Spiracle (vertebrates)
Spiracles are openings on the surface of some animals, which usually lead to respiratory systems.
The spiracle 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 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 requiem sharks, hammerhead sharks, and chimaeras, and is found in some primitive bony fishes. 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.
Blowholes in cetaceans are also sometimes referred to as spiracles.
