4. THE CIRCULATORY SYSTEM
139
TROUT
venosus
SHARK
Fig. 1. Diagrams of the heart in a teleost (trout) and an elasmobranch (shark).
From D. J. Randall (1968), Am Zoologist 8, 179-189.
incomplete atrial and ventricular septa and the spiral folds in the conus
arteriosus (bulbus chordis). The ventral aorta is either very short or nonexistent, and, as in amphibia, the branchial arteries arise from the anterior
end of the conus arteriosus. The degree of subdivision of the heart
varies between different genera of lungfish and is correlated with the
prevalence of air breathing. Neoceratodus, the Australian lungfish,
breathes air only when the oxygen content of the water is low, the
African lungfish, Protopterus aetlziopicus, and the South American lungfish, Lepidosiren paradoxa, are predominantly air breathers and spend
several months each year on land. Division of the heart is least complete in Neoceratodus and best developed in Lepidosiren (Johansen and
Hanson, 1968). The pulmonary vein and sinus venosus represent separate
pathways for venous blood returning to the heart. Blood from the lungs