It is therefore a haemocoel, the coelom of the developed insect being represented only by the cavities of the genital glands and their ducts.
The cells which line them and also the cavities of the tubes contain urates, which are excreted from the blood in the surrounding bodycavity.
In Hirudo and the Gnathobdellidae there is only one system of cavities which consist of four principal longitudinal trunks, of which the two lateral are contractile, which communicate with a network ramifying everywhere, even among the cells of the epidermis.
The gonads and their ducts in the Hirudinea invariably form a closed system of cavities entirely shut off from the coelom in which they lie.
The successive cavities are not, however, completely closed from each other; there is some communication between adjoining segments, and the septa are sometimes deficient here and there.