| An open circulatory system is one where blood bathes the internal organs directly. There is no difference between blood and interstitial fluid, and the general body fluid is called hemolymph. One or more hearts pump the hemolymph into an interconnected system of sinuses, which are spaces surrounding the organs.
Insects, other arthropods and most mollusks have an open circulatory system, as opposed to one like ours, a closed circulatory system.
e.g.,
In grasshoppers and other arthropods, the heart is an elongated tube located dorsally. When the heart contracts, it pumps hemolymph through vessels out into sinuses.
When the heart relaxes, it draws hemolymph into the circulatory system through pores called ostia. Body movements that squeeze the sinuses help circulate the hemolymph.
2002.01.08 SEF says re: open circulatory system: check out Horseshoe crab for an interesting sidelight on all this | 
