Abstract Siliceous skeletal remains, principally of diatoms and radiolarians with smaller amounts of sponge spicules and silicoflagellates, constitute up to approximately 40 percent of some Recent pelagic sediments. The main factors controlling the contribution made by siliceous organisms to marine sediments are apparently (1) the rate of production of siliceous organisms in the overlying waters, (2) the degree of dilution of siliceous remains by terrigenous, volcanic and calcareous organic contributions to the sediment and (3) the extent of solution of the siliceous skeletons, most of which apparently occurs shortly after deposition. Depth of water is apparently not an important direct factor, except insofar as it affects the amount of calcareous contributions to the sediments. The evidence at present available indicates that the production of siliceous organisms is high in the regions of the equatorial current systems and around the edges of the central water masses of the oceans, especially in areas of upwelling, and is much lower in those parts of the oceans in which the near-surface waters are more stably stratified.

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....