What evolutionary features have allowed amphibians to land on land? Why does their livelihood directly depend on water? What caused the need for a second circle of blood circulation and a three-chambered heart?
1) breathing with atmospheric air (respiratory organ – lungs), three-chambered heart, two circles of blood circulation, development of limbs (the appearance of joints and belts of the upper and lower extremities) and the skeleton as a whole (the appearance of the cervical spine, which provides head mobility);
2) fertilization (external) and the development of the larval stage of all amphibians occurs in water, they also have skin respiration, the skin has many glands and is covered with mucus, its drying out leads to the death of the animal;
3) the second circle of blood circulation (pulmonary) is associated with the reduction of the branchial apparatus and the transition to pulmonary gas exchange; the presence of two circles of blood circulation is impossible with a two-chambered (like in fish) heart, therefore all terrestrial vertebrates (including amphibians) have two atria in the heart – one receives venous blood from organs (veins of the systemic circulation), and the other – arterial through the veins of the small circle.
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