When their hostLineus ruberis hyperinfested, numerous male and femaleIntoshia lineihaving reached sexual maturity leave the host spontaneously to lead a period of free life in seawater during which sexual reproduction occurs. The latter involves a mating process which leads to internal fertilization. The fertilized ova give birth to embryos then, 24 h later, to swimming ciliated larvae which, once out of the female parasite, constitute infecting agents.