Gill
Gill of parasitic being
For the cultural observer, please see John Gill.
A gill was an opening in the side of an aquatic lifeform's head that allowed it to breathe. Gill slits were a feature of amphibians. (TNG: "Genesis")
Sur-snakes had a dual respiratory system consisting of both lungs and gills. The Aquans also had gills. (TAS: "The Ambergris Element")
The parasitic beings which invaded Starfleet Command in 2364 breathed through a blue gill at the back of the host's neck. To fool the other parasites, Beverly Crusher simulated such a gill on William T. Riker's neck. The ruse was so well-executed that even Jean-Luc Picard believed it. (TNG: "Conspiracy")