D. circulatory system
that is the answer.
The open circulatory system is found in arthropods and mollusks. The body cavity of these animals is called a hemocoel. The heart pumps a blood-like substance directly into the hemocoel where it moves freely and the organs are able to directly access the nutrients. In the hemocoel, blood and interstitial fluid (a special fluid which surrounds cells) is combined into a substance called hemolyph. This is the goo which sometimes squishes out of bugs when they get stepped on.
