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Any of various marine protozoans of the group Radiolaria, having rigid skeletons usually made of silica. The skeletons are usually spherically symmetrical and structurally complex, containing elaborate patterns of perforations (through which pseudopods extend) and often spicules. Skeletal remains of radiolarians sink to form ooze on the ocean floor, and prehistoric radiolarian ooze has fossilized to become chert and flint.
any protozoan of the class Polycystinea (superclass Actinopoda), found in the upper layers of all oceans. Radiolarians, which are mostly spherically symmetrical, are known for their complex and beautifully sculptured, though minute, skeletons, referred to as tests. Usually composed of silica, the test is elaborately perforated in a variety of patterns, forming a series either of latticelike plates or of loose needle-shaped spicules. Pseudopodia extend through the perforated skeleton. A chitinous central capsule encloses the nuclei and divides the cytoplasm into two zones. The outer cytoplasm contains many vacuoles that control the organism's buoyancy.