1. A RIGID VESSEL COMPRISING AT LEAST TWO SHELLS EACH OF WHICH COMPRISING A FILAMENT WOUND SIMILARLY SHAPED HOLLOW BODY, CORRESPONDING TO A SURFACE OF REVOLUTION AND INCLUDING TWO POLES, SAID SHELLS BEING DISTINCT FROM ONE ANOTHER, FITTING INTO ONE ANOTHER AND SUPPORTING ONE ANOTHER OVER SUBSTANTIALLY THEIR ENTIRE SURFACE AREA, SAID SHELLS BEING SECURED TO ONE ANOTHER ONLY ADJACENT TO ONE OF SAID POLES, SAID SHELLS BEING FREE OF ATTACHMENT TO ONE ANOTHER OVER THE REMAINDER OF THEIR SURFACE AREA AND THE UNSECURED POLE OF SAID INNER SHELL HAVING A RELATIVELY LARGE POLAR OPENING FORMED THEREIN.