What are the features of the internal structure of amphibians, by which one can judge the complication

What are the features of the internal structure of amphibians, by which one can judge the complication of their organism during evolution.

Amphibians are the heirs of the ancient cross-finned fish, respectively, the comparison will be carried out from the initial species. The organism of amphibians really got complications, namely, – instead of a 2-chamber heart, it became a 3-chamber one, which, accordingly, entailed the appearance of another circle of blood circulation. Unlike fish, where fins are present, amphibians have paired limbs of the articular type. The respiratory organs also succumbed to changes, lungs appeared in amphibians, and gills remained in fish.