118. The Lattice-Spheres of the Acantharia.—The lattice-shells or Sphæroid skeletons of the Acantharia are immediately distinguishable from those of all other Radiolaria by their centrogenous development and the central union of the radial spines by which they are supported; the only exception is furnished by the remarkable genus Cenocapsa (Pl. [133], fig. 11), in which the radial spines are absent, not primitively, however, but in consequence of degeneration; for the twenty cross-shaped perspinal pores, originally due to the twenty radial spines, are still present. In the most nearly allied genera, Porocapsa (Pl. [133], fig. 7) and Cannocapsa (Pl. [133], fig. 8), the proximal part of the twenty radial spines is still present, while their distal portion has degenerated; hence in this case they do not stand in direct communication with the spherical shell. On the other hand, this primitive connection persists in the genera Astrocapsa (Pl. [133], figs. 9, 10), and Sphærocapsa (Pl. [135], figs. 6-10). The five genera just mentioned form the peculiar family Sphærocapsida (pp. [795]-[802]); the spherical shell is in these cases composed of very numerous small plates disposed like a pavement, each plate or aglet being perforated by a pore canal; in addition to which there are twenty larger (perspinal) pores (or twenty cross-shaped groups each of four aspinal pores) at those important points where primitively the twenty radial spines penetrate the calymma. This peculiar porous "pavement shell" has probably been developed (independently of the twenty radial spines) upon the calymma of the Acanthonida (Acanthonia, p. [749]) by the action of the sarcodictyum; it has, therefore, quite a different morphological significance from the spherical lattice-shell of the Dorataspida, which is composed of tangential apophyses of the twenty Acanthonid spines (pp. [802]-[847], Pls. [134]-[138]). Each radial spine here forms either two opposite or four crossed transverse processes, and since their branches spread over the surface of the spherical calymma and are united suturally at their extremities, the peculiar lattice-sphere of the Dorataspida arises. This extensive family is again divided into two subfamilies:—the Diporaspida (Pls. [137], [138]) possess always only two opposite apophyses, and form by the union of their branches two opposite primary apertures or aspinal meshes. The Tessaraspida, on the other hand (Pls. [135], [138]), have always four crossed transverse processes, and form by their union four primary aspinal meshes. From the Diporaspida are probably to be derived the Phractopeltida (p. [847], Pl. [133], figs. 1-6), the only Acantharia which possess a double lattice-sphere; their double concentric spherical shell may be compared with that of the Dyosphærida.
119. The Lattice-Spheres of the Phæodaria.—The lattice-spheres or Sphæroid skeletons of the Phæodaria, which are generally developed quite regularly, though occasionally in a modified form, fall in the order Phæosphæria into two groups of very different structure, each of which includes two families. The first group (Phæosphæria inarticulata) contains the families Orosphærida (Pls. [106], [107]) and Sagosphærida (Pl. [108]); the lattice-work of the former consists of irregular polygonal meshes and very coarse, partially hollow trabeculæ; in the latter, on the other hand, it consists of triangular meshes and very slender filiform trabeculæ; in both families the whole sphæroid skeleton forms a single unsegmented piece as in most Sphæroidea. In the second group of Phæosphæria (Phæosphæria articulata), on the other hand, the lattice-sphere is segmented in quite a peculiar manner, and composed of hollow cylindrical tangential tubes, which are separated by astral septa at the nodal points of the network; this remarkable structure characterises the two families, Aulosphærida (Pls. [109]-[111]) and Cannosphærida (Pl. [112]); the segmented lattice-sphere of the former is simple and hollow; while that of the latter is connected by centripetal radial tubes with a simple concentric inner shell, which is sometimes solid, sometimes latticed, and provided with a main-opening corresponding to the astropyle of the enclosed central capsule. Since in the Aulosphærida also, hollow centripetal radial tubes project from the segmented lattice-sphere, it is possible that they have been derived from the Cannosphærida by the loss of the primitive internal shell. A special peculiarity of many Phæosphæria (Oroscena, Sagoscena, Auloscena, &c.) consists in the fact that the whole surface of the lattice-sphere is regularly covered with pyramidal or tent-shaped prominences (Pl. [106], fig. 4; Pl. [108], fig. 1; Pl. [110], fig. 1). A simple lattice-sphere quite similar to that of most Monosphærida also constitutes the skeleton of the Castanellida (Pl. [113]), but since it possesses a special main-opening, it must be referred promorphologically to the Cyrtoid shells of the Phæogromia.
