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Radiolaria are exclusively marine organisms inhabiting all seas. They are pelagic, and are known to occur from surface waters to depths of several thousand meters. Although planktonic, they are able to move vertically by means of a frothy, protoplasmic hydrostatic apparatus by which the specific gravity of the organism is altered—thus some Collodaria and Acantharia, occurring in great numbers at the sea surface during warm, calm weather, descend to depths of 10–200 m. during storms. The main food of most radiolarian groups apparently comprises unicellular algae and protozoans, though some nourishment (or, at least, assistance in assimilation) is probably also derived from symbiotic xanthellae living in their protoplasm. In some groups, reproduction of forms normally inhabiting surface waters apparently occurs at depths of several hundred meters. The warmer waters are richer in species (and in some groups also in individuals) than the colder waters.

Most of the Acantharia, Spumellaria, and Nassellaria are found only at depths of 0–400 m., but many groups of the Tripylea are restricted to well-defined depth zones within the approximate limits 50–5000 m. Little is known of the detailed ecology of the Spumellaria and Nassellaria, but the Tripylea have been much more thoroughly studied. Species of the last-mentioned group have been found to have unipolar and bipolar (often uni- and bi-polar submergent) and warm-water patterns of distribution, and possess varying degrees of stenothermy or eurythermy. Distribution of many species is dependent upon certain current systems, which may expatriate individuals to unfavorable regions....

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