What structures allow hemolymph to enter the heart from the pericardial cavity?

• A. Dorsal aorta
• B. Pericardial sinus
• C. Ostia
• D. Venous sinus

Answer: (C)

In an open circulatory system, the heart reclaims hemolymph from the surrounding pericardial sinus through one-way openings called ostia. These pores are in the heart wall and have valves that allow hemolymph to enter the heart when it relaxes and the pressure in the heart drops, but prevent backflow when the heart contracts. This setup lets the heart pull hemolymph from the pericardial sinus and then pump it into the dorsal vessels and onward to tissues. The pericardial sinus is the cavity around the heart that contains the circulating hemolymph; the dorsal aorta carries hemolymph away from the heart, not into it, and the term venous sinus isn’t the standard entry point in this context.