110. The Skeleton of the Acantharia.—The skeletons of the Acantharia or Actipylea are distinguished from those of all other Radiolaria by two very important peculiarities; in the first place, they consist not of silica but of a peculiar organic substance, Acanthin, and secondly, their development is centrogenous, numerous radial spines or acanthin spicules being formed which are united in the middle of the central capsule. Hence the Acantharia are the only Radiolaria in which the skeleton originates from the first in the middle of the central capsule. The number of radial spines is primitively indefinite, variable, and often considerable (more than a hundred), but in the great majority it is limited to twenty. In accordance with this the legion may be divided into two orders, the more archaic small group Adelacantha, with an indefinite number of spines, and the more recent group, Icosacantha, which has been developed from them and possesses twenty regularly disposed spines; of the three hundred and seventy-two species of Acantharia which have been hitherto described, about five per cent. belong to the former, about ninety-five per cent. to the latter division (see note A, below). The numerous genera of Icosacantha may then be again divided into two suborders, of which the Acanthonida (p. [740], Pls. [130]-[132]) produce no complete lattice-shell, and thus agree with the Actinelida, with which they may be united as Acanthometra in the broader sense (or Acantharia without a lattice-shell). The Acanthophracta, on the other hand (p. [791], Pls. [133]-[140]), produce a complete lattice-shell, usually by means of two opposite or four crossed transverse processes, which arise from each radial spine and unite with each other (see note B, below). In most Acanthophracta the lattice-shell remains single; only in the Phractopeltida does it consist of two concentric lattice-spheres (p. [847], Pl. [133], figs. 1-6). Furthermore, the whole order Acanthophracta may be subdivided into two suborders according to the different ground-form of the lattice-shell; this remains spherical in the Sphærophracta (the three families Sphærocapsida, Dorataspida, Phractopeltida, Pls. [133]-[138]). On the other hand, it assumes another form in the Prunophracta; it becomes ellipsoidal in the Belonaspida (Pl. [136], figs. 6-9), discoidal or lentiform in the Hexalaspida (Pl. [139]); and finally takes the shape of a double cone in the Diploconida (Pl. [140]).
A. The group Adelacantha consists only of the suborder Actinelida, with the three families Astrolophida, Litholophida, and Chiastolida (p. [728], Pl. [129], figs. 1-3); the number of the radial spines is very different and variable, sometimes only from ten to sixteen, but usually from thirty to fifty, and often more than one hundred; they are generally irregularly distributed, and not as in the second main division. This latter, the Icosacantha, always possesses twenty radial spines, which are regularly disposed according to a constant law, the so-called "Müllerian" or "Icosacanthan" law; the twenty spines are always so placed between the poles of a spineless axis that they form five zones each of four spines; the four spines of each zone are equidistant from each other, and also from the same pole, and alternate with those of the neighbouring zones, so that the whole twenty lie in four meridian planes, which cut out an angle of 45° (compare pp. [717]-[722], Pls. [130]-[140]). In spite of the manifold variations in form which are developed in the Icosacantha, they may all be derived from a common stem-form, Acanthometron (p. [742]), since the law of distribution of the twenty spines is constantly inherited.
B. An exception is found in the peculiar family Sphærocapsida (p. [797], Pl. [133], figs. 7-11; Pl. [135], figs. 6-10). Here the shell is composed of innumerable small, perforated plates, which arise on the surface of the calymma independently of the spines.
111. The Skeleton of the Nassellaria.—The skeletons of the Nassellaria or Monopylea consist of silica, and are never composed of separate portions, but constitute always a single continuous piece. The ground-form is originally monaxon, corresponding to that of the central capsule, with a constant difference between the two poles of the vertical main axis. The ground-form is never spherical or polyaxon as in the lattice-shells of the Spumellaria, and the skeleton never consists of hollow tubes, as in the Phæodaria. The legion Nassellaria may be divided into two orders; in the Plectellaria (three suborders Nassoidea, Plectoidea, Stephoidea) the skeleton does not form a complete lattice-shell; in the Cyrtellaria, on the other hand, which are derived from these, the siliceous skeleton forms a complete lattice-shell enclosing the central capsule. The number of forms thus developed is astonishingly great, so that among the Nassellaria no less than two hundred and seventy-four genera and sixteen hundred and eighty-seven species may be distinguished, almost as many as in the Sphærellaria. In spite of this great variety of forms the legion Monopylea is probably monophyletic; at least all the different skeletal forms may be derived from three elements which are combined in the most manifold fashion; (1) the sagittal ring, a simple siliceous ring, which lies vertically in the sagittal plane of the body, encircles the central capsule and comes into contact with it at the basal pole of the main axis (§ [124]); (2) the basal or oral tripod, composed of three diverging radial spines, which meet in the middle of the basal pole of the central capsule (or in the centre of the porochora) (§ [125]); (3) the cephalis, or lattice-head, a simple ovoid or subspherical lattice-shell, which encloses the central capsule and stands in connection with it at the basal pole of its main axis. Any one of these three important structural elements of the Nassellarian skeleton may possibly be the starting-point for all the remaining forms of the Monopylea; the great difficulty in their phylogenetic derivation lies in the facts that, on the one hand, any one of the three elements may alone constitute the skeleton, and on the other hand, in the great majority of the legion, two or three are united together (compare §§ [182]-[185]).
