Vertebrates
The spiracle (/ˈspɪrəkəl/ or /ˈspaɪərəkəl/ is a small hole behind each eye that opens to the mouth in some fish. In the primitive jawless fish the first gill opening immediately behind the mouth is essentially similar to the other gill opening. With the evolution of the jaw in the early jawed vertebrates, this gill slit was "caught" between the forward gill-rod (now functioning as the jaw) and the next rod, the hyomandibular bone, supporting the jaw hinge and anchoring the jaw to the skull proper. The gill opening was closed off from below, the remaining opening was small and hole-like, and is termed a "spiracle". The spiracle is still found in all cartilaginous fish except chimaeras, and in bony fishes (coelacanth, sturgeon, paddlefish and bichirs). It is also seen as an otic notch in the skull of the extinct labyrinthodonts, and is thought to be associated with the ear opening in amniotes and frogs.
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