44. Principal Axes.—From the foregoing consideration of the statical conditions and their direct causal connection with the geometrical ground-forms of the Radiolaria, the great mechanical significance of the differentiation of definite axes in these unicellular free-swimming organisms will be manifest. The most important of these is the primary main axis (axis principalis, or protaxon), which in all cases has a vertical direction. It is wanting in the Centrostigma (spheres and endospherical polyhedra), and in the Anaxonia (acentra). It is isopolar in the phacotypic forms (Monaxonia isopola), and in the double pyramids (Stauraxonia isopola). It is allopolar in all monastatic ground-forms, in the conotypic forms (Monaxonia allopola), pyramids (Stauraxonia allopola), and the Centroplana (or bilateral forms).
45. Secondary or Transverse Axes.—In contrast to the vertical main axis all the other constant axes differentiated in the body may be called "secondary axes," or "transverse axes," since they cross the former at definite points. All ground-forms whose vertical axis is crossed by a fixed number of such axes at definite angles may be called "Stauraxonia." They are divided into two groups, double pyramids and single pyramids; in the former the two poles of the main axis (or the two halves of the body separated by the equatorial plane) are similar (Stauraxonia homopola), in the latter dissimilar (Stauraxonia heteropola). If all the secondary axes be equal, the stauraxon ground-form is regularly radial. If some of them be unequal they are arranged in certain relations towards two primary transverse axes, perpendicular to each other, to which all the other secondary axes are subsidiary; the ground-forms are then either amphithect or bilateral. The two primary transverse axes, which may also be designated "ideal transverse axes" (euthyni), divide the vertical main axis in its centre; one of them is the sagittal, the other the frontal. These three dimensive axes give the factors which accurately determine the ground-form and the dimensions in most Radiolaria; the vertical main axis determines the length (principal axis); one horizontal transverse axis determines the thickness (sagittal axis), and the other the breadth (frontal axis). Those ground-forms in which the transverse axes are isopolar are termed "amphithect," and those in which the one (frontal or lateral) is isopolar and the other (sagittal or dorso-ventral) is allopolar, are termed "bilateral," or better "zeugitic."
46. Primary and Secondary Ground-Forms.—The geometrical sphere must be regarded as the original ground-form of the Radiolaria; it being understood that its monophyletic derivation from a single stem-form, Actissa, is correct. The simplest forms of Actissa (Procyttarium, Pl. [1], fig. 1) are in fact geometrically perfect spheres; indeed even the individual parts which compose their unicellular bodies (nucleolus, nucleus, central capsule and calymma) are concentric spheres. But in addition the central capsules of most other Spumellaria, especially the Sphæroidea, as well as of many Acantharia are true spheres. Furthermore the simple or concentrically composed lattice-spheres of Sphæroidea, Sphærophracta, and Phæosphæria may be regarded as spheres, although strictly speaking they are endospherical polyhedra. From the primary spherical form of the Radiolaria all other secondary forms may be derived in the following order:—1. By the development of a main axis the Monaxonia arise. 2. By the development of transverse axes the Stauraxonia arise. 3. In both groups (Monaxonia and Stauraxonia) the two poles (or upper and lower halves of the body) are at first similar (Isopola). 4. By differentiation in the two poles or halves of the body (distinction between the basal pole and the apical) the forms with different poles (Allopola) arise. 5. The transverse axes of the Stauraxonia are at first equal (regular pyramids and double pyramids). 6. By differentiation in the transverse axes (distinction between the sagittal and the frontal axis) the amphithect pyramids and double pyramids arise. 7. From the amphithect pyramids the Amphipleura arise by differentiation of both poles of the sagittal axis. 8. The zygopleural ground-form appears last, as the simplest form of the Amphipleura.
47. The Ground-Forms of the Spumellaria.—The Spumellaria, being the oldest and most primitive Radiolaria, have for the most part either indifferent or multistable equilibrium; e.g., all Colloidea and Beloidea which have a spherical central capsule, and also most Sphæroidea. Among these primitive Centrostigma true spheres and endospherical polyhedra are represented in the utmost variety, and the regular polyhedra in particular. By the development of a vertical main axis these Centrostigma have also given rise to very numerous Centraxonia, which are usually isopolar, very rarely allopolar. Sometimes they are Monaxonia (circular in transverse section), sometimes Stauraxonia (polygonal in transverse section). The vertical main axis is longer in the Prunoidea, shorter in the Discoidea than any of the other axes. The Larcoidea are distinguished by their lentelliptical or triaxial ellipsoid form; the three different but isopolar axes corresponding with those of the rhombic octahedron; but even among the Sphæroidea, Prunoidea, and Discoidea, this form is sometimes produced by the differentiation of two different transverse axes at right angles to each other. Whilst these ground-forms (Centraxonia and Centrostigma) occur in the utmost variety among the Spumellaria, the centroplanar (or true bilateral) ground-form is entirely wanting.
