I'm currently reading Treatise on Invertebrate Paleontology (A) and don't understand the following fragment:
The affinity for oxygen in lower animals is many times that in higher ones, whereas conditions are the reverse with regard to carbon dioxide.
What does it mean by "affinity"? Affinity of hemoglobins (in blood)? Affinity of oxygen-transporting molecules, whatever they are? Something else? The book doesn't provide a source of its claim here.
EDIT: @Will - I'm more interested in what kind of affinity they are speaking about in regards to carbon dioxide (I can't comment due to not having enough reputation yet.)