In contrast to heliozoa, radiolaria exhibit a "central capsule" which separates the intra- from the extracapsular cytoplasm. Nucleus or nuclei are always located in the intracypsular cytoplasm. Most radiolaria possess a species' specific skeleton , mostly with star-like appearance, which is made out of silicic acid or strontium sulfate. Most radiolaria are found in the deeper planktonic strata of warm-water seas. Acantharia are found mainly in the upper plankton. Famous are the radiolarian sediments of Barbados.
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Collozoum inerme
is a colonial radiolarian without skeleton. The white spheres represent individual central capsules kept together by a common jelly