Wicked cool! Platynereis dumerilii has two different types of eyes: one type found in insects, and another found almost exclusively in vertebrates.

Insect eyes are known to consist of an array of compound lenses, whereas vertebrate eyes contain a single lens. But they are also made of different types of cells: insects' eyes are built up with cells called rhabdomeric photoreceptors; vertebrates use ciliary photoreceptors.

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So if this worm has both kinds of photoreceptor, does that mean that the two types of eyes, insect and vertebrate, both originated in an ancestor of this species? If the animal had two copies of the genes needed to make one kind of photoreceptor, speculates Wittbrodt, then the extra set would have been free to evolve into the other photoreceptor. Different animals would subsequently evolve to use the two options in different ways.

The issue of the evolution of so complex a structure as an eye was dealt with in a particularly charming manner in Richard Dawkins' Climbing Mount Improbable, chapter 5.