Parasitic crustaceans: https://bogleech.com/bio-paracrust
Parasitic jellyfish: https://blogs.scientificamerican.com/artful-amoeba/when-jell...
Another parasitic jellyfish (its larva develops inside-out, then turns itself outside-in when it bursts out of its host!): https://en.m.wikipedia.org/wiki/Polypodium_hydriforme