In newts and salamanders, metamorphosis is less dramatic than in frogs. This is because the larvae are already carnivorous and continue to feed as predators when they are adults so few changes are needed to their digestive systems. Their lungs are functional early, but the larvae do not make as much use of them as do tadpoles. Their gills are never covered by gill sacs and are reabsorbed just before the animals leave the water. Other changes include the reduction in size or loss of tail fins, the closure of gill slits, thickening of the skin, the development of eyelids, and certain changes in dentition and tongue structure. Salamanders are at their most vulnerable at metamorphosis as swimming speeds are reduced and transforming tails are encumbrances on land. Adult salamanders often have an aquatic phase in spring and summer, and a land phase in winter. For adaptation to a water phase, prolactin is the required hormone, and for adaptation to the land phase, thyroxine. External gills do not return in subsequent aquatic phases because these are completely absorbed upon leaving the water for the first time.
in nEwts anD sALamaNDers metAMorphOsIs is LESs dramatic thAN iN froGs this is BEcAUsE tHe larVae Are aLReady cArNivoROUs And contInUe To feed as preDatorS whEn tHey are adults so Few chAngeS Are neeDed tO their digesTiVE systEms TheIr lunGS aRE fUNCtIOnal eaRlY bUt the LaRVae do NOT mAkE as mUch use Of tHem aS dO taDpoles thEiR gillS are nEVer cOvERed by giLl sacs and aRE reaBsorBEd jUsT beForE the animals leAVE tHe WATeR otHER changEs include The redUctiOn in size Or loss Of taIl fiNs the cLOsURe OF gilL SlitS THIckEninG oF the skin THe DeveLopMEnt of EYelids aNd cErtAin ChaNgeS iN DenTItion ANd tongUe sTRuctUrE salamAnders arE AT their mOSt VULNErAble aT metAmOrphosis as SwimmINg spEeds aRe rEdUced and trANSFormINg tails are enCumbrANcES on lANd adult saLAmanders often hAvE An aqUaTiC phAsE in sPriNg And sUMMer AND A lAnD PhasE iN winter for adaPtAtion to a watEr phase prolactIn IS thE RequireD hoRmone AnD fOr AdaptatION tO the LAnd PHASe tHyrOxine EXternAL gIlls do noT ReTuRN in sUBsequEnt aQuATIC phaSes beCaUse thEse are complEtEly AbsORbed UPoN leavinG THE waTer For The first tiMe