112. The Skeleton of the Phæodaria.—The skeleton of the Phæodaria or Cannopylea is always extracapsular, usually consists of a silicate of carbon (more rarely of pure silica), and in the majority of the legion is composed of hollow cylindrical tubes, whose siliceous wall is very thin, and whose lumen is filled with gelatinous material (§ [127]). The manifold and remarkable skeletal forms occurring in this legion are not monophyletic, since they cannot be derived from a common stem-form; they are, on the contrary, polyphyletic, various skeletonless Phæodaria (Phæodinida) have independently acquired skeletons of different form and composition. The legion Phæodaria can be subdivided into four orders, the skeletons of which present the following important distinctions:—(1) The Phæocystina possess only incomplete Beloid skeletons (§ [115]), composed of many separate pieces, sometimes tangentially (Cannorrhaphida, Pl. [101]), sometimes radially arranged (Aulacanthida, Pls. [102]-[105]). (2) The Phæosphæria form Sphæroid skeletons (§ [116]), usually only a simple lattice-shell without special aperture (Pls. [106]-[111]); two concentric shells united by radial bars occur only in the Cannosphærida (Pl. [112]). (3) The Phæogromia are distinguished by the formation of a simple Cyrtoid skeleton (§ [123]) resembling that of the Monocyrtida; the monothalamus lattice-shell is usually ovoid or helmet-shaped, more rarely polyhedral or almost spherical; a vertical main axis can always be distinguished, at the basal pole of which is an aperture usually armed with teeth or spines (Pls. [99], [100], [113]-[120]). (4) The Phæoconchia are distinguished from all other Radiolaria by the possession of a bivalved shell like that of the Conchifera; the two valves of this Conchoid skeleton must be distinguished as dorsal and ventral, as in the Brachiopoda (Pls. [121]-[128]). The fifteen families of Phæodaria which are arranged in the four orders just mentioned, present such great differences among themselves, that the skeleton must be regarded as probably polyphyletic even within the limits of each order.
113. Types of Skeletal Formation.—No less than twelve different principal forms may be distinguished as morphological types of the formation of the skeleton in the Radiolaria; some of these are peculiar to a single legion or even to a smaller group; but sometimes the same form occurs in several legions. Some types occur only in an isolated manner, independently of the others, but most exist in various combinations with other types. Of the twelve described below the Conchoid and Cannoid occur only in the Phæodaria; the Plectoid and Circoid only in the Nassellaria; the Astroid only in the Acantharia; the remaining seven types are found in several legions in the same form and hence are polyphyletic.
114. The Astroid Skeleton.—Under the name "Astroid" we place the peculiar star-shaped skeletons of the Acantharia in opposition to those of all other Radiolaria, for they are separated from them not only fundamentally by reason of the chemical nature of their substance (Acanthin, § [102]), but also by their centrogenous origin, and the resulting stellate form (Pls. [129]-[140]). The Acantharia are the only Radiolaria in which the skeleton arises within the central capsule by the formation of numerous rays or radial spines of acanthin which project on all sides from the centre. Originally these are united at this point, their conical or pyramidal points meeting and being supported one upon another. In the great majority of Acantharia this loose apposition is constant, so that when the soft parts are destroyed the skeleton falls to pieces. Only in a few forms in this legion are the central ends of the spines fused so that the whole skeleton forms a connected star (Astrolithium). The small group Chiastolida (or Acanthochiasmida) is characterised by the fact that the two rays which are opposite to one another in each axis unite and form a diametral bar. The skeleton is almost always composed of twenty radial spines, which are regularly disposed (Icosacantha), only in the small primitive group Actinelida is the number variable (Adelacantha, § [110]).
