Choana (plural: Choanae) latinization from the Greek choanē meaning funnel is the posterior nasal aperture. It is the opening between the nasal cavity and the nasopharynx.

It is therefore not a structure but a space bounded anteriorly and inferiorly by the horizontal plate of palatine bone, superiorly and posteriorly by the sphenoid bone and laterally by the medial pterygoid plates. The choanae are separated by the vomer. In human and zoological anatomy (sometimes called zootomy), several terms are used to describe the location of organs and other structures in the body of bilateral animals. ... The anatomical planes The anatomical position is a schematic convention for describing the relative morphology of the human body. ... The palatine bone is a bone situated at the back part of the nasal cavity between the maxilla and the pterygoid process of the sphenoid. ... Figure 1 : Sphenoid bone, upper surface. ... Pterygoid can refer to: a plate near the Vomer bone a muscle such as Lateral pterygoid muscle or Medial pterygoid muscle This is a disambiguation page — a navigational aid which lists pages that might otherwise share the same title. ... The vomer bone is one of the unpaired facial bones of the skull. ...

The only animals with choana are the tetrapoda, and they could might as well be called Choanata (they are also the only ones with a vomeronasal organ, which has an embryonic origin from the olfactory structure). These internal nasal passages evolved while the vertebrates still lived in water. At this point they already needed to gulp air to get enough oxygen, and instead of open their jaws each time to do this, small openings to breath through would have been a better design. And that's what happened for some group. The vomeronasal organ (VNO) or Jacobsons organ (sometimes misspelled Jacobsens) is an auxiliary olfactory sense organ in some vertebrates, all of which are tetrapods. ...

Fish don't have choana, instead they have a pair of external nostrils, like two tubes where the frontal opening lays close the upper jaw and the posterior opening further behind near the eyes.

A 400 million years old fossil lobe-finned fish called Kenichthys campbelli has something between a choana and the external nostrils seen on other fish, which makes it look like it has a cleft palate or cleft lip. The reason seems to be that the frontal opening of the external nostrils has migrated into the mouth for some reason. This migration is still seen in the tetrapod embryo, and which sometimes can cause a baby to be born with a cleft palate. Why it should migrate is a mystery since the nostrils would be useless as a breathing device before they end up inside the mouth.

Tetrapods are also equipped with a lacrimal duct, or tear duct. How it evolved is not known, but it has an internal connection with the choana. If the choana had started like a natural crack between maxillan and the premaxillan bacause of an unclomplete fusion that turned out handy for air breathing animals (and this gap got wider and deeper with time), the frontal part of it would have to fuse together not to weaken the upper jaw, creating a small opening on the upper lip. Some more migrating, and they would meet the anterior pair of the external nasal openings. The posterior pair of the openings was then free to form the lacrimal duct if a migration caused them to come in contact with the eyes. This wouldn't been the first time the jaws evolved some sort of opening. For instance, the snakes have evolved a cleft in the lower jaw, allowing them to stick out their tounge without having to open their jaw. For an animal living in water, the formation of a paired cleft on the upper jaw would be most logic. Terrestrial vertebrates would either way find a way to breath without needing to open their jaws each time.

Some fossil species is said to have both conventional external nostrils and a choana. But only more fossils will give a real answear how the choanas evolved.

Besides tetrapods, the lungfishes has internal nostrils too. These seems to have a different origin than those of the tetrapods, and they have no tear duct either. Hagfishes have a single internal nostril that opens inside the mouth cavity, while Chimaeras have open canals that leads water from their external nostrils into their mouth and through their gills. Orders see text Lungfishes are sarcopterygian fish that can breathe air (and in some species are obligate air-breathers), and have limb-like appendages instead of fins. ... Genera Eptatretus Myxine Nemamyxine Neomyxine Notomyxine A hagfish is any of several marine chordates of the class Myxini, also known as Hyperotreti. ... Families Callorhynchidae Rhinochimaeridae Chimaeridae Other meanings, based on a fantastic animal, are at Chimera. ...