- Kentrogonida
- Akentrogonida
Rhizocephala are degenerate crustaceans, parasitic on decapod crustaceans. Rhizocephalans are derived from barnacles, a relationship which is unrecognisable from the adult forms, but can be seen by comparison of the larvae. As adults they lack appendages, segmentation, and all internal organs except gonads and the remains of the nervous system. Other than the minute naupliar stages, the only distinguishable portion of a rhizocephalan body is the externa or reproductive portion. A female nauplius settles on a host and metamorphoses as it penetrates the internal portion of the animal. It then ramifies, or grows in a similar manner to a root system, through the host, centering on the digestive system. Once mature, the female produces a sac-like externa on the abdomen of the host. The externa is immature until a male nauplii settles on it and fuses with it. The externa then produces two types of eggs, small ones that will become female and large ones that become males. Because the externa is located in the same location as the host's egg sac would be, the host is fooled into thinking that the parasitic externa is its own egg sac. It cares for the rhizocephalan externa as if it were its own egg sac and never molts again (crustaceans don't molt until they release their eggs or young from the brood pouch). The host is so well fooled by the externa that even male hosts, which would never have carried eggs or young in a brood pouch, care for the externa as if they were females.
