103. The Physical Properties of the Skeleton.—The skeletons of all Radiolaria are characterised pre-eminently by a high degree of firmness, which fits them to serve as protective and supporting apparatus. This is obvious in the case of the pure siliceous shells of the Polycystina; but the acanthin framework of the Acantharia also possesses a degree of stiffness but little inferior, whilst the silicate skeletons of the Phæodaria seem on the whole to be not so firm. The hollow skeletal tubes of the last-named, which are filled with gelatinous material, are very brittle on account of the delicacy of their walls. Their elasticity also is very small, whilst that of the acanthin spines is considerable. The thin long needles of many Acantharia are very elastic, as are also the bristle-like siliceous spicules of many Spumellaria. The refractive power of the skeleton in the various legions is very different, depending upon the chemical constitution. The siliceous skeleton of the Polycystina (Spumellaria and Nassellaria) and the silicate skeleton of the Phæodaria have the same refractive index as glycerine, and hence become invisible when mounted in that fluid; they then become visible only on addition of water, and are clearer in proportion to the quantity of water which is added. The refractive index of acanthin is, however, very different from that of glycerine, so that the skeletons of Acantharia are readily visible when mounted in this fluid. In water, the skeletons of all Radiolaria appear about equally refractive, as also in Canada balsam. The substance of the skeleton appears almost entirely hyaline, colourless, and transparent. Very rarely it is faintly coloured (in some Acantharia). A cloudy opaque constitution is seen in some Phæodaria (especially in the "porcellanous shells" of Tuscarorida and Circoporida, Pls. [100], [114]-[117]); when dried, these appear by reflected light milky-white or yellowish-white; the cause of this opacity lies partly in the peculiar "cement-like structure" of these porcellanous shells, partly in their fine porosity, and the minute air-bubbles contained in their thick walls.

104. The Elementary Structure of the Skeleton.—The general constitution of the skeleton—or more accurately expressed, of the morphological elements of which the skeleton consists—is of such a nature that it may be termed structureless. Both the organic acanthin skeletons of the Acantharia and the silicate skeletons of the Phæodaria, as well as the inorganic siliceous skeletons of the Spumellaria and Nassellaria, appear under the microscope perfectly homogeneous, transparent, colourless, and crystalline. Only very rarely do they show traces of a concentric striation, which arises from the deposition of the skeletal substance in layers; as, for example, the thick spines of some Phæodaria (Pls. [105]-[107], &c.). Some of the Phæodaria, however, form an exception to this rule, inasmuch as their partially tubular skeletal elements possess a remarkable porcellanous structure. In the tubular or Cannoid skeleton, which occurs in most Cannopylea, the lumen of the thin-walled flinty tube is filled with jelly, and frequently a thin siliceous thread runs in its axis, and is connected with the wall by transverse threads (§§ [127], [139]). The elementary structure of the opaque porcellanous shells, which distinguish the two families Circoporida (Pls. [114]-[117]) and Tuscarorida (Pl. [100]), is quite peculiar. Numerous fine siliceous spicules lie scattered irregularly in a finely granular or porous matrix.

105. Complete and Incomplete Lattice-Shells.—In the great majority of Radiolaria (in all four legions) the skeleton has the form of a delicate lattice-shell or a receptacle in which the central capsule is enclosed. In a small minority, however, this is not the case. The skeleton then consists only of isolated rigid pieces (radial or tangential spicules), or of a simple ring (sagittal ring of the Stephoidea), or of a basal tripod with or without a loose tissue of trabeculæ, &c. (Plectoidea); the central capsule is then not surrounded by a special latticed receptacle, but only rests upon the skeletal trabeculæ. According to these different arrangements, two principal groups or sublegions may be distinguished in each legion, of which one set (Cataphracta) are characterised by a complete lattice-shell, whilst the others (Aphracta) are without it. The Radiolaria aphracta, then, or Radiolaria without a complete skeleton, are the Collodaria (p. [9]), the Acanthometra (p. [725]), the Plectellaria (p. [895]), and the Phæocystina (p. [1543]). On the other hand, the Radiolaria cataphracta, or Radiolaria with a complete skeleton, are the Sphærellaria (p. [49]), the Acanthophracta (p. [791]), the Cyrtellaria (p. [1015]), and the Phæocoscina (p. [1590]).

Upon this basis the first subdivision of the Radiolaria was made by Johannes Müller, who recognised three groups:—"I. Thalassicolla, without receptacle, naked or with spicules; II. Polycystina, with a siliceous receptacle; III. Acanthometra, without receptacle, but with siliceous radial spines" (L. N. [12], p. 16).

106. The Ectolithia and Entolithia (Extracapsular and Intracapsular Skeletons).—The relation of the skeleton to the central capsule in the Radiolaria is very various in many respects; in the first instance two great groups, Ectolithia and Entolithia (see note A), may be distinguished topographically by mere external observation; in the former the skeleton lies entirely outside the central capsule; in the latter, partially at all events, within it. The Ectolithia, with a completely extracapsular skeleton, include all Nassellaria and Phæodaria, as well as a great part of the Spumellaria (all Collodaria and the most archaic forms of Sphærellaria); the Entolithia, on the other hand, in which the skeleton lies partly within, partly without the central capsule, include all Acantharia and the majority of the Spumellaria (most Sphærellaria, see note B).