115. The Beloid Skeleton.—As Beloid or spicular skeletons are grouped together all those which consist of several disconnected portions; these always lie outside the central capsule, either within the calymma or on its surface. Such extracapsular Beloid skeletons are entirely wanting in the Acantharia and Nassellaria; they occur only in the Beloidea among the Spumellaria, and in the Phæocystina among the Phæodaria; the individual Beloid portions of the former are solid, those of the latter hollow. In both groups the simplest forms of the separate portions are simple unbranched needles (Thalassosphæra, Thalassoplancta, Physematium, Belonozoum, among the Spumellaria; Cannobelos and Cannorrhaphis among the Phæodaria); usually these spicules are disposed tangentially over the surface of the calymma. Among the Beloidea branched spicules occur more commonly than these simple ones; they are either stellate (with many rays united in a centre) or twin-like, with a tangential bar, from each pole of which two or three (seldom more) radial branches project (Pls. [2], [4]). Among the Phæodaria the subfamily Dictyochida is characterised by the annular shape of its Beloid portions, either simple rings, or hat-shaped or pyramidal bodies with a latticed cap over the ring (Pl. [101], figs. 3-14; Pl. [114], figs. 7-13). The family Aulacanthida among the Phæodaria, alone possesses hollow radial tubes, which penetrate the whole calymma, and project distally over its surface, whilst their proximal ends rest upon the surface of the central capsule. Although in these cases the enclosed proximal end is always simple, the free distal end develops the most various processes in adaptation to its prehensile functions (Pls. [102]-[105]).
116. The Sphæroid Skeletons or Lattice-Spheres.—The "lattice-spheres" or sphæroid skeletons are the simplest and most primitive forms of lattice shells, and are widely distributed in the three legions Spumellaria, Acantharia, and Phæodaria, whilst they are entirely wanting in the Nassellaria. The round lattice-shell is either a true sphere in the geometrical sense, or an endospherical polyhedron, i.e., a polyhedron, all whose angles lie in the surface of a sphere (§ [25]). In general, primary and secondary lattice-spheres may be distinguished, of which the former are secreted on the outer surface of the primary, the latter on that of the secondary calymma (§ [85]). Furthermore, simple and compound lattice-spheres may be distinguished, the latter of which consist of two or more concentric lattice-spheres firmly united by radial bars; in such cases the innermost lattice-sphere is always to be regarded as the oldest or primary, all the succeeding ones as secondary, and the outermost as the youngest (§ [129]). The simple lattice-spheres are usually to be regarded as primary; they may, however, occasionally be secondary, in which case the primary shell, originally enclosed, has been lost by degeneration (as, for example, in the case of the Aulosphærida and some Sphærellaria).
117. The Lattice-Spheres of the Spumellaria.—The lattice-spheres or Sphæroid skeletons of the Spumellaria exhibit in spite of their simple type of structure, an extraordinary variety in the formation of the lattice-work and radial apophyses, so that in the systematic portion of this work no less than one hundred and seven genera and six hundred and fifty species are distinguished; these are united in one suborder, the Sphæroidea (pp. [50]-[284], Pls. [5]-[8], [11]-[30]). It may be divided into two main divisions, the Monosphærida with a single primary lattice-sphere (Pls. [12]-[14], [21], [26], [27]), and Pliosphærida (or Sphæroidea concentrica) whose skeleton consists of two or more concentric lattice-spheres united by radial bars. The latter are subdivided into Dyosphærida with two concentric lattice-spheres (Pls. [16], [19], [20], [22], [28]); Triosphærida, with three spheres (Pls. [17], [24], [29]); Tetrasphærida, with four (Pls. [23], [30]); Polysphærida, with five or more (Pls. [15], [23]); and Spongosphærida, with spongy lattice-spheres (Pls. [18], [25]). A special group is made up of the simple lattice-spheres of the social Collosphærida (or Sphæroidea polyzoa) (Pls. [5]-[8]); these are usually more or less irregular, and characterised by the development of peculiar tubular processes; the latter are generally wanting in the Sphæroidea monozoa, whose lattice-shell is very regularly formed. This distinction is interesting and important, inasmuch as the regular lattice-spheres are explained by the independent development of the free-swimming Monozoa, whilst the irregular spheres are due to the mutual dependence of the social Polyzoa.