Respiration — lungs, gills, and no lungs at all
Reference: Respiratory system
Six different ways of getting oxygen into a body, including two vertebrate groups that threw their lungs away entirely and breathe through their skin.
The modes
- Lungs, two-way — mammals, reptiles, amphibians. Air goes in and comes back out of the same passages, so fresh and stale air mix.
- Lungs, one-way — birds. Air sacs drive a single-direction flow through rigid parabronchi, so the gas exchange surface always sees fresh air. It is a straightforwardly better design than the mammal lung, and it is why birds function at altitudes that would kill a mammal.
- Gills — fish, using counter-current flow, extracting oxygen from water where there is far less of it.
- Both — lungfish, and amphibians across their life cycle: gills as larvae, lungs as adults.
- Tracheae — insects have no lungs and no oxygen-carrying blood. Air tubes open along the body and pipe oxygen directly to the tissues. This works beautifully and caps insect body size, because diffusion down a tube has a hard range limit.
- Stomata — plants, through adjustable pores on the leaf underside.
The kinds with no lungs
Worth stating on its own, because it sounds wrong: there are air-breathing vertebrates with no lungs.
The lungless salamanders (Plethodontidae) are the largest salamander family in the world — hundreds of species — and not one of them has lungs. They exchange gas entirely through skin and the lining of the mouth. The Bornean flat-headed frog is in the same position. Both are limited to cool, wet, oxygen-rich conditions, because skin only works as a lung when it stays damp and the demand stays low.
So “breathes air” and “has lungs” are separate questions, and the catalogue keeps them separate.
Every creature on this axis
| Creature | Respiration |
|---|---|
| Horse | Lungs, two-way |
| Bat | Lungs, two-way |
| Whale | Lungs — in water |
| Eagle | Lungs, one-way |
| Shark | Gills |
| Octopus | Gills |
| Dragonfly | Tracheae (no lungs) |
| Frog and salamander | Gills, then lungs, skin — or no lungs |
| Snake | Lungs (often one only) |
| Oak | Stomata |
| Dragon Traditional | Lungs, plus unspecified fire mechanism |
| Pegasus Traditional | Lungs, two-way |
| Griffin Traditional | Lungs |
| Centaur Traditional | Lungs — and a second thorax |
| Sphinx Traditional | Lungs |
| Phoenix Traditional | Lungs |
| Mermaid Traditional | Unresolved — lungs above, water below |