There are three basic types of internal circulation found in invertebrates: circulation in a gastrovascular cavity, closed circulatory system, and open circulatory system.
The simplest method is diffusion in a gastrovascular cavity. In this system a body wall of about only two cells thick encloses a central gastrovascular cavity, which serves the dual functions of digestion and distribution of substances throughout the body. The fluid inside the cavity is continuous with the water outside through a single opening; thus both inner and outer layers of tissue are bathed by fluids. Sometimes flagella may line the cells of the cavity which help stir the contents, aiding in the distrubtion of materials throughout the organism. In this system, the cells closest to the cavity have direct access to the nutrients, but the nutrients are able to diffuse a short distance to cells further away.
Animals: Planarians (flatworms) and cnidarians (jellyfish)
Open Circulatory System
For animals with many layers of cells such as arthropods and other insects, a more complex system is needed beyond the gastrovascular cavity. In an open circulatory system, blood bathes the internal organs directly. Thus hemolymph, which serves as both blood and interstitial fluid, circulates throughout the body. Chemical exchange between the fluid and body cells occurs as the hemolymph exudes through sinuses, which are openings surrounding organs. Hemolymph is circulated throughout the body by body movements that squeeze the sinuses and by the contraction of hearts, usually parts of a dorsal vessel. Contraction of the hearts pumps hemolymph through vessels, which open into the interconnected system of sinuses. When the hearts relax, they draw hemolymph in through pores which are equipped with valves that close when the heart contracts.
Animals: Insects, arthropods, and most mollusks
Closed Circulatory System
In closed circulatory systems, the blood is confined within blood vessels. Thus blood does not mix freely with body fluids. Materials pass into and out of the circulating fluid by diffusion. The most basic closed circulatory system can be seen in earthworms. There are usually two major vessels, one dorsal and one ventral, which branch into smaller vessels to supply blood to all organs. The dorsal vessels functions as the main heart, pumping blood forward by waves of peristalsis. Toward the anterior end, pairs of vessels loop around the digestive tract, connecting the dorsal and ventral vessels. Five pairs of these vessels function as auxiliary hearts by pulsating. Blood exchanges materials with the interstitial fluid, which bathes the cells.
Animals: Annelids (earthworms), some mollusks (squids and octupuses), and vertebrates (but in a more complex system)