If cockroaches have closed circulatory system than how can the blood circulate in whole body??
@Olivezz, although your friends have given some information, i would like to add few more points to make it more clear.
There are 13 heart present in a continuous longitudinal manner. The heart pumps the haemolymph into the blood sinuses. From the pericardial sinus, the haemolymph enters into the heart through ostia. When the heart gets filled it contracts from behind forwards and then the heart becomes normal. Then the next systole follows after a short interval. During systole (contraction), the valve like ostia close, preventing back flow of haemolymph into the pericardial sinus. Therefore, some of its haemolymph is pumped into segmental vessels, while most of it is poured into the head sinus through the terminally opening anterior aorta. From the head sinus, the haemolymph flows backward into the thorax and abdomen. While flowing backwards from head sinus, the haemolymph remains in the ventral part due to presence of Oesophagus in dorsal part and so it fills into the perineural sinus. From the perineural sinus, the haemolymph, now, flows into the perivisceral sinus through the pores of ventral diaphram in abdominal region. Then from perivisceral sinus, it flows into pericardial sinus through the pores of dorsal diaphram. Then, during heart's diastole, it fills in the heart through the ostia.