This is the most numerous of all vertebrate classes. Their swim bladder is an air sac used to control buoyancy, thus unlike the sharks, bony fish can hold still at any depth and not sink. They were the first land vertebrates. Frogs, especially, go through metamorphosis . Their eggs have no egg shells, so the sperm can swim through the water to the eggs, and the embryos must develop in water.
Reptiles have scales and are dry to the touch.
Their eggs have leathery shells. Some dinosaurs may have been endothermic. It is thought that birds are descended from dinosaurs, as evidenced, in part, by the scales on their feet. Birds bones are light weight for flight. Birds have shelled eggs and so must have internal fertilization the egg muct be fertilized before the hen s reproductive tract secretes an eggshell.