120. The Prunoid Skeleton or Lattice-Ellipsoid.—The "lattice-ellipsoids" or Prunoid skeletons have arisen from the lattice-spheres or Sphæroid skeletons by more energetic growth and elongation of one axis; this is the main axis of the body and is probably always vertical; its two poles are commonly equal. The Prunoid skeleton is either a true ellipsoid in the geometrical sense or an "endellipsoidal polyhedron" (i.e., a polyhedron, all the angles of which lie in an ellipsoidal surface). By further elongation of the main axis, the ellipsoidal form passes over into the cylindrical, the polar surfaces of the cylinder being usually rounded, rarely truncated. The rich order Prunoidea (pp. [284]-[402]) contains numerous modifications of this form of shell which arise on the one hand by the formation of transverse constrictions, on the other by the apposition of concentric secondary shells. In respect of the latter, simple and compound Prunoid shells can be distinguished as in the case of the Sphæroid shells. In the compound Prunoid shells either all the concentric lattice-shells may be ellipsoidal or the inner may be spherical. More important differences are found in the transverse annular constrictions, which give the Prunoid skeleton a segmented appearance; in this respect, three principal forms may be distinguished (p. [288]):—(A) Monoprunida, with unsegmented shell, having no transverse constriction (Pls. [15]-[17]); (B) Dyoprunida, having a shell with two segments and one (equatorial) transverse constriction (Pl. [39]); (C) Polyprunida, with three or more parallel transverse constrictions, by means of which the shell is divided into four or more segments (Pl. [40]). In the same manner as the Prunoidea have arisen from the Sphæroidea among the Spumellaria by greater development of the vertical main axis, the ellipsoidal Belonaspida have arisen from the spherical Dorataspida among the Acantharia (p. [859]; Pl. [136], figs. 6-9; Pl. [139], figs. 8, 9). The main axis of the ellipsoid in this case is always occupied by the opposite equatorial spines of the hydrotomical axis (pp. [719], [860]). In the legion Phæodaria a similar prolongation of the main axis rarely occurs; it is found, however, in Aulatractus (Pl. [111], figs. 6, 7), the lattice-shell of this Aulosphærid being sometimes truly fusiform, sometimes rather ellipsoidal or even double-conical.
121. The Discoid Skeletons or Lattice-Discs.—The "lattice-discs" or Discoid skeletons are characteristic of the Spumellarian group Discoidea, and have arisen from the lattice-spheres of the Sphæroidea by a less development of one axis, which is the main axis of the body, and is probably usually vertical; its two poles are always equal. The Discoid lattice-shell is either a biconvex lens (with a thin margin), or a plane disc (a shortened cylinder with thick margin), or some form intermediate between the two. All Discoid shells show a horizontal median plane or equatorial plane, by which they are divided into two equal halves, an upper and lower; the margin of the lens itself is originally the equator. The main axis, the shortest of all the axes of the shell, stands vertically in the centre of the equatorial plane. Among the Phæodaria Discoid shells rarely occur (Aulophacus), as also among the Acantharia (Hexalaspida).
