Hall, Richard P. University of California, Los Angeles, California.
A subclass of the Actinopodea. Unlike Radiolaria, these protozoans have no central capsule. Most species live in fresh water. Pseudopodia may be either slender with an axial filament surrounded by cytoplasm (axopodia) or filamentous (filopodia). Axopodial filaments can be extended or retracted rapidly by mechanisms not yet explained. Certain floating species can roll along on the tips of their axopodia and also swim by moving their axopodia. Some species are naked; others have skeletal elements ranging from siliceous scales or spicules embedded in a gelatinous capsule to a reticulate chitinous skeleton often impregnated with silica. A centroplast may or may not be present. The subclass has three orders. Actinophryida lack skeletons and centroplasts and include both uninucleate and multinucleate genera (illus. a). In the Centrohelida, a centroplast is present or else is assumed to be because the nucleus is eccentric (illus.b). The organisms may be covered with a gelatinous sheath or with a skeleton composed of discrete siliceous elements embedded in such a layer. The Desmothoracida have a perforate continuous skeleton which is composed of chitin, and sometimes impregnated with minerals (illus.c). See also: Actinophryida; Actinopodea; Centrohelida; Desmothoracida; Protozoa; Sarcodina; Sarcomastigophora
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