orthonectid worm is a crab parasite in a ocean that defies explanation

it is shaped like a worm and is microscopic having a few hundred cells

however having dozen cells instead of just one, makes it multicellular and being multicellular it must be an animal since its capabilities of movement are far greater than in fungus

once inside a host they swim in host body fluids and eat/drink some of it

they inhabit the host as an un-adult member of species

when they mature they actually exit the host to find other worms to reproduce with

adult worms die after reproducing

the cycle begins again

a fertilized female worm will spread out multiple offspring which is smaller than the adults and they will seek out crabs and whatnot and have only limited time to find them until their resources run out

it is the smallest multicellular animal