Physiology of Finfish and Shellfish (2+1)

2.2.21. Shellfishes

The open circulatory system, examples of which are common to molluscs and arthropods. Blue blood, containing the pigment hemocyanin as its oxygen-capturing molecule instead of red hemoglobin, is pumped from the heart through blood vessels that lead to the various organs of the body. But then it leave the blood vessels and enter body cavities, where the organs are bathed in blood, or sinuses or lacunae (spaces) within the organs or in other words tissues are surrounded by the blood. Such a circulatory system is called an open circulatory system because the blood is not contained within blood vessels all the time. The resulting blood flow is sluggish because there is no blood pressure after the blood leaves the blood vessels. The animal must move its muscles to move the blood within the spaces. This type of circulatory system can be associated only with an inactive animal because it is an inefficient means of transporting oxygen and nutrients throughout the body.

Arthropods and most mollusks (except cephalopods: nautilus, squid, octopus) have an open circulatory system.

In a closed system, blood remains within blood vessels, pressure is high, and blood is therefore pumped faster.