In the evolution of the vertebrate circulatory system, a transitional step led to separate pulmonary and systemic circuits. This step can be observed in
A) lungfish.
B) the ancient ostracoderm.
C) birds.
D) amphibians.
E) reptiles.

Question

Quizplus · Accepted Answer

Lungfish are intermediate between aquatic and terrestrial animals and have a circulatory system that is transitional between fish and amphibians. They have both a pulmonary circuit for oxygenating blood through the lungs and a systemic circuit for carrying oxygenated blood to the body's tissues. This is a transitional step towards the complete separation of pulmonary and systemic circuits seen in tetrapods such as amphibians, reptiles, birds, and mammals.

In the Evolution of the Vertebrate Circulatory System, a Transitional