Radiolaria, a class of Protozoa, (q.v.) belonging to the Rhizopoda, and including some of the most specialised of the Protozoa. They nearly all have a skeleton, which is formed usually of silica, but in one group (the Acantharia) it consists of a substance allied to chitin and called "Acanthine." The structure may be simple, consisting only of spicules, as in the Beloidea, which may be fused at one end, or be embedded in some organic matrix; as a rule the skeleton is complex, being composed of a series of rings, spines, or variously-shaped arrangements of lattice-work. The body is divided into two parts, a central capsule and an extra-capsular region. These are usually separated by a membrane pierced by numerous fine pores (Porulosa), or one or more apertures (Osculosa). The extra-capsular region is supported by a gelatinous skeleton known as the calymna in addition to the siliceous or acanthin skeleton which it usually surrounds. Reproduction is effected by budding, fission, or the development of spores; the first is rare and the last is the normal method. There are two kinds of spores: isospores and anisospores, of which the latter are sexual and the former asexual; an "alternation of generations" (q.v.) therefore takes place. The Radiolaria are all marine, and are commonest in tropical seas far from land, living either on the surface or at great depths. They occur less sparingly even in Arctic seas. They are known from very early geological ages; rich faunas are known in the Lower Silurian (Ordovician) rooks, and it has been recently claimed that they occur in Pre-Cambrian rocks; in that case they would be the oldest form of life. They are amall in size, the simple ones being rarely more than 1/25 inch in diameter; a few, however, are an inch in diameter, while some compound ones (Collozoum) may be a foot in length. Nevertheless, they are of great geological importance, as they form thick deposits as in the Barbados (late Tertiary in age), in Jurassic, and Palaeozoic oherts, and in some "phthanites" in the schists of the Cottian Alps. Their occurrence in some crystals of felspar (albite) has also been proved. The classification is us follows: -
I. Porulosa: Capsular membrane penetrated by numerous fine pores.
1. Spumellaria: Pores evenly divided: skeleton of silica or absent.
2. Acantharia: Pores in lines or groups: skeleton of acanthin.
II. Osculosa; Capsular membrane with one or more large pores.
3. Nassellaria: a porous aperture at one end of the axis.
4. Phaeodaria: the main aperture is surrounded by a phaeodium - a mass of rounded green bodies which may be algae living with the radiolarian.