Different organisms occupy different environment, which requires them to adapt to those conditions to ensure their survival. In this sense, there exist vast physical forms between fish and mammals due to their opposing habitats (water and air respectively). However, the main distinguishing characteristic is the structure of their gaseous exchange system.
Fish use water as their respiratory medium. Hence they require an efficient respiratory system. This is because water has less oxygen than air. Fish require a complex respiratory system that will extract oxygen from water. The respiratory system of fish consists of evaginations of epithelium which are known as ...
