 In the Amphiumas , metamorphosis is incomplete , and they retain one pair of gill slits as adults , with fully functioning internal gills . Some species that lack lungs respire through gills . In most cases , these are external gills , visible as tufts on either side of the head . Some terrestrial salamanders have lungs used in respiration , although these are simple and sac @-@ like , unlike the more complex organs found in mammals . Many species , such as the olm , have both lungs and gills as adults . 
