Amphibians: The limitations
In general amphibians are only partly successful in their colonization of land, since their limbs are short and they need to flex their body laterally in order to take reasonable strides. Amphibian skin is permeable and in a dry atmosphere would quickly dehydrate, they even do not have the mechanisms to drink water. A moist skin is also required to supplement respiration, since the lungs are comparatively simple and not totally adequate for its needs. These limitations restrict amphibians to moist environments. For reproduction amphibians are also almost entirely dependent on water since, like fish, their eggs have no water-proof covering and their larvae (tadpoles) are quite fish-like. These larvae initially have no legs swim using a long tail and breathes using external feathery gills.