National Geographic this month has an article with some great pictures of nudibranchs, marine invertebrates with some amazing color patterns and characteristics. Their flashy coloration and sharp contrasts have evolved as a warning to potential predators to "stay away," something called aposematic coloration. This isn't false advertising, though: Nudibranchs munch on toxic corals, hydroids and sponges, and instead of being harmed, they take the defense mechanisms of their food and excrete them from their own skin. Some are highly poisonous, while others take structures called nematocysts (the same potentially-deadly stingers found on jellyfish and hydroids) and project them out of their own skin.
With a defensive arsenal like this, they can afford to be as flashy and conspicuous as they want. Ironically, however, they can't check themselves in the mirror--they have no eyes. The sensory organs on their heads (towards the front) act sort of like our noses, in the way that they can pick up chemicals on the water.
Like other gastropods (slugs and snails, etc.), they are hermaphroditic, carrying both eggs and sperm. When they mate with each other, they share both sets of sex cells, and each carries on with fertilized eggs. Their name comes from the Latin for "naked gill", referring to the respiratory organ sticking out of their back.
Check out the article, with accompanying slideshow and photos, at http://ngm.nationalgeographic.com/2008/06/nudibranchs/holland-text
(p.s. "Whence did these pictures come?" you may ask, "they're just gummi worms in a studio, or Photoshopped!" In fact, they were taken underwater, on the reefs where they live, using special photography apparatus (yes, that's how big they are, down in the corner there):