Q. Why can the amphibian circulation be classified as incomplete and double?
The amphibian circulation is double because it is composed of pulmonary and systemic circulations respectively, heart-tissues-heart and heart-lungs- heart. Since amphibians have only one ventricle in the heart arterial blood coming from the lungs and venous blood taken from the tissues are mixed in the ventricle that then pumps the mixture back to the systemic and to the pulmonary circulations. The amphibian circulation is classified as incomplete because arterial and venous bloods mix in the circuit.
The blood oxygenation in amphibians take place also in the systemic circulation since their skin is a gas exchange organ.