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FIGURE 1. The Foraminifera, closely related to the Radiolaria, are exclusively marine single-celled animals. The approximately 45 living planktonic species (many more are benthic, living on the bottom), possess a calcareous shell formed by several chambers. Photos courtesy of Silvia Watanabe.
FIGURE 2. As opposed to the radiolarians, the skeletons of the Acantharia are composed of strontium sulfate (rather than silica). Some representative species are illustrated here. Photos courtesy of Renate Bernstein.
FIGURE 3. Many surface-dwelling Polycystina host symbiotic algal cells in their cytoplasm; both organisms, the host and the alga, benefit from this association. The micrograph of this surface-dwelling radiolarian from the Caribbean about 300 micrometers (approximately 1/8th of an inch) across shows its cytoplasmic pseudopodia (thin rays radiating from the specimen), and the symbiotic algae - tiny yellow-green spherules. Photo courtesy of Atsushi Matsuoka (from Matsuoka, A., 1993. Observations of living radiolarians from the surface water in the Caribbean Sea. News of Osaka Micropaleontologists, Special Volume 9, pp. 349-363).
Figure 4. All known organisms are classified within successively larger categories according to the traits they have in common. Thus, the Kingdom Protista encompasses single-celled animals, the Phylum Sarcodina applies to those single-celled protists that possess cytoplasmic extensions or pseudopodia used for locomotion and prey capture, and so on. The figure illustrates the position of the Polycystina within this system, as well as its subdivisions into lower categories.
Figure 5. Skeletons of some representative Phaeodaria, a group closely associated with the Polycystina; the two are known collectively as Radiolaria. Photos courtesy of Renate Bernstein.
Figure 6. Distribution of radiolarian oozes in the World Ocean. Upon death Polycystine shells settle to the sea floor. While these shells are not immune to dissolution, in many areas input from the water column exceeds the rate of dissolution, so that skeletons pile up on the bottom forming layers tens to hundreds of meters thick. These sediments where radiolarian shells are a major component are known as radiolarian oozes. Modified from Lisitzin, A.P., 1974. Osadkoobrazovanie v okeanakh. Nauka, Moskva, pp. 1-438.
Figure 7. Evolutionary transformations of a polycystine lineage through time. The fact that radiolarian shells preserve in the sediments for millions of years makes them especially useful for the investigation of evolutionary changes. These figures show selected representatives from a continuum of morphological change that took place in the subgenus Podocyrtis (Lampterium) from 47 to 41 million years ago. After the last form illustrated, the lineage became extinct, leaving no descendants. Assembled by W. R. Riedel, using illustrations published in Micropaleontology, vol.24, p.89, courtesy of Micropaleontology Press.
Figure 8. A 5-shelled (5 spherical concentric shells) polycystine undergoes several growth stages before reaching the adult stage, starting with the small internal sphere, and ending with the full grown 5-shelled specimen. Upon death this skeleton sinks to the sea floor and starts dissolving slowly; because the internal spheres are usually thinner than the external ones, they disappear first. Inadequate taxonomic frameworks and ignorance of these processes are responsible for the erection of many "new species" on the basis of these incomplete skeletons.

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