An open circulatory system is common in most molluscs and anthropods. In this type of ciruclatory system, a fluid called hemolymph ( a mix of blood and lymph), is bathed on organs and body tissues in a cavity called the homecoel. Hemolymph travels through circulatory vessels into cavities or sinuses, which are open spaces surrounding an organ. The reason why this fluid is bathed on organs is so the tissues receive oxygen and nutrients directly.
The circulation of hemolymph is done with muscle contractions because there is no blood pressure, which is the nature of an open system like this.
There are advantages and disadvantages to an open circulatory system. The simplicity of this system makes it suitable for small organisms. An open system requires less energy to distribute nutrients and oxygen throughout organs and tissues. There are also unique features that an open system can give to certain organisms. For an example, spiders use hydrostatic pressure made by their open circulatory system to extend their legs. Some disadvantages to this system is the fact that the animals cannot control their velocity of their blood flow, and oxygen cannot be filtered in and out as easily.