- Gills are organs of water respiration. The gills of fish sit on the gill arches in the oropharyngeal environment and are closed from the outside by gill covers. In other animals, the gills have a different arrangement.
- The gill sacs and gill glands of fish later evolved into the tonsils, thymus, and Eustachian auditory tubes of other animals. Development in ontogeny of some other organs is also associated with embryonic gill pockets.
- In the Early Cambrian, gill arches gave rise to the prototype of jaws of chordates (first presented in the Metaspriggin skeleton).
- Relatively recently, organs that act as gills in the anus region began to develop in some turtles, although they have lungs for air breathing. An example of such an organism is the Fitzroy tortoise (Rheodytes leukops), which lives in the Fitzroy River basin in the Australian state of Queensland: in its cloaca there are two bags filled with water and assimilating oxygen from the water.
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