Closer examination of the animals' anatomy revealed that the sea slugs had a large part of their penis coiled up in a spiral inside their bodies, which they would then use to replenish their missing part.

BBC: Sea slug's 'disposable penis' surprises

Scientists have said that designing a bionic eye has been much more difficult than developing aids like cochlear implants for hearing, in part because visual information is two-dimensional, and because of the anatomy of the eye.

WSJ: FDA Approves Bionic Eye

And though the basic anatomy of the brain is known, it is not always clear what role each little part plays.

ECONOMIST: Out-of-body experiences