The incurrent ostia are vertical slits in the sidewalls of the heart. There may be up to twelve pairs of these incurrent ostia in some forms. The front and back edges of each hole are molded into little lips that form valves. These valves allow blood to enter the heart during diastole, the filling phase of the heart. During diastole incoming blood forces the lips apart, and hemolymph flows into the heart from the hemocoel. The valves prevent backflow through the ostia when the heart contracts and pumps during systole. Systole is the ejection phase of the heart. The pressure of hemolymph in the heart squeezes the lips of the incurrent ostia together as the heart contracts and the lips remain shut during systole. In some insects the heartbeat reverses, and when reversal of flow occurs as the heartbeat reverses, hemolymph can flow backwards into the hemocoel as the heart contracts.

In the groups where the excurrent ostia occur, little clumps of cells, called papillae, sit at the entrances of the ostia into the heart. These papillae expand during systole forcing hemolymph out of the heart into the hemocoel. During diastole, the papillae contract preventing hemolymph from flowing from the hemocoel back into the heart.

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