External gills

External gills are respiratory structures that project outside an animal’s body and exchange oxygen and carbon dioxide directly with surrounding water.

Cutaneous respiration: Gas exchange across an animal’s skin or other body surface. External gills extend respiratory exchange across thin surfaces exposed directly to water.
Axolotl: A neotenic salamander native to the lake system of central Mexico that retains larval traits as an adult. Its conspicuous feathery external gills persist throughout its aquatic adult life.
Internal gill: A respiratory structure housed within a body cavity or chamber through which water is circulated. Internal gills gain protection and controlled water flow, unlike exposed external gills.
Amphibian metamorphosis: The developmental transformation by which an amphibian larva changes into a juvenile or adult form. Metamorphosis often removes external gills as the animal shifts to another respiratory arrangement.
Countercurrent exchange: A transfer process in which two fluids flow in opposite directions, sustaining a concentration gradient along their interface. Opposing water and blood flows can maintain oxygen uptake across gill surfaces.
Salamander larva: The aquatic juvenile stage of a salamander, often equipped with gills before metamorphosis. Many salamander larvae breathe with external gills that are lost or reduced at metamorphosis.
Fish gill: A vascular respiratory organ in most fishes, usually protected beneath the operculum or within gill slits. Most fish gills exchange gases internally rather than projecting freely into the water.
Neoteny: The retention of juvenile traits in a sexually mature organism. Neoteny explains why some adult salamanders retain larval external gills.
Diffusion: The net movement of particles from higher concentration to lower concentration. Oxygen and carbon dioxide cross external-gill surfaces down concentration gradients.
Mudpuppy: A fully aquatic North American salamander that retains larval features throughout life. Its branched external gills remain functional in adulthood.

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