48. The Ground-Forms of Acantharia.—In the small family Astrolophida, which contains the most archaic forms of the legion (Actinelius, Astrolophus), the Acantharia show a direct relation to the most primitive Spumellaria (Actissa), and like these have indifferent equilibrium; their central capsule is a sphere, their calymma an endospherical polyhedron, whose angles are indicated by the distal ends of the numerous equal radial spines. In the great majority of Acantharia, however (all Acanthonida and Acanthophracta), twenty radial spines are present, regularly distributed, according to Müller's icosacanthan law, in five parallel circles, each containing four crossed spines (p. [717]). Usually the twenty spines are equal, and the ground-form is the quadratic octahedron, or a regular double pyramid with sixteen sides. But in some groups (the Amphilonchida and Prunophracta) two opposite equatorial spines are much more strongly developed than the other eighteen, and therefore the hydrotomical axis in the equatorial plane is larger than the geotomical axis (p. [719]); the isopolar stauraxonian form passes over into the allopolar, and the ground-form becomes the rhombic octahedron or the amphithect double pyramid (compare §§ [33] and [34], and p. [720]). The centroplanar ground-form is entirely wanting in the Acantharia.
49. The Ground-Forms of the Nassellaria.—The Nassellaria all possess monostatic ground-forms, inasmuch as by the very structure of their monopylean central capsule a vertical main axis is necessitated, whose basal pole occupies the porochora. The same arrangement is also for the most part clearly recognisable in the corresponding structure of the skeleton, which is generally either centraxon or centroplanar. Among their manifold skeletal forms different larger groups of ground-forms may be recognised according as the vertical allopolar main axis is crossed by differentiated transverse axes or not (Stauraxonia or Monaxonia); the former are either triradial or multiradial. The triradial, with three lateral or terminal radial apophyses, constitute the greater part of the Nassellaria, and have probably been derived originally from the triradial Plectoidea (Triplagia, Triplecta); a more careful examination, however (especially with reference to the structure of the cortinar septum), reveals the fact that the ground-form is not strictly regularly pyramidal (with three equal radii), but amphipleural (with two paired ventral and one unpaired dorsal radius), and that it usually passes over into a distinctly zygopleural form. The same holds true of the multiradial Nassellaria, where for the most part three interradial or six adradial (sometimes more) apophyses are intercalated between the three primary perradial ones; sometimes here also the ground-form is a quite regular hexagonal or nonagonal pyramid, but usually it is more or less amphithect or amphipleural. Among the eradial Nassellaria, which have no radial apophyses, the ground-form is sometimes allopolar monaxon (conical, ovoid, hemispherical, &c.), sometimes amphithect pyramidal (even in the simplest Stephanida, Archicircus, &c.), or sometimes distinctly zygopleural or bilateral (many Plectellaria).
50. The Ground-Forms of the Phæodaria.—The Phæodaria agree with the Nassellaria in the possession of a primitively centraxon ground-form, and like them are monostatic, since a vertical main axis whose basal pole passes through the astropyle is present, owing to the characteristic structure of their cannopylean central capsule. In the great majority of Phæodaria the spheroidal central capsule also possesses a pair of parapylæ near the opposite apical pole of the main axis (Tripylea), and these determine (as the right and left secondary openings) an isopolar frontal axis. Hence, strictly speaking, in most Phæodaria the central capsule has the geometrical ground-form of the amphithect pyramid (as in the Ctenophora), with an allopolar vertical main axis, and two unequal, but isopolar, horizontal transverse axes. In many Phæodaria the skeleton also has this amphithect pyramidal ground-form, e.g., the bivalved Phæoconchia and part of the Phæogromia. On the contrary, in the rest of the Phæodaria the skeleton exhibits very various geometrical ground-forms, independent of that of the central capsule. In the Phæosphæria it forms preferably spheres or endospherical polyhedra, as also in the Castanellida and Circoporida among the Phæogromia; among the Circoporida there are also seen with remarkable distinctness the regular polyhedra (especially the dodecahedron and icosahedron). Isopolar monaxonia are found among the Aulosphærida (Aulatractus) and Orosphærida; allopolar monaxonia among the Challengerida (Lithogromia). The Medusettida and Tuscarorida show various forms of regular pyramids (allopolar Stauraxonia); and finally, the Challengerida are for the most part centroplanar or bilateral. Thus the Phæodaria present a great wealth of different geometrical ground-forms in the development of their skeleton, not in that of their central capsule.
Chapter II.—THE CENTRAL CAPSULE.
51. Components of the Central Capsule.—In all Radiolaria without exception, at some period of life or other, the central portion of the soft body is separated from the peripheral portion by an independent, anatomically recognisable membrane; this membrane with all its contents is designated the central capsule, and is the peculiar central organ of the unicellular body, which distinguishes the Radiolaria most clearly from the other Rhizopoda. In the great majority of the Radiolaria the volume of the central capsule is less than that of the surrounding peripheral soft body which we place in opposition to it as "extracapsulum." The "capsule-membrane," which separates these two constituents, arises very early in most Radiolaria, and persists throughout their whole life. In some species, however, the membrane only appears later, immediately before the formation of the spores, and hence is absent for a considerable period. Regarded as a whole, then, the capsule consists of the following parts:—(1) the capsule-membrane; (2) the enclosed endoplasm, or intracapsular protoplasm; (3) the nucleus. But in addition, many other non-essential structures may be enclosed in the central capsule, especially hyaline spheres (vacuoles), fatty spheres, pigment granules, crystals, &c.
The central capsule was first described in my Monograph in 1862 (pp. 69-82) as the most characteristic component of the Radiolarian organism, and distinguished from the whole extracapsular soft body. The fact that it has recently been reported as absent by various authors is due to their having observed young or unripe specimens, before the formation of the spores. In some species of Polycyttaria and Acantharia the membrane persists only a very short time.