OH: "I should be at work and not thinking about which animal has the most countercurrent exchangers in it. (Canada geese have four, one in each lung and one in each leg. I think they're currently leading)"
@whitequark I studied these in university (physiology) but did not know about geese! Kind of surprised this was left out as it was a McGill B.Sc. Shark was a notable example IIRC.