. Cell division by binary fission is rare, and cells divide instead via endogeny. In 2020, the myxozoan Henneguya salminicola was found to lack a mitochondrial genome, and thus be incapable of aerobic respiration; it was the first animal to be positively identified as such. Its actual metabolism is currently unknown. Phylogenetics Myxozoans were originally considered to be protozoans, and were included among other non-motile forms in the group Sporozoa. As their distinct nature became clear through 18S ribosomal DNA (rDNA) sequencing, they were relocated in the metazoa. Detailed classification within the metazoa was however long hindered by conflicting rDNA evidence: although 18S rDNA suggested an affinity with Cnidaria, other rDNA sampled, and the HOX genes of two species, were more similar to those of the Bilateria. The discovery that Buddenbrockia plumatellae, a worm-like parasite of bryozoans up to 2 mm in length, is a myxozoan initially appeared to strengthen the case for a bilaterian origin, as the body plan is superficially similar. Nevertheless, closer examination reveals that Buddenbrockia longitudinal symmetry is not twofold, but fourfold, casting doubt on this hypothesis. Further testing resolved the genetic conundrum by sourcing the first three previously identified discrepant HOX genes (Myx1-3) to the bryozoan Cristatella mucedo and the fourth (Myx4) to northern pike, the respective hosts of the two corresponding Myxozoa samples. This explained the confusion: the original experiments had used samples contaminated by tissue from host organisms, leading to false positives for a position among the Bilateria. More careful cloning of 50 coding genes from Buddenbrockia firmly established the clade as severely modified members of the phylum Cnidaria, with medusozoans as their closest relatives. Similarities between myxozoan polar capsules and cnidarian nematocysts had been drawn for a long time, but were generally assumed to be the result of convergent evolution. Taxonomists now recognize the outdated subgroup Actinosporea as a life-cycle phase of Myxosporea. Molecular clocks suggest that myxozoans and their closest relatives, the polypodiozoa, shared their last common ancestor with medusazoans about 600 million years ago, during the Ediacaran period. Taxonomy Myxozoan taxonomy has undergone great and important changes in its levels of generic, family and suborder classification. Fiala et al. (2015) proposed a new classification based on spores. References External links Animal classes