Radiolaria

Radiolaria, a class of marine Rhizopod Protozoa. Thread-like processes of living matter radiate outwards on all sides; a membranous capsule with fine pores, or with one or more apertures, separates an internal nucleated region of the cell from an outer part supported by a gelatinous framework; there is a hard skeleton of silica or of a 'horny' material called acanthin. They multiply by dividing or by forming spores within the central capsule. Most of the Radiolarians are minute, under $\backslash frac\{1\}\{100\}$ of an inch, but a few which form colonies may measure an inch or even 4 inches in length. Most of them include 'yellow cells' or algae, with which they live in a partnership known as Symbiosis (q.v.). Radiolarians occur in all seas, in every latitude, and at all depths, though many kinds predominate on the surface, where they are wafted about by currents. The siliceous skeletons form most of the ooze which has been dredged from depths of 2000 to 3000 fathoms. Fossil forms are abundant in certain deposits—'Barbadoes earth,' 'Tripoli powder,' and various marls, &c. See Haeckel in the Challenger Reports (1887).