Tadpoles are the primary stage of amphibian individual development. Early tadpoles were round or oval, looked like fish and had lateral organs.
Because there are no pores in the mouth, you can't eat; After that, eyes and nostrils appeared one after another; There is a sucker under the head, which can be used to adsorb on aquatic plants. There are external gills on both sides of the head, which have respiratory function. The tail is large and flat, with segmented caudal muscles inside, and thin-film upper and lower caudal fins above and below the sarcomere, which can help tadpoles swim in the water.
After the mouth appeared, I scraped off the seaweed with the horny teeth of my lips and began to live independently in the water. When the sucker disappears, the outer gill will also shrink; With the healing of pharyngeal skin folds and body wall, gill cover is formed, and 1 outlet hole is left on the left side of body surface or in the middle or back of ventral surface. Internal gill breathing in gill cavity can also breathe free oxygen on the water surface with the occurrence of lung. After a certain period of development, some of them grow hind limbs first, differentiate into five toes at the end, and then grow forelimbs from the gill cover.
Like frogs. Some grow forelimbs first and then hind limbs, such as salamanders. With the tail gradually shrinking, the mouth also changed obviously, and gradually developed into a young man who can live on land. Eels and salamanders in the order Ceratopoda have gills all their lives and live in water. The process of tadpoles developing into adults through metamorphosis is called metamorphosis.
Extended data
Frogs are amphibians, belonging to Chordata, Amphibia, Anura and Ranidae. Its adult has no tail. Its eggs are laid in water, fertilized in vitro, hatched into tadpoles, and breathed through gills. After the mutation, adults mainly breathe through the lungs and skin.
Most frogs reproduce by in vitro fertilization, and fertilized eggs hatch outside the mother's body into tadpoles. Only 10 to 12 frog species in the world have evolved into in vivo fertilization, and some of them will expel fertilized eggs and hatch into tadpoles. But in zoology, the frog refers to the black-spotted lateral fold frog. Frogs have always been regarded as egg-laying animals, but scientists have found that a frog living in the rainforest of Sulawesi Island in Indonesia can give birth to tadpoles. This frog is the only frog that can "drop tadpoles" among more than 6,000 kinds of frogs in the world.
Frogs are slim and good at swimming. The neck is not obvious, and there are no ribs. The ulna and radius of the forelimb healed, the tibia and fibula of the hind limb healed, the claws could not rotate flexibly, but the limbs were muscular. The most primitive frogs began to evolve in the early Triassic. The earliest frog with jumping action appeared in Jurassic. Because frogs feed on insects and other invertebrates, they must live by the water. There are about 130 species of frogs in China, and almost all of them are experts in exterminating forest and farmland pests.
References:
Frog _ Baidu encyclopedia