What adaptations to environmental conditions appeared in amphibians in connection with their access to land?

Amphibians lead aquatic and terrestrial lifestyles. In this regard, they have formed such structural features and physiology as:
1) mucus-coated skin in which atmospheric oxygen dissolves when breathing;
2) lever limbs for movement on land;
3) the skeleton is clearly differentiated into departments;
4) lungs for atmospheric breathing;
5) two circles of blood circulation and a three-chambered heart, providing a partial separation of blood into venous and arterial;
6) there is a middle ear, closed outside the eardrum.