Large animals need a circulatory system because the cells in the interior of their bodies are too distant from the surface for absorbed oxygen to diffuse to them sufficiently. By circulating fluid carrying oxygen to these deeper areas, the distance oxygen has to diffuse is greatly reduced.
There are two types of circulatory systems: open and closed. Open circulatory systems are found in many invertebrates with circulatory systems, including arthropods and most mollusks. In insects, however, this system only serves to circulate nutrients, not oxygen. Insects instead use a system of tubes for air that brings oxygen gas closer to their internal organs directly. An open circulatory system pumps blood, or its equivalent, out into the body, but after a short time in vessels, the fluid empties into cavities, bathing the internal organs. It is not under pressure from the heart, so only body movement causes it to circulate.
Closed circulatory systems are found in cephalopods such as squid and segmented worms as well as in all vertebrates. In these systems, the blood is kept in pressurized vessels at all times, enabling it to circulate more efficiently. Unlike open circulatory systems, closed circulatory systems usually have blood cells that carry oxygen, as well as fluid that carries nutrients.