 The superclass Tetrapoda is divided into four classes of vertebrate animals with four limbs . Reptiles , birds and mammals are amniotes , the eggs of which are either laid or carried by the female and are surrounded by several membranes , some of which are impervious . Lacking these membranes , amphibians require water bodies for reproduction , although some species have developed various strategies for protecting or bypassing the vulnerable aquatic larval stage . They are not found in the sea with the exception of one or two frogs that live in brackish water in mangrove swamps . On land , amphibians are restricted to moist habitats because of the need to keep their skin damp . 
