But wait —- I said that we do retain a gill slit as adults. The first embryonic gill cleft in forms the spiracle in many fish today (mainly cartilaginous fish such as rays, that use it to inhale water while they rest on the bottom of the sea — most bony fish lose the structure as adults). In us (and all other tetrapods) it forms the Eustachian tube and the middle ear cavity. So, if you puncture your eardrum, there you have a fully-functional gill slit leading from the outside into the back of your throat.