 While most types of <unk> are distinguished on the basis of features in mature specimens , several are known from juvenile and larval specimens . Metamorphosis is seen in <unk> , <unk> , and <unk> , with aquatic larvae developing into adults capable of living on land . Several types of <unk> do not fully metamorphose , but retain features of juveniles such as gills and small body size in what is known as neoteny . <unk> and the <unk> <unk> were also neotenic because they retained gills , but they are only known from adult specimens . 
