We usually don't think of babies that grow inside their mothers as parasites, but sometimes the lines get very blurry. This is especially true in Narcomedusae, a group of poorly known jellyfish found throughout the world's oceans. Some species of Narcomedusae (affectionately called narcos by the people that study them) can grow inside their own mother, who provides nourishment and a safe environment for her.

 

The narco babies can then leave their mother, find another jellyfish of an entirely different species, attach to its flesh, and thrive on the nourishment and safe environment it provides. The physiological interaction of baby and host is similar in both cases - the host provides, the baby takes. But in one case the host is providing for its own offspring, in the other it is providing for somebody else's offspring.