radiolarian UK /ˌɹeɪ.di.ə(ʊ)ˈlɛəɹi.ən/US /ˌɹeɪ.di.oʊˈlɛɹi.ən/ noun1 •Any of many marine amoeboid protozoa, of subclass Radiolaria, having filamentous pseudopodia; they have intricate silica skeletons.Water wells inkblack in the streets repeating the polelamps in glozy rosettes that dish and slide in the wash like radiolarians pale with phosphorous on a midnight sea. — 1979 formsradiolarians