ocr: Protozoa olarian. Radiolarians (ray-dee-o-lair-ce-ans) are protozoans of relatively large size found at various depths in the ocean. Some occur in colonies; some contain symbiotic algae; and many are bioluminescent. Because they form skeletons of glass (silica), they can be readily found as fossils. Radiolarians float because a frothy layer called the calymma, situated in the outer portion of their cytoplasm, holds trapped bubbles of carbon dioxide. 55