From: Star Trek: The Original Series
Caption: Kelvans aboard the Enterprise
Planet: Kelva. Humanoid in appearance, the Kelvans' true appearance remains a mystery and remains unexplained. As refugees of a galaxy poisoned by heavy radiation, the Kelvans have assumed humanoid form, and are finding themselves beginning to show characteristics of the bodies they have assumed. The impression is that a Kelvan in true form, would never conduct themself in the manner that most humanoids do.