Gill
With the exception of some aquatic insects, the filaments and lamellae (folds) contain blood or coelomic fluid, from which gases are exchanged through the thin walls.
Gills or gill-like organs, located in different parts of the body, are found in various groups of aquatic animals, including mollusks, crustaceans, insects, fish, and amphibians.
Semiterrestrial marine animals such as crabs and mudskippers have gill chambers in which they store water, enabling them to use the dissolved oxygen when they are on land.
Galen observed that fish had multitudes of openings (foramina), big enough to admit gases, but too fine to give passage to water.
[2] Many microscopic aquatic animals, and some larger but inactive ones, can absorb sufficient oxygen through the entire surface of their bodies, and so can respire adequately without gills.
[3] A high surface area is crucial to the gas exchange of aquatic organisms, as water contains only a small fraction of the dissolved oxygen than air does, and it diffuses more slowly.
[4] Rather than using lungs, "[g]aseous exchange takes place across the surface of highly vascularised gills over which a one-way current of water is kept flowing by a specialised pumping mechanism.
Most species employ a countercurrent exchange system to enhance the diffusion of substances in and out of the gill, with blood and water flowing in opposite directions to each other.
[10] Most sharks rely on ram ventilation, forcing water into the mouth and over the gills by rapidly swimming forward.
In slow-moving or bottom-dwelling species, especially among skates and rays, the spiracle may be enlarged, and the fish breathes by sucking water through this opening, instead of through the mouth.
The gills' large surface area tends to create a problem for fish that seek to regulate the osmolarity of their internal fluids.
Therefore, freshwater fishes must utilize their gill ionocytes to attain ions from their environment to maintain optimal blood osmolarity.
[9] Lungfish larvae also have external gills, as does the primitive ray-finned fish Polypterus, though the latter has a structure different from amphibians.
A current of water is maintained through the gills for gas exchange, and food particles are filtered out at the same time.
[16] Respiration in the echinoderms (such as starfish and sea urchins) is carried out using a very primitive version of gills called papulae.
The gills of aquatic insects are tracheal, but the air tubes are sealed, commonly connected to thin external plates or tufted structures that allow diffusion.
A plastron is a type of structural adaptation occurring among some aquatic arthropods (primarily insects), a form of inorganic gill which holds a thin film of atmospheric oxygen in an area with small openings called spiracles that connect to the tracheal system.
The plastron typically consists of dense patches of hydrophobic setae on the body, which prevent water entry into the spiracles, but may also involve scales or microscopic ridges projecting from the cuticle.
[18][19] A somewhat similar mechanism is used by the diving bell spider, which maintains an underwater bubble that exchanges gas like a plastron.
Other diving insects (such as backswimmers, and hydrophilid beetles) may carry trapped air bubbles, but deplete the oxygen more quickly, and thus need constant replenishment.
