What Is An Amphibian? (1962)

[Applause] [Applause] amphibians are the most primitive land animals a group believed to be the ancestors of all vertebrates that live on land they are something like fish out of water their movement over the land is reminiscent of the movement of fish in water this is not surprising in view of the fact that the ancestors of amphibians probably lived only in the water during a time when ponds were drying up in order to survive such animals had to have the ability to move from a drying Pond to one that was not yet dry how did animals capable of making such a transition happened to originate about 350 million years ago During the devonian period when large areas of land were swampy there existed fish that had some amphibian like characteristics such fish still exist they had bony fins which we now know were capable of evolving into legs within each fin were three basic Bones the basic bones of such a fin are comparable with leg bones of a primitive amphibian other bones of the fin were irregular we now know they were evolving into toes as changes occurred certain lobe finned fish became modified became capable of existing on land they were the first amphibians fossil skeletons indicate some of the intermediate steps in the development of amphibia although many stages in the transition are unknown we do know that the skin came to be an accessory breathing organ furthermore improvements occurred in the blood supply system amphibians evolved A system that delivered blood from the lungs to the brain the control center of the body a system having a three-chambered heart to keep fresh fresh oxygenated blood separate from blood returning from the tissues Evolution taking place over millions of years produced the amphibians found on Earth today animals that are imperfectly adapted to both land and water their limbs sprawling on both sides do not succeed in lifting the animal's abdomen from the ground indeed amphibians have several characteristics in common with fish though most of them breathe air all are capable of existing in water for extended periods of time more important nearly all amphibians must lay their eggs in water the eggs much like those of a fish develop into tiny embryos then into larvas since there is little Yol in an egg for a larvae to absorb an egg must hatch while the larvae inside is still quite small it hatches the emerging larvae lives a fish-like existence there are gills for respiration and there is a well-developed tail for swimming as normal maturation proceeds gills disappear and limbs Bud out and become functional soon the amphibian is nearly mature ready to live in air but not all species of amphibians behave in this manner some like the aelole never leave the water such species fail to mature completely their gills never disappear their skins never thicken and their limbs never become welldeveloped for land Locomotion as they grow they merely become old larvas having gills and capable of reproducing experimentation reveals a clue as to why the gills of some amphibians do not disappear it appears that the thyroid gland plays an important role in maturation when an old larvae is injected with thyroid extract it begins to mature in a few days the gills are absorbed into the body the salamander has become an air breathing land animal amphibians are found in and near almost all the fresh waters on Earth in freshwater areas one may find several orders of amphibians in the water are salamanders some like the Congo eel are highly specialized for existence in water their legs never become suitable for movement on land others like the triturus are not so highly specialized when mature they are able to move onto land there they spend most of their lives in damp places such amphibians must remain in damp places for you recall their skins act as accessory breathing organs another order the toads and frogs are different from other amphibians they lack tails in the mature stage like nearly all amphibians their laral environment is aquatic their larvas commonly called tadpoles are found in freshwater ponds during the Spring months tadpoles are vegetarians though their bodies are shaped differently from those of other amphibians they have fish-like tails as they mature the larvas develop legs since some species Take 2 years to mature several Generations may exist together mature frogs are capable of quick hopping movement though their sprawling amphibian legs make plop Landings inevitable several members of this order have attained some degree of freedom from the water they have moved onto land into trees which provide a humid atmosphere they are the tree frogs this South American tree frog secretes a protective [Music] poison some tree frogs lay their eggs on the undersides of leaves some toads are adapted to live in dry desert environments in order to survive their larvas must at some stage in their development live in a wet environment a third order of amphibians the Sicilians have no legs like all amphibians they are of interest to biologists in their study of the biotic World in particular scientific experimentation upon amphibians has increased our knowledge of embryology and development embryologists work with developing eggs of amphibians they may remove add or transplant portions of an egg then they examine the developing amphibian over a period of Time how will the embryo develop will it mature how will it differ from the norm such experiments give us important information about the process of animal development as well as about the biology of amphibians animals that are something like fish out of water the lowest vertebrate animals that have adapted to life on land [Music] [Applause]