Rhizopod Protozoa, with the body divided into a central part or capside lodging the nucleus, and an extra-capsular region, by a membranous capsule only exceptionally absent. Capsular membrane pierced by fine pores, or by one or several apertures. Extra-capsidar region supported by a gelatinous skeleton or calymna. Pseudopodia for the most part radiant, sometimes branching and anastomosing. Skeleton seldom absent, either spicular or continuous, composed of acanthin or of silica. Reproduction by flagellate uninucleate spores produced in the central capsule. Solitary or colonial; exclusively marine.

Skeletal elements are rarely absent, as in Colloidea, Nassoidea, and Phaeodinida. They consist in Acantharia of an organic substance, acanthin, destroyed by a red heat and by acids; it is supposed to be either identical with vitellin (Brandt) or akin to chitin (Haeckel). This Acantharian skeleton takes the form of solid radial spines, which meet in the centre of the central capsule and are connected at their bases in one of four ways - (1) by simple apposition, (2) combined also with leaf-like ad-central processes, (3) by fusion of all, or (4) of opposite pairs, the last a mode seldom found. In number the spines in question are usually twenty, disposed in a regular manner according to Miiller's law1; indefinite in number and arrangement only in the Acanthometrid family Actinelida. They are often provided with lateral outgrowths (apophyses), which may be incomplete or united in the Acanthophracta to form a lattice-shell. But in the family Sphaerocapsida the shell is composed of small plates, each pierced by a pore and united by a cement; the spines undergo partial atrophy in two genera, complete atrophy in a third, twenty apertures, however, marking the places where they should protrude.

In all other Radiolaria the skeleton consists of silica, which is probably deposited in an organic matrix2; that of Phaeodaria is blackened by heat, stained by carmine, and destroyed by boiling caustic alkalies; and in two families of the same sub-group (Cireo-porida, Tuscaroridd) it has a porcellanous aspect by reflected light, and consists of fine siliceous needles imbedded in a granular matrix. Its elements are solid, except in most Phaeodaria, where they are hollow siliceous tubes filled with a gelatinous substance. The shapes taken by the skeleton are numerous. It may be spicular (beloid), as in the Beloidea among Spumellaria, and the Phaeocystina among Phaeodaria, the spicules being of various shapes and disposed more or less tangentially, radially only in the Phaeodarian family Aulocanthida. In all other instances it forms a connected whole; a simple ring or several rings united in various planes, the Nassellarian Stephoidea; radial spines united at a centre which lies ex-centrically near the oral pole, the Nassellarian Plectoidea; a lattice-shell, the bars of which lie either in one plane, or in different planes, and in this case spongy in texture, as is not uncommon among Spumellaria, in shape spherical, ellipsoidal (= prunoid), discoidal, lent-elliptical (=larcoid) i.e. with three principal axes of unequal length, cyrtoid i.e. with two dissimilar poles, an apical and basal, or finally conchoid.

The last-named consists of two valves, and is confined to a few Phaeodaria. The cyrtoid skeleton may be monothalamous, and then ovate or cap-shaped, as in some Nassellaria, where it is termed cephalis, a few Phaeodaria, e.g. Challengerida, and very rarely in Spumellaria; or it may be in some Nassellaria poly-thalamous, the cephalis being simple, bilobate, or multilobate, with, especially when simple, new joints added at the basal pole; incomplete internal septa correspond to the external constrictions. The shell is rarely multiple, except in the Spumellarian Sphaeroidear where there may be two spheres, three, four, five, or even ten, rarely more, one within the other, all united by radial bars1.

1 This law may be thus expressed. Imagine a globe with an axis of rotation, and five circles inscribed on it, an equatorial, two tropical and two polar. The twenty spines lie four in each of these circles, the equatorial and polar spines in the same meridian lines, 900 apart, i. e. at o°, 900, 1800, 2700; the tropical in meridian lines exactly intermediate, i.e. at 450, 1350, 2250, 3150. The twenty spines are rarely all equal. See Haeckel, 'Challenger Reports,' xviii. pp. 717-20, for a summary of the variations known.

2 For the organic matrix, see Brandt, 'Kolonie-bild. Radiolarien,' Fauna und Flora des Golfes von Neapel, xiii. p 63. Calcareous bodies, shaped like the rowel of a spur, occur in the calymna of some Radiolaria. They are supposed by Haeckel, who terms them 'calcastrellae,' not to be skeletal elements, but either unicellular algae, or of foreign origin. See Haeckel, op. cit. supra, p. lxx, note D.

The division of the body into a peripheral or extra-capsular, and a central or intra-capsular portion is constant. The central capsule is delimited by a resistent chitinoid (?) membrane, a skeletal structure which is double in Phaeodaria, and is absent in the young stages of some forms, but appears just before sporulation. Collozoum inerme is said to develope it only when forming isospores, and in Sphaerozoum neapolitanum it is permanently absent (Brandt). When it is thus absent the contour of the body is variable. The capsule is normally spherical, but it becomes adapted to the general shape of the skeleton as well as to its peculiarities. Its surface is lobed in many Spumellarian Sphaeroidea; and in the Nassellarian Spyroidea and Cyrtoidea it has 3-4 lobes projecting through the basal or cortinar plate of the shell. When the shell is double or multiple the capsule generally incloses the inner or two or three of the inner shells during its growth. Its membrane is always perforated either by innumerable fine pores, evenly distributed as in Spumcllaria, or aggregated in groups or lines with imporous intervals, as in Acantharia; or, as in Nassellaria, by pores confined to a special membrane or porochora, situated at the basal pole of the capsule, primitively circular in shape, but lobed or even broken up into tracts according as the lobes of the capsule itself are more or less prominent (supra); or again there is a main oral aperture or astropyle, with or without the addition of secondary apertures or parapylae, as in Phaeo-daria. The astropyle consists of a tubular proboscis, rising from the centre of a radially striated disc or operculum.