Hemolymph (or haemolymph) is the blood analogue used by all arthropods and most mollusks that have an open circulatory system. In these animals there is no distinction between blood and interstitial fluid. The liquid fills all of the interior (hemocoel) of the body and surrounds all cells.
Hemolymph is composed of water, inorganic salts (mostly Na+, Cl-, K+, Mg2+, and Ca2+), and organic compounds (mostly carbohydrates, proteins, and lipids). The primary oxygen transporter molecule is hemocyanin.
There are free-floating cells, the hemocytes , within the hemolymph. They play a role in the arthropod immune system.