<P> The gastrovascular cavity is the primary organ of digestion and circulation in two major animal phyla: the Cnidaria (including jellyfish and corals) and Platyhelminthes (flatworms). The cavity may be extensively branched into a system of canals . In cnidarians, the gastrovascular system is also known as the coelenteron, and is commonly known as a "blind gut" or "blind sac", since food enters and waste exits through the same orifice . </P> <P> The radially symmetrical cnidarians have a sac - like body in two distinct layers, the epidermis and gastrodermis, with a jellylike layer called the mesoglea between . Extracellular digestion takes place within the central cavity of the sac - like body . This cavity has only one opening to the outside which, in most cnidarians, is surrounded by tentacles for capturing prey . </P>

What is the advantage of a gastrovascular cavity