[3] Their body plan is uniquely reduced in an extreme adaptation to their parasitic lifestyle, and makes their relationship to other barnacles unrecognisable in the adult form.

[4] Adult rhizocephalans lack appendages, segmentation, and all internal organs except gonads, a few muscles, and the remains of the nervous system.

[6] The female cypris larva in Kentrogonida settles on a host and metamorphoses into a specialized juvenile form, a kentrogon.

[7] The vermigon grows into a network of root-like threads through the host's tissue, centering on the digestive system and especially the hepatopancreas, and absorb nutrients from the hemolymph.

But it soon molts to a second stage that contains an orifice known as the mantle departure, and which leads into the two receptacle passageways — once assumed to be the testes in hermaphroditic parasites before the realization that they were actually two separate sexes — and starts releasing pheromones to attract male cyprids.

[11][12][13] In the order Akentrogonida, which form a monophyletic group nested within the paraphyletic Kentrogonida,[14] the male does not develop into a trichogon, and the cypris injects its cell mass through its antennule and directly into the body of the immature externa.

In species like Clistosaccus paguri, the male injects its cluster of cells which migrates through the connective tissue of the mantle and into the receptacle.

[8] Following an updated classification of barnacles by Chan et al. (2021), the subgroups Akentrogonida and Kentrogonida were dropped, leaving 13 families as children of the infraclass Rhizocephala.

[3] Triangulidae (4) Mycetomorphidae (2) Peltogastridae (47) Peltogasterellidae (10) Polyascidae (8) Clistosaccidae (2) Polysaccidae (2) Thompsoniidae (24) Pirusaccidae (1) Duplorbidae (5) Chthamalophilidae (4) Parthenopeidae (3) Sacculinidae (177)