RHIZOCEPHALANS are barnacles (Cirripedia), but are extremely specialized for parasitic life on decapod crustaceans. A cypris larva settles and develops into a new instar, the kentrogon, which inoculates the host with the parasite. The very early primordial parasite has;been argued to consist solely of embryonic stem cells or even eggs(1), hut the true nature of this unknown stage has remained a puzzle for more than a century(2). We present data from in vitro experiments on the rhizocephalan Loxothylacus panopaei documenting that, unlike previous postulations, the recently injected parasite is not naked embryonic cells, but has the form of a motile, vermiform body, enclosed in an acellular sheath, After a period of maturation the vermiform body splits up into a number of naked and independently moving cells, which in our in vitro experiments disperse by amoeboid movements. This suggests that, in vivo, the cells disperse in the haemolymph of the host crab, where each has the potential to develop into an adult parasite, although in most cases only one will succeed.