You Broke My Heart, Good Thing I Have A Couple Backups

You think the platypus is an interesting animal?  They've got nothing on the cephalopods.  The squid, octopuses, nautiluses and cuttlefish are way more interesting than any old monotreme.  I could probably do a week of octopus trivia without effort. Probably won't though.

Anyway, in an effort to keep this one relatively short and sweet, I'm going to get on with it.

The cephalopods are somewhat unusual amongst the molluscs (snails, clams, cephalopods, etc) in that its circulatory system is fully enclosed.  That means that their blood never leaves its veins and arteries (same thing with mammals like you and I), but in the rest of the group the blood goes out from the heart and lungs and basically just sloshes around.  Doesn't seem very efficient to me, but they seem to be doing fine, so I won't pressure them to change.

(click to embiggen, clicky-poppy)

Big deal, you say.  Lots of things have a closed circulatory system.  Basically, if it's not an insect or mollusc, it has a closed circulatory system.  Or none at all, in the case of flatworms, but that's a topic for another day.  Since so many things have a closed circulatory system, why am I wasting your time talking about the closed circulatory system of the octopus?  Because, like a great many things in cephalopod anatomy, even though it does the same job as what we have, it goes about it in a strange way.

Namely, cephalopods have three hearts.  Where mammals have evolved a four chambered heart, with two chambers responsible for getting de-oxygenated blood to the lungs and two chambers responsible for getting the oxygenated blood from the lungs to the rest of the body, cephalopods have developed a heart for each bit of work.  For each set of gills (there are two sets of gills, like we've got two lungs), there's a heart that pumps their blood through it.  After their blood (also fascinating, also a topic for another time) is oxygenated, it gets pumped through the body by their third heart. In the diagram above (figures A & B) the gill hearts are called Branchial Hearts and they only label one, but you can see that there's two.  The heart that pumps to the body is called the Systemic Heart (figure C).  Even though none of them are technically redundant, they can survive if one of them's not working properly, since the other two will still provide enough pressure to keep them alive.

Cephalopods: Creepy, disgusting sea beasties, with heart enough to spare.

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Sources:
Image from The Octopus News Magazine Online forums, original source unknown.
Cephalove blog article on cephalopod hearts (not some gross furry fetish site, I swear, although this particular article is about mating)