Trip Tucker's wrist nipples
A nipple is a structure from which a fluid emanates. In biological terms, it is the section of the body through which nourishment is transferred from parent to infant.
In 2151, Commander Tucker was accidentally impregnated by a female Xyrillian without his knowledge. The first indication of the pregnancy was the appearance of a growth on his wrist. This growth turned out to be a nipple. (ENT: "Unexpected")
Nipples were not generally shown in Star Trek: The Animated Series, as evidenced for example by such topless figures as Stavos Keniclius 5 or Lucien.