Cytoplasmic fine structure of nassellarian Radiolaria

Abstract

Light and electron microscopic examinations were made of the cytoplasm, axopodial system, zooxanthellae, and skeletal-cytoplasmic relationship in two species of nassellarian Radiolaria. The organization of the axopodia and the fine structure of sinusoidal-membranous-tubular elements in the fusule are presented. These membranous tubules surround the microtubules at the periphery of the fusule and provide continuity between endoplasmic reticulum inside and outside of the capsular membrane. Their possible role in communication between ectoplasm and endoplasm is discussed. The axoplast lies in close proximity to the nucleus which may provide genetic control of microtubule organization. The apex of the axoplast is surrounded by mitochondria whose chemical products may regulate microtubule assembly and disassembly. The fine structure of the dinoflagellate zooxanthellae, their control by the radiolarian host, and their possible nutritional role is discussed. These findings are related to previous research on nassellarian Radiolaria and other Rhizopoda such as benthic and planktonic Foraminifera.