The main body cavity of arthropods and molluscs, which forms part of the open circulatory system (see circulation). It consists of a network of spaces (sinuses) that are filled with haemolymph, which performs the roles of both blood and tissue fluid. The haemocoel is an enlarged blastocoel (see blastula), which greatly reduces the coelom (this is restricted to the cavities of the gonads and excretory organs). The haemocoel can also act as a hydrostatic skeleton.