What is an open circulatory system?
Open circulatory system is the one in which blood does not circulate only in blood vessels but it also falls in cavities that irrigate tissues. In the open circulatory system the blood pressure is low and normally the blood (called hemolymph) has low cellularity.
Arthropods, molluscs (the cephalopods are exception) and protochordates have open circulatory system.
The Circulatory System - Image Diversity: open circulatory system