Order Anura　　　　　　 　　　　　　　Order Caudata The ancestors of amphibians can be traced back to 350,000,000 years ago-the Crossopterygian。 They're the pioneers of all vertebrates. These ancient amphibians have evolved into reptiles like dinosaurs, snakes lizards…etc. Amphibians are noted for they have both the properties of 水棲 and 陸棲. They usually lay their eggs in or near the water, and their juveniles grow up in the water and breath with their cheeks. After sometime, they become frogs and land on the land, thus in their life, there are two periods coexisting. They lay tens to thousands of eggs. The eggs usually stick together and so was called 卵塊。Eggs of frogs has no shell, therefore, preserving water will be the major challenge. This task is complete by two ways; one, lay the eggs directly into, or near the water; two, to cover all the eggs with some transparent liquid bag. In this bag, eggs or even embryos can stay in this special swimming pool and needless to mention the protection it can also provide. After embryos hatch into tadpoles, some sort of enzyme will be produced and will brake the bag to let out the tadpoles. The remaining organization will become the first treat for the newly hatched tadpoles. There're also several factors that are affecting the length of tadpole period. Low temperature could be one reason to prolong it. Also, when the foods are not sufficient, tadpole period also tends to prolong. Drought, on the other hand, urges tadpoles to metamorphosis into frogs. Generally, tadpole period lasts for thirty or forty days. The process in the between of tadpoles and frogs is named metamorphosis. Tadpoles grow out their back limps first, and until the final end of tadpole period we can see the fore limps hiding in the transparent bags in the chest and stretch out until the last minute before metamorphosis. These stretched-out fore limps will then block the vent of cheeks and force the frogs to breath with lungs and skin. Meanwhile their body structure and digesting system will be enormously different than it was in the tadpole period to fit in the life on the dry land. Before all these are done, frogs are not ready to eat or to digest. Then, what can they do to preserve their life? The tail can tell the answer! This shrinking tail of new frogs can provide the nutrition in this critical period. These astonishing phenomena are the results of 350,000,000 years evolution. And they're also helpful for the amphibians to fit in all kinds of conditions on earth except the poles.