Have a Heart
Figure 1. Shark Heart (author's original image) Hearts have been on my mind lately. We, humans that is, of course, have a four-chambered heart. We have two atria and two ventricles. The right side carries blood low in oxygen, and the left side, having just returned from the lungs, carries blood high in oxygen. We share this heart anatomy with the other mammals, birds, and crocodiles. Found in most amphibians and some reptiles is a three-chambered heart with two atria and one ventricle, which allows mixing of oxygenated and deoxygenated blood. That is a bit freaky in my view: oxygenated blood and deoxygenated blood mix in the central chamber; whoever thought this was a good design? Most animals in the kingdom do not have hearts with 3 or 4 chambers. The most speciose vertebrates, scaly fishes, have a two-chambered heart. Suppose we follow the blood flow in 18-spined sculpin (AKA long-horned sculpin), Myoxocephalus octodecimspinosus...