Open and Closed Type of Circulatory Systems
There are two categories of circulatory system found in higher metazoans. In one type the original blastocoel carries on to be the main perivisceral space. Either there is no true coelom at all as in pseudocoelomates, or the coelom may be extremely obliterated or reduced and restricted to gonads and excretory organs like in arthropods and most molluscs. In these animals there is no closed network of capillaries connecting the arteries and veins. The arterial blood flows into large spaces or sinuses (or into smaller spaces termed as lacunae), in which various tissues or organs are bathed through blood. From these spaces blood passes into large open veins or by ostia into the heart directly. This is called open type of circulatory system.
In the another type, a system of closed capillaries connect the smaller arteries along with veins, fluid consisting of nutrients filter through the capillary walls into tissue spaces, from which cells in turn derive their nutrients. This is called closed type of circulatory system. This type of circulatory system is found in annelids, cephalopod molluscs and chordates.