The Periplaneta has an open circulatory system, which means that blood does not flow through blood vessels and instead flows through haemocoel (body cavity). All vertebrates and annelids (such as earthworms) and cephalopods (squids and octopus), have closed circulatory system.
Related Theory
In all vertebrate organisms, as well as some invertebrates, this is a closed-loop system in which the blood is not moving freely in a cavity. In a closed circulatory system, blood is contained inside blood vessels, circulating unidirectionally (in one direction) from the heart around the systemic circulatory route, then returning to the heart again. In contrast to a closed system, arthropods (including insects, crustaceans, and most mollusks) have an open circulatory system. In an open circulatory system, the blood is not enclosed in the blood vessels, but is pumped into a cavity called a hemocoel. The blood is called haemolymph because it mixes with the interstitial fluid.