The tissue layer between epidermis and gastrodermis. In polypoid forms, usually functioning in the manner of an elaborate basement membrane and as a flexible skeleton. In medusoid forms, it has its function in smaller organs such as tentacles and manubrium but, across most of the umbrella, it is typically much thickened, well organised, muscular and contractile. In most medusae it is transparent and forms the jelly of jellyfish.