question: How come tadpoles can breathe underwater but frogs can’t?
comment: Tadpoles have gills, so they breathe underwater like a fish. As they mature into frogs, they develop lungs.
passages: Some amphibians have gills, either in the early stages of their development (e.g. tadpoles of frogs), while others retain them into adulthood (e.g. some salamanders).
  
  Section::::Fish.
   The lungs in amphibians are primitive compared to those of amniotes, possessing few internal septa and large alveoli, and consequently having a comparatively slow diffusion rate for oxygen entering the blood. Ventilation is accomplished by buccal pumping. Most amphibians, however, are able to exchange gases with the water or air via their skin. To enable sufficient cutaneous respiration, the surface of their highly vascularised skin must remain moist to allow the oxygen to diffuse at a sufficiently high rate. Because oxygen concentration in the water increases at both low temperatures and high flow rates, aquatic amphibians in these situations can rely primarily on cutaneous respiration, as in the Titicaca water frog and the hellbender salamander. In air, where oxygen is more concentrated, some small species can rely solely on cutaneous gas exchange, most famously the plethodontid salamanders, which have neither lungs nor gills. Many aquatic salamanders and all tadpoles have gills in their larval stage, with some (such as the axolotl) retaining gills as aquatic adults.
   Amphibians have three main organs involved in gas exchange: the lungs, the skin, and the gills, which can be used singly or in a variety of different combinations. The relative importance of these structures differs according to the age, the environment and species of the amphibian. The skin of amphibians and their larvae is highly vascularised, leading to relatively efficient gas exchange when the skin is moist. The larvae of amphibians, such as the pre-metamorphosis tadpole stage of frogs, also have external gills. The gills are absorbed into the body during metamorphosis, after which the lungs will then take over. The lungs are usually simpler than in the other land vertebrates, with few internal septa and larger alveoli; however, toads, which spend more time on land, have a larger alveolar surface with more developed lungs. To increase the rate of gas exchange by diffusion, amphibians maintain the concentration gradient across the respiratory surface using a process called buccal pumping. The lower floor of the mouth is moved in a "pumping" manner, which can be observed by the naked eye.
   Frogs have a highly developed nervous system that consists of a brain, spinal cord and nerves. Many parts of frog brains correspond with those of humans. It consists of two olfactory lobes, two cerebral hemispheres, a pineal body, two optic lobes, a cerebellum and a medulla oblongata. Muscular coordination and posture are controlled by the cerebellum, and the medulla oblongata regulates respiration, digestion and other automatic functions. The relative size of the cerebrum in frogs is much smaller than it is in humans. Frogs have ten pairs of cranial nerves which pass information from the outside directly to the brain, and ten pairs of spinal nerves which pass information from the extremities to the brain through the spinal cord. By contrast, all amniotes (mammals, birds and reptiles) have twelve pairs of cranial nerves.
   Section::::Life history.:Life cycle.:Tadpoles.
  
  The larvae that emerge from the eggs, known as tadpoles (or occasionally polliwogs), typically have oval bodies and long, vertically flattened tails. As a general rule, free-living larvae are fully aquatic, but at least one species ("Nannophrys ceylonensis") has semiterrestrial tadpoles which live among wet rocks. Tadpoles lack eyelids and have cartilaginous skeletons, lateral line systems, gills for respiration (external gills at first, internal gills later), and vertically flattened tails they use for swimming.
   Many of the true frogs are aquatic or live close to water. Most species lay their eggs in the water and go through a tadpole stage. However, as with most families of frogs, there is large variation of habitat within the family. Those of the genus "Tomopterna" are burrowing frogs native to Africa and exhibit most of the characteristics common to burrowing frogs around the world. There are also arboreal species of true frogs, and the family includes some of the very few amphibians that can live in brackish water.
  
  Section::::Systematics.
   Section::::Reproduction.
  
  For the purpose of reproduction most amphibians require fresh water although some lay their eggs on land and have developed various means of keeping them moist. A few (e.g. "Fejervarya raja") can inhabit brackish water, but there are no true marine amphibians. There are reports, however, of particular amphibian populations unexpectedly invading marine waters. Such was the case with the Black Sea invasion of the natural hybrid "Pelophylax esculentus" reported in 2010.
   Section::::Life history.
  
  Like other amphibians, the life cycle of a frog normally starts in water with an egg that hatches into a limbless larva with gills, commonly known as a tadpole. After further growth, during which it develops limbs and lungs, the tadpole undergoes metamorphosis in which its appearance and internal organs are rearranged. After this it is able to leave the water as a miniature, air-breathing frog.
  
