Molluscs (Mollusca) are a group of invertebrates that includes squid, octopuses, cuttlefish, nudibranchs, snails, slugs, limpets, sea hares, mussels, clams, oysters, scallops, as well as many less well-known animals. Scientists estimate that there are more than 100,000 species of molluscs alive today.
Excluding cephalopods, Mollusks have an open circulatory system, meaning the blood does not circulate entirely within vessels but is collected from the gills, pumped through the heart, and released directly into spaces in the tissues from which it returns to the gills and then to the heart. Such a blood-filled space is known as a hemocoel or blood cavity. In the mollusks, the hemocoel has largely replaced the coelom, which is reduced to a small area around the heart and to the cavities of the organs of reproduction and excretion.