A. The difference between Ectolithia and Entolithia was applied in my Monograph in 1862 (p. 222) to separate the Monocyttaria into two main groups. The arrangement was, however, quite artificial, being contrary to the natural relations of the larger groups, as was shown seventeen years later by the discovery of the different structural relations of the central capsule.

B. Among the Acantharia, which all possess primitively an intracapsular and centrogenous skeleton, the remarkable Cenocapsa (Pl. [133], fig. 11), seems to furnish the single exception; in it the skeleton consists of a simple spherical shell which encloses the concentric central capsule. The exception is, however, only apparent; the twenty perspinal pores of the shell show that they were originally in connection with twenty centrogenous acanthin spines, and that those have disappeared by retrograde metamorphosis.

107. Perigenous and Centrogenous Skeletons.—Much more important than the topographical relation of the skeleton to the central capsule, according to which the Ectolithia and Entolithia are separated from each other (§ [106]), is the original development of the skeleton within or without the central capsule, which gives rise to the distinction between perigenous and centrogenous skeletons. Centrogenous skeletons are found only in the Acantharia, which are further distinguished from all other Radiolaria by their skeleton being formed of acanthin; in all Acantharia the formation of the skeleton begins in the middle of the central capsule, from which twenty (the number is inconstant only in the small group Actinelida) radial spines are centrifugally developed. The three other legions, on the contrary, possess on the whole a perigenous skeleton, which originally develops outside the central capsule and never in its middle. In the Nassellaria and Phæodaria the skeleton retains this extracapsular position, as also in the Beloidea and part of the Sphærellaria among the Spumellaria; in the great majority of the latter, however, the primary perigenous skeleton is subsequently enveloped by the growing central capsule, so that it lies partially within it (§ [109]).

108. Polyphyletic Origin of the Skeleton.—The skeleton of the Radiolaria has undoubtedly originated polyphyletically, for it is impossible to derive its manifold varieties from a single ground-form, or to regard them as modifications of one type. It is much more probable that the different skeletonless Radiolaria have entered upon different ways of skeleton formation quite independently of each other. At the outset it is quite clear that the skeletons of the four legions have originated independently of each other. Further, it is certain that within the legion of the Spumellaria the Beloid skeletons of the Collodaria are not connected with the Sphæroid skeletons of the Sphærellaria and the forms derived from them (see § [109]). In the same way the skeletons of the Phæodaria are polyphyletic; probably in this legion the Beloid, Sphæroid, Cyrtoid, and Conchoid skeletons have been developed quite independently (see § [112]). In the Nassellaria, on the other hand, it is possible that all the skeletal forms are to be derived monophyletically from a single simple primitive form (either the sagittal ring or basal tripod?) (see § [111]). Still more probable is it that the Acantharia have arisen monophyletically, for all the forms of their acanthin skeleton may be derived without violence from Actinelius (see § [110]).

109. The Skeleton of the Spumellaria.—The skeletons of the Spumellaria or Peripylea consist of silica, and are very different and of independent origin in the two orders of this legion. The first order, Collodaria, have either no skeleton whatever (Colloidea, p. [10], Pls. [1], [3]), or their skeleton is Beloid, a loose extracapsular envelope of spicules, consisting of numerous unconnected portions; the separate parts are usually disposed tangentially, either as simple or compound siliceous spicules (Beloidea, p. [28], Pls. [2], [4]). The second order of Spumellaria, on the other hand (Sphærellaria, p. [49]), develops a siliceous lattice-shell which consists of a single piece, and is remarkable for the extraordinary variety of its forms (pp. [50]-[715], Pls. [5]-[50]). To this order belong not less than three hundred genera and seventeen hundred species of the Challenger Radiolaria (that is, about two-fifths of all the genera and species). In spite of this extreme richness in different forms this large group must be regarded as monophyletic, since all its forms may be quite naturally derived from a common stem-form, a simple lattice-sphere (Cenosphæra, p. [61], Pl. [2]). The twenty-eight families of Sphærellaria may be distributed in four suborders, among which the Sphæroidea constitute the stem-forms, since they retain the original spherical shape (Pls. [5]-[8], [11]-[30]). In the other three suborders a vertical main axis is developed, which in Prunoidea is longer, in Discoidea shorter than the other axes of the shell. Hence the shell of the Prunoidea (p. [284], Pls. [13], bis, [17], [39], [40]) is ellipsoidal or cylindrical, that of the Discoidea, on the other hand, lenticular or discoidal (p. [402], Pls. [31]-[38], [41]-[48]). Finally, the shell of the fourth suborder, Larcoidea, is lentelliptical; it has the ground-form of a triaxial ellipsoid, and is characterised by the possession of three unequal dimensive axes, or three isopolar axes of different lengths perpendicular to each other (p. [599], Pls. [9], [10], [49], [50]).