The movement of the blood through, the vascular organs of animals. In birds and mammals, the air-breathing animals, the circulation is double (pulmonary and systemic), and is carried on through the heart (q.v.), the arteries, and the capillary tubes. In reptiles the heart consists of two auricles, and one ventricle, and there is an incomplete double circulation. In fishes, which are water-breathing animals, there is a simple circulation the heart consisting of a single auricle and a single ventricle, the blood taking in oxygen and giving out carbonic acid in the gills instead of the lungs, which are absent in fishes.