Flatworms and sea slugs
Flatworms (left) often mimic sea slugs (right), but the species are quite different. Flatworms lack body cavities, respiratory organs and circulatory systems — they're pretty much just flat animals with holes for food to go in and out. "Sea slugs" is a term that describes many differents kind of marine creatures including shell-less saltwater snails and nudibranchs. Nudibranchs are conspicuously colorful mollusks that can secrete toxins as a defense mechanism, so it's easy to see why flatworms might evolve to look like them.