Rhizocephala

:  an order of Cirripedia comprising extremely degenerate forms that live as parasites on crabs and hermit crabs, are hermaphroditic though complementary males occur, when young pass through stages similiar to those of a developing barnacle, and afterward attach themselves to suitable hosts where after a complex series of changes they become limbless unsegmented tumid sacs attached by a short peduncle from which rootlike processes penetrate to all parts of the host and absorb its juices — see peltogaster, sacculina