An animal cage comprising an enclosure having a shape capable of rolling movement. The enclosure has openings therein, preferably formed of mesh, for passage of gases and matter therethrough and to prevent egress of the animal. The enclosure further includes a door for passage of the animal and may include a food reservoir and fluid reservoir to provide food and fluid to an animal housed within the cage. A second enclosure may also be provided within the first enclosure to prevent injury of the animal due to movement of the cage. The enclosures may be formed on a support frame having one or more longitudinal rings or half-rings and one or more latitudinal rings or half-rings. The enclosures may be completed by individual section members which cover sections of the enclosure and which are held in place by outer tension bars and inner compression bars. The outer tension bars and inner compression bars are held in relative tension and compression by a polar cap assembly having a tension cone and a compression cone for acting on the tension and compression bars.