  Section::::Life history.:Reproduction.
   The skin of a frog is permeable to oxygen and carbon dioxide, as well as to water. There are blood vessels near the surface of the skin and when a frog is underwater, oxygen diffuses directly into the blood. When not submerged, a frog breathes by a process known as buccal pumping. Its lungs are similar to those of humans, but the chest muscles are not involved in respiration, and no ribs or diaphragm exist to help move air in and out. Instead, it puffs out its throat and draws air in through the nostrils, which in many species can then be closed by valves. When the floor of the mouth is compressed, air is forced into the lungs. The fully aquatic Bornean flat-headed frog ("Barbourula kalimantanensis") is the first frog known to lack lungs entirely.
   Amphibians have a juvenile stage and an adult stage, and the circulatory systems of the two are distinct. In the juvenile (or tadpole) stage, the circulation is similar to that of a fish; the two-chambered heart pumps the blood through the gills where it is oxygenated, and is spread around the body and back to the heart in a single loop. In the adult stage, amphibians (especially frogs) lose their gills and develop lungs. They have a heart that consists of a single ventricle and two atria. When the ventricle starts contracting, deoxygenated blood is pumped through the pulmonary artery to the lungs. Continued contraction then pumps oxygenated blood around the rest of the body. Mixing of the two bloodstreams is minimized by the anatomy of the chambers.
   Most amphibians have an aquatic larval stage and thus are at least semiaquatic by virtue of this fact. Many adult amphibians are also semiquatic (while others are fully aquatic or terrestrial). However, some amphibians lack an aquatic larval stage. Some frogs, such as most leiopelmatids, most ranixalids, some leptodactylids, some myobatrachids, Darwin's frog and the Seychelles frog, have nonaquatic tadpoles. Some caecilians, many frogs such as saddleback toads, most sooglossids and the greenhouse frog, and most plethodontid salamanders lay eggs on land in which the larvae develop into adult form before they hatch. The alpine salamander and African live-bearing toads ("Nectophrynoides" and "Nimbaphrynoides") are ovoviviparous and give birth on land. Additionally, about 75% of caecilians are viviparous.
   The lungs of most frogs and other amphibians are simple and balloon-like, with gas exchange limited to the outer surface of the lung. This is not very efficient, but amphibians have low metabolic demands and can also quickly dispose of carbon dioxide by diffusion across their skin in water, and supplement their oxygen supply by the same method. Amphibians employ a positive pressure system to get air to their lungs, forcing air down into the lungs by buccal pumping. This is distinct from most higher vertebrates, who use a breathing system driven by negative pressure where the lungs are inflated by expanding the rib cage. In buccal pumping, the floor of the mouth is lowered, filling the mouth cavity with air. The throat muscles then presses the throat against the underside of the skull, forcing the air into the lungs.
   Tadpoles of this genus have adapted to a variety of conditions. Most species (e.g. Blyth's river frog "L. blythii" or the fanged river frog "L. macrodon") develop normally, with free-swimming tadpoles that eat food. The tadpoles of the corrugated frog ("L. laticeps") are free-swimming but endotrophic, meaning they do not eat but live on stored yolk until metamorphosis into frogs. Before, "L. limborgi" was assumed to have direct development (eggs hatching as tiny, full-formed frogs), but more careful observations have showed it has free-swimming but endotrophic larvae; this probably applies to the closely related "L. hascheanus", too. "L. larvaepartus" is the only known species of frog that gives live birth to tadpoles. Parental care is performed by males.
   Section::::Lifecycle.
  
  Hochstetter's frogs can live to 30 years old. Adults do not breed until they are three years old, laying up to 20 eggs each season. While all four species develop as tadpoles inside the egg, hatching as froglets with developed back legs, Hochstetter's frogs, as the only semiaquatic species, continue to develop in water while the three other species are cared for by their parents.
  
  Section::::Distribution.
   In typical amphibian development, eggs are laid in water and larvae are adapted to an aquatic lifestyle. Frogs, toads, and newts all hatch from the eggs as larvae with external gills but it will take some time for the amphibians to interact outside with pulmonary respiration. Afterwards, newt larvae start a predatory lifestyle, while tadpoles mostly scrape food off surfaces with their horny tooth ridges.
   Like many amphibian species' tadpoles, those of the pinewoods tree frog are susceptible to parasitic infection by trematode flatworms. However, tadpoles swim in a manner that does not necessarily reduce the intensity of infection, but reduces the likelihood of these parasites infecting in the head and body region; instead, their swimming behavior shunts parasites to the tail area, where infection by parasites causes fewer harmful effects.
  
