the commonest form of hydatid cyst in humans, caused by Echinococcus granulosus and found in the liver, lungs, or any other site where the hexacanth embryo may settle if it passes the hepatic or pulmonary capillary filters; characterized by large balloonlike forms lined internally with a germinative membrane, enclosed externally in a laminated membrane within a host-parasite capsule, and filled with fluid (hydatid fluid) and infectious scoleces of the young tapeworms (hydatid sand).