Amphibians were not the first terrestrial animals, but they were the first terrestrial vertebrates. Just as special physical adaptations were required in order for plants to make the transition from life in water to life on land, so were they required for animals. The density of air and water are very different, as is the concentration of oxygen in air when compared to that of water. Bodies surrounded by air instead of water benefit greatly from an internal supporting framework (endoskeleton) and an expanded, internalized air exchange mechanism, such as (lungs).
Frogs have a bony endoskeleton, a distinct head (cephalic) region, four limbs (they are tetrapods), a closed_circulatory_system with a three-chambered heart (two atria (sing: atrium), one ventricle), separate sexes (they are dioecious), kidneys, and ten pairs of cranial_nerves.
The dorsal external cephalic region of a frog includes eyes, nostrils, and tympanic_membranes. Male frogs have tympanic membranes that are larger than their eyes, but female tympanic membranes are about the same size as their eyes. Males have one thickened thumb pad on each hand to assist with the clasping of the female during reproduction.
If you open the frog's mouth you can see (and feel) the vomerine_teeth on the upper front surface of the mouth. You can also identify the tongue and its attachment point at the front of the lower portion of the mouth. Eustachian tubes are also visible, as is the glottis. Eustachian tubes connect the tympanic membranes to the throat and assist with regulation of air pressure within the auditory mechanism. The glottis leads to the esophagus, which is turn connects to the stomach.