  Section::::Description.:Adult.
   Most amphibians go through metamorphosis, a process of significant morphological change after birth. In typical amphibian development, eggs are laid in water and larvae are adapted to an aquatic lifestyle. Frogs, toads and salamanders all hatch from the egg as larvae with external gills. Metamorphosis in amphibians is regulated by thyroxine concentration in the blood, which stimulates metamorphosis, and prolactin, which counteracts thyroxine's effect. Specific events are dependent on threshold values for different tissues. Because most embryonic development is outside the parental body, it is subject to many adaptations due to specific environmental circumstances. For this reason tadpoles can have horny ridges instead of teeth, whisker-like skin extensions or fins. They also make use of a sensory lateral line organ similar to that of fish. After metamorphosis, these organs become redundant and will be reabsorbed by controlled cell death, called apoptosis. The variety of adaptations to specific environmental circumstances among amphibians is wide, with many discoveries still being made.
   Section::::Chordata.:Amphibians.:Frogs and toads.
  
  With frogs and toads, the external gills of the newly hatched tadpole are covered with a gill sac after a few days, and lungs are quickly formed. Front legs are formed under the gill sac, and hindlegs are visible a few days later. Following that there is usually a longer stage during which the tadpole lives off a vegetarian diet. Tadpoles use a relatively long, spiral‐shaped gut to digest that diet.
   Section::::Sound communication.:Mode of sound communication.
  
  Calls are often sent through the air, but other mediums have been discovered. Some species call while they are underwater and the sound travels through the water. This is adaptive in a region with many species competing for air time. Narins has found female frog species that use solid surfaces, such as blades of grass and logs, upon which they tap rhythmically to attract mates. Also, Feng has found that some species of frogs use ultrasound.
  
  Section::::Sound production.
   The behavior of an amphibian hatchling, commonly referred to as a tadpole, is controlled by a few thousand neurons. 99% of a "Xenopus" hatchling's first day after hatching is spent hanging from a thread of mucus secreted from near its mouth will eventually form; if it becomes detached from this thread, it will swim back and become reattached, usually within ten seconds. While newt hatchlings are only able to swim for a few seconds, "Xenopus" tadpoles may be able to swim for minutes as long as they do not bump into anything. The tadpole live from remaining yolk-mass in the gut for a period, before it swims off to find food.
   See also reptiles for more detailed descriptions of the respiratory system in these animals.
  
  Section::::Respiratory systems.:Amphibians.
  
  Both the lungs and the skin serve as respiratory organs in amphibians. The skin of these animals is highly vascularized and moist, with moisture maintained via secretion of mucus from specialized cells. While the lungs are of primary importance to breathing control, the skin's unique properties aid rapid gas exchange when amphibians are submerged in oxygen-rich water.
  
  Section::::Respiratory systems.:Aquatic birds.
   Section::::Range and habitat.
   Many environmental scientists believe amphibians, including frogs, are good biological indicators of broader ecosystem health because of their intermediate positions in food chains, their permeable skins, and typically biphasic lives (aquatic larvae and terrestrial adults). It appears that species with both aquatic eggs and larvae are most affected by the decline, while those with direct development are the most resistant.
   Amphibians such as frogs need both terrestrial and aquatic habitats in which to reproduce and feed. While tadpoles control algal populations, adult frogs forage on insects. Frogs are used as an indicator of ecosystem health due to their thin skin which absorbs both nutrient and toxins from the surrounding environment resulting in an above average extinction rate in unfavorable and polluted environmental conditions.
   Frog larvae are known as tadpoles and typically have oval bodies and long, vertically flattened tails with fins. The free-living larvae are normally fully aquatic, but the tadpoles of some species (such as "Nannophrys ceylonensis") are semi-terrestrial and live among wet rocks. Tadpoles have cartilaginous skeletons, gills for respiration (external gills at first, internal gills later), lateral line systems and large tails that they use for swimming. Newly hatched tadpoles soon develop gill pouches that cover the gills. The lungs develop early and are used as accessory breathing organs, the tadpoles rising to the water surface to gulp air. Some species complete their development inside the egg and hatch directly into small frogs. These larvae do not have gills but instead have specialised areas of skin through which respiration takes place. While tadpoles do not have true teeth, in most species, the jaws have long, parallel rows of small keratinized structures called keradonts surrounded by a horny beak. Front legs are formed under the gill sac and hind legs become visible a few days later.
  