The Radiolaria are found in all seas and in every latitude; their wide distribution is due to oceanic currents. The variety of species is greatest in tropical regions. The Pacific Ocean has an extremely rich fauna, both in species and numbers, and is probably closely approached by the Indian Ocean. They are found at all depths, pelagic, i. e. at or near the surface, zonarial and abyssal. The Spumellaria and Acantharia predominate at the surface, the Nassellaria and Phaeodaria at abyssal depths; the zonarial fauna being a mixed one. It is probable that the young of a given species are found at a greater depth than the adult. The organic skeletons of Acantharia perish at death, but the siliceous skeletons of Spumellaria and Nassellaria ( = Polyeystina of Ehrenberg), and of Phaeodaria, to a very slight extent, occur in marine deposits. They make up often more than three-quarters of the Radiolarian ooze which has been dredged in the Pacific and Indian Oceans at depths of 2-3000 fathoms. In Globigerina ooze (p. 893 post) they are frequently present in great numbers. Nor are they wanting in the deepest and most extensive of oceanic deposits, the Red Clay, which is probably derived to a great extent from the decomposition of their shells.
They are also found in varying proportions in muds near the shores. The same groups occur fossil, the Phaeodaria being represented by the family Dictyochida, which has a perfectly solidified skeleton. Simple Spumellaria and Nassellaria have been detected in various Palaeozoic strata. Among Mesozoic formations they are found principally in Jurassic strata, in quartzites of various kinds (fossilised Radiolarian ooze) and in coprolites, scantily in the Chalk and Trias. The Spumellarian Sphaeroida and Discoidea constitute the minority, the Nas-sellarian Cyrtoidea the majority; the skeletons are simple and massive, somewhat larger than the Radiolaria of the Tertiary strata. The deposits of the latter are apparently of Miocene age, and contain either living species or species akin to them. They are found plentifully in Barbados as marls, sometimes almost pure Radiolarian ooze, sometimes Globigerina ooze, or containing clay, pumice, etc.; in the Nicobar Islands as a Radiolarian clay; and in the Mediterranean round its coasts, both South European and North African (Oran to Tripoli), and in its islands, as marls, chalky deposits, powdery tripoli, sometimes coherent as whetstone or polishing slate.
Haeckel classifies the Radiolaria as follows: A. Holotrypasta s. Porulosa: central capsule with innumerable fine pores: spherical or derived from a sphere.
Pores of capsule evenly distributed; skeleton siliceous or wanting; contains (i) Collodaria, with no skeleton, Colloidea, e.g. Tha/assicolla, Collozoum, or a spicular skeleton, Beloidea, e. g. Physematium, Sphaerozoum; (ii) Sphaerellaria with a lattice or spongy shell always complete, spherical in Sphaeroidea, which contains the colonial Collosphaerida among other forms, cylindrical or elliptical in Prunoidea, discoidal in Discoidea, lent-elliptical in Larcoidea.
Pores in groups or lines, skeleton composed of acanthin in the form of radial spines, with or without a lattice-shell; contains (i) Acanthometra with spines, lattice-shell if present incomplete, the former indefinite in number and irregularly arranged in Actinelida, or twenty and regularly arranged in Acanthonida; (ii) Acanthophracta with twenty spines regularly arranged, and complete lattice-shell; subdivided according to character of shell and size of spines.
Central capsule with one principal aperture, and with or tvithout accessory openings; derived from an egg-shape, or perhaps bilateral.
A porous area = porochora at one pole of main axis with an intracapsular podocone; skeleton siliceous: contains (i) Plectellaria, without shell, Nassoidea (Cystidium, Nassella), or with an incomplete one, either a basal tripod without ring, Plectoidea, or a sagittal ring usually without tripod, Stephoidea; (ii) Cyrtellaria, with a complete lattice-shell or cephalis, bilocular with sagittal constriction, Spyroidea, multilocular with two or more constrictions and lobes, Botryodea, or simple, Cyrtoidea.
Pansolenia, including Tripylea of R. Hertwig; membrane of central capsule double; an astropyle with or without parapylae; a phaeodium surrounding astropyle; skeleton siliceous: contains (i) Phaeocystina, with skeleton wanting, Phaeodinida (Phaeocolla, Phaeodina), or composed of scattered pieces, Cannorhaphida, or hollow radial tubes, Aulacanthida; (ii) Phaeosphaeria, a simple lattice-shell, or one of hollow tangential tubes with nodal septa, single or double; (iii) Phaeogromia, a simple spherical, ovate or polyhedral shell, with large mouth opposite astropyle surrounded by teeth or feet, diatomaceous, Challengerida, alveolate, Medusettida, latticed, Castanellida: or porcellanous with fine needles imbedded in a punctate cement, Circoporida, Tuscarorida; (iv) Phaeoconchia, a bivalved lattice-shell, thick and firm, Concharida, or thin, fragile, scarcely latticed, with a conical cupola or helmet-shaped galea at the centre of each valve, and- with hollow tubes, Coelodendrida, Coelographida.
The number of genera and species known is very large.
Biitschli, Bronn's Klass. und Ordn. des Thierreichs, i. Protozoa, pp. 332-478; Haeckel, 'Report on the Radiolaria,' Challenger Reports, xviii. 1887; Id. 'Die Radiolarien,' Berlin, 1862; Brandt, 'Die Kolonie-bildende Radiolarien,' etc, Fauna und Flora des Golfes von Neapel, xiii. 1885; R. Hertwig, 'Organismus der Radiolarien,' Dk. Med. Wiss. Ges. Jena, ii. 1880; Id. 'Zur Histologie der Radiolarien,' Leipzig, 1876.
Coelothamnus Davidoffii and structure of skeleton, especially in Cyrtida, Biitschli, Z. W. Z. xxxvi. 1882; Thalassicolla coerulea, Eberth, A. M. A. xxx. (1), 1887.
Extracapsular bodies; spore-formation; phosphorescence and other points of physiology, Brandt, op. cit. supra.
Regeneration from central capsule, Schneider, Archiv fur Anat. und Physiol. 1867, p. 509.
Yellow cells, Brandt, op. cit. supra, pp. 65-71; Id. Mitth. Zool. Stat. Naples, iv. 1883, p. 220, p. 235; Geddes, Nature, xxv. 1881-2, p. 303.
Distribution and Fossil forms, Haeckel, Challenger Reports cited supra, pp. cxlvi-clxxv. with lit. quoted.