A colonial radiolarian consisting of numerous radiolarian cells and their dinoflagellate symbionts enclosed within the translucent wall of a hollow gelatinous spheroid. Each radiolarian cell is surrounded by a finely porous, siliceous shell which segregates the host cell body from the zooxanthellae in the peripheral ectoplasm. A large lipid body occurs in the center of each radiolarian cell and is surrounded by a layer of endoplasm containing numerous nuclei with cord-like strands of chromatin, tubular cristae mitochondria, and endoplasmic reticulum. The zooxanthellae are sequestered in large vacuoles within the rhizopodial network. Positive evidence for host assimilation of photosynthetically derived organic substances is presented and may partially explain the remarkable abundance of these organisms in oligotrophic regions of the ocean.