INFRACLASS RHIZOCEPHALA MÜLLER, 1862

Diagnosis: Here given in abbreviated form after Høeg et al. (2020). Monophyletic by molecular analyses. All parasitic stages without segmentation, appendages and alimentary canal. Specialized cypris antennules; parasites on Crustacea; with parasitic phase starting with an initial internal phase. The adult parasite consists of an internal ramifying and nutrient-absorbing root system and an external reproductive sac (externa). Separate sexes, with the female externa hosting and nourishing one or several dwarf males reduced to sperm-producing tissue and a few somatic cells.

Comment: Høeg et al. (2020) provided diagnoses of all the families; these can only exceptionally be characterized as monophyletic by morphology alone, but the molecular analyses support the monophyly of all almost presently recognized families.