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Above is a diagram of an open circulatory system. An open circulatory system is made up of a heart, vessels, and hemolymph. This diagram shows how the hemolymph, fluid present in most invertebrates that is equivalent to blood, is circulated throughout the body of a grasshopper.
The heart is an organ. It is made up of different tissues. It has muscle tissue called myocardium. It has connective tissue on the inside (endocardium), and on the outside (pericardium). The heart has valves that make sure the blood goes the right way through the heart. So the heart is an organ made from several tissues.