Polyclad flatworms and dorid nudibranchs are unrelated animals. One is a flat, unsegmented worm with a branching gut; the other is a shell-less gastropod mollusc. On a reef they can be difficult to separate at a glance, and the giveaways are subtle: nudibranchs carry rhinophores at the front and a gill tuft at the rear, while flatworms have neither and swim by rippling their whole margin.
Both groups carry chemical defences. Many polyclads sequester or produce distasteful compounds, and some contain tetrodotoxin; many nudibranchs retain defences from their sponge or hydroid prey. Where several defended species converge on one warning pattern, each benefits from a predator's single learning experience, the arrangement usually called Mullerian mimicry.
The difficulty is establishing direction. Demonstrating mimicry requires showing that a predator's response to one species transfers to the other, and such experiments are rare for reef flatworms. Much of the literature rests on photographic resemblance and shared habitat, so which resemblances are true mimicry and which are independent convergence on a generally conspicuous signal is unsettled.