h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Simple enough, really. Do bees have hearts? Ants? How small are you before you don't have a heart, or does every animal have some kind of vascular system?

David Cameron.

Some kind of insect, I imagine... I remember reading somewhere that while they do not have veins and whatnot they do have hearts...

Depends waht you mean by a heart... Basically any multicellular animal, once it gets beyond a certain size need some kind of vbascular system, ultimately, this evolved into the type of mammalian heart plus veins/arteries/capiliaries, but in other animals you can see its progression towards this...
In humans, for example, and other mammels, we've effectively got two hearts well, you've the left hand side, opperating as a unit, and the right hand side opperating as a unit (though of course the two interact), If I remember correctly, you can find a three chambered heart in birds, and then back to amphebians you find a two chambered heart, further back still, and to some of the worms and beyond, you find simple open circulation/vascular systems; the blood basically sits in a single cavity (the haemocoel I think (though probably spelt correctly)... and there is a system to help move this blood about, but not in the very highly structured way as occurs when they're is an actual heart and vascular sytem of veins/arteries etc... I'm trying to remember this all from a very long while back though hang on

Tiny critters don´t use blood quite the way we do. I believe that insects use it for transporting food but not oxygen. They get the oxygen from an independent network of air tubes.

ahh ha... thought so
Yep, strickly speaking the simplist, adn therefore I'd guess smallest hearts are those found in invertibrates, certain worms etc; these types of heart are less concerned with transport of oxygen as they are in vertibrates....
getting on to vertibrates, the fish heart would probably be the simplist (and kind of oldest evolutionary), and so I'm guessing the smallest heart (as we kinda understand it, involved with transporting oxygen and* nutrients round the creatures body), would be found in... whatever the smallest type of fish is These have the simple two chambered heart, an atrium which pumps blood into the ventricle (rather like in humans and other mammels) The vnetricle, in fish, then pumps blood into the conus (err think its spelt like that), which is a kinda weird elastic bag/compartment, which fills up with blood from the ventricle, I guess by just the amoutn of blood it holds, this then squeezes the blood straight out to the gills where it's obviously oxygenated... This then moves round the body until it eventually returns to the atrium, from where its back into the ventricle, thence the conus, and back to the gills for enrichment with oxygen...

Yes, Insects have a fasinating way of dealing with oxygen, a set of tubes, open at the ends (to the outside of the body), (called spiracles I seem to recall but I might be wrong....), these open and just passively, by the differnce in airpressure between the outside and inside of the tubes, take air into the tubes, which effectively takes the air, and its oxygen, to all the parts of the insects body

Not just insects. Most spiders have spiracles as well. Which is why they drown so easily.