Fishes constitute three of the seven classes of vertebrates (the others being amphibian, reptiles, birds and mammals). Fishes are different from the other vertebrates because they have gills for inhalation and exhalation, while the other classes uses lungs. Amphibians also start with gills, but later develop lungs and actually breathe through the skin (because their lungs are small) and that is why their skin has to be kept moist. Fishes practice external fertilization, since the eggs mate with sperm cells in the water. Amphibians also practice the same; all the other vertebrates have internal fertilization of eggs. External fertilization leaves the offspring more prone to predation, as compared to internal fertilization. Fishes are among the oldest existing vertebrate classes and have more species than any of the other vertebrate classes.
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