There are two subclasses of bony fish. One, ray-finned fish, is the largest group of living vertebrates with over 20,000 species. The other, lobe-finned fish, has few living species but has great significance as the group ancestral to land vertebrates. These two subclasses are distinguished by different skeletal structures in their paired fins. During the Devonian, lobe-finned fish were common in fresh water. They had nostrils which opened into their mouth cavities and they had lungs. This allowed them to supplement gill respiration in water with respiration in the air, an important adaptation for animals living in warm water, which is low in oxygen. Their muscular fins may have supported them when lifting their bodies to breathe air. Several explanations have been offered as to why some lobe-finned fish became increasingly adapted to life on land. One is the diversity of terrestrial arthropods which offered an abundant food supply. Another is periodic drying of their fresh-water habitats. The first amphibians appeared in the late Devonian. They had legs in place of fins, but many features of their anatomy suggest they spent much of their time in water. These first terrestrial vertebrates share a number of features with the lobe-finned fish thought to be their ancestors, including the arrangement of bones in their paired appendages and a unique folding of their tooth enamel. This second feature lends its name to these early amphibians-Labyrinthodont or labyrinth tooth.