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