 While most types of <unk> are distinguished on the basis of features in mature specimens , several are known from juvenile and <unk> specimens . <unk> 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 <unk> , but retain features of juveniles such as gills and small body size in what is known as <unk> . <unk> and the <unk> <unk> were also <unk> because they retained gills , but they are only known from adult specimens . 
