The frog is covered by thin, a soft, moist skin composed of 2 layers, an inner dermis & an outer epidermis. The skin does not only protect the frog but also helps in respiration
A broad network of blood vessels runs throughout the frog's skin. Oxygen pass through the membranous skin, thereby enters directly into the blood. When a frog immerse beneath the water, all frog’s respiration takes place through the skin. Oxygen can be obtained directly from the water.
The frog does not breathe through its skin alone. Adult frogs have saclike lungs, paired simple, as in man, air enters the body through 2 nostrils, passes through the windpipe, & is received by the lungs. The mechanism of breathing is different in the frog from that in man. In human beings breathing is aided by the ribs, the diaphragm, & the chest muscles. The frog has no ribs or diaphragm, & its chest muscles are not involved in breathing.
A frog may breathe by simply opening its mouth & letting air flow into the windpipe. However, it may also breathe with its mouth closed. The floor of the mouth is lowered, causing the frog's throat to "puff out." When the nostrils open, air enters the enlarged mouth. Then, with nostrils closed, the air in the mouth is forced into the lungs by contraction of the floor of the mouth.
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