Mark H
"Within the basic groups of animals, nudibranchs, like cephalopods, are molluscs. Within the molluscs, they are part of a larger group called the gastropods, along with snails of land and sea, and various slug-like animals. [...] Their name means “naked gill,” where the gill is the tuft-like part on their back." - Mark H from Bookmarklet
"Why are they so beautifully colored? They are announcing their inedibility. Nudibranchs accumulate toxic substances, in some cases from what they eat and in some cases by making the chemicals themselves. Predators avoid them. The evolution of this arrangement raises some puzzles, and the literature I’ve looked at so far is cautious about how it all works. The main puzzle is how the bright coloration can initially become established. The first individuals in a species to be brightly colored will be very conspicuous. Whether they are toxic or not, they will be more easily seen by predators. A predator might become sick, or die, from eating the brightly colored mutant, but that does not do the colored individual any good." - Mark H