- any minute, marine protozoan of the class Radiolaria, or, in some classification schemes, the superclass Actinopoda, having an amebalike body with radiating, filamentous pseudopodia and a usually elaborate outer skeleton.
- any of various marine protozoans constituting the order Radiolaria, typically having a siliceous shell and stiff radiating cytoplasmic projections: phylum Actinopoda (actinopods)
1862, from Radiolaria, Modern Latin classification name, from Latin radiolus, diminutive of radius (q.v.).
- 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.