122. The Larcoid Skeleton or Lentelliptical Lattice-Shell.—The lentelliptical lattice-shells, which may be shortly designated "Larcoid," are especially characteristic of the Larcoidea, a large order of Spumellaria (pp. [599]-[715]; Pls. [9], [10], [49], [50]). In addition they recur among the Acantharia, in the small family Hexalaspida (p. [872], Pl. [139]), and the family Diploconida (p. [881], Pl. [140]), which is derived from it. These lentelliptical lattice-shells are all characterised by the clear differentiation of three unequal, but isopolar dimensive axes, i.e., the three geometrical axes, perpendicular to one another, which determine the form of the shell, are of unequal length; the two poles of each are, however, equal. The geometrical ground-form is, therefore, a triaxial ellipsoid (§ [34]). In the rich order Larcoidea the lentelliptical lattice-shell shows many variations in its development.
123. The Cyrtoid Skeleton.—Cyrtoid skeletons are those lattice-shells which possess a vertical main axis with two different poles (Monaxonia allopola); the upper pole is usually termed the apical, the lower the basal. Such Cyrtoid shells are characteristic of the great majority of the Nassellaria or Monopylea (and especially of the Cyrtellaria); they are also found in a large division of the Phæodaria (the Phæogromia), and in some Spumellaria. In general the manifold Cyrtoid shells may be divided into two large groups, those with one and those with several chambers. The monothalamous Cyrtoid shells are usually ovoid, conical, cap- or helmet-shaped; their internal cavity is simple, without constrictions or septa. Among the Nassellaria they occur in the Monocyrtida (Pls. [51]-[54], [98]), where they have received the name "Cephalis." A form of shell, essentially the same, is found amongst the Phæodaria in the order Phæogromia, more especially in the Challengerida (Pl. [99]), Medusettida (Pls. [118]-[120]), and Tuscarorida (Pl. [100]), many of these latter closely resembling many Monocyrtida. Such monothalamous Cyrtoid shells occur much more rarely among the Spumellaria (e.g., among the Prunoidea in Lithapium, Lithomespilus, Druppatractus, Pls. [13], [14], &c.). Polythalamous Cyrtoid shells (Pls. [55]-[80]) occur exclusively in the Nassellaria, and exhibit in this legion an astonishing variety of structure; they are distinguished from the monothalamous forms by the development of internal septa, or of annular incomplete diaphragms, which usually correspond to the external constrictions; their interior is thus divided into two or more communicating compartments. Among the polythalamous Cyrtoid shells may be distinguished three principal groups, the Stichocyrtid, Zygocyrtid, and Polycyrtid. Zygocyrtid shells are characteristic of the Spyroidea (Pls. [84]-[90]), and are distinguished by a bilobate cephalis (cephalis bilocularis); the median sagittal ring, or a corresponding constriction, divides the shell into right and left compartments. Polycyrtid shells (Pl. [96]) are peculiar to the Botryodea, and characterised by a multilobate cephalis (cephalis multilocularis). Stichocyrtid shells are those in which the primary cephalis remains simple, and new joints are successively added to its basal pole; such shells occur in the majority of the Cyrtoidea. Secondary chambers are sometimes added in the other two groups (Botryodea and Spyroidea). When, as often happens in these polythalamous Cyrtoid shells, two or three distinct joints follow each other, the first is called the "cephalis," the second the "thorax," and the third the "abdomen" (Tricyrtida Pls. [64]-[75]).
124. The Circoid Skeleton.—This is a very important and remarkable type of skeletal formation, which occurs exclusively in the legion Nassellaria, where it plays a very prominent part; its characteristic element is the "sagittal ring," a simple, vertical, siliceous ring, which surrounds the central capsule in its sagittal plane, and is specially differentiated in its basal portion. This "primary sagittal ring" whose vertical allopolar main axis coincides with that of the Monopylean central capsule embraced by it, is characteristic of all members of the order Stephoidea (p. [931], Pls. [81]-[83], [92]-[94]); here it forms by itself the skeleton of the Stephanida (Pl. [81]); in the Semantida (Pl. [92]) it is combined with a horizontal basal ring, in the Coronida (Pls. [82], [93]) with a vertical frontal ring and in the Tympanida (Pls. [83], [94]) with two horizontal rings, an upper mitral and a lower basal. In the great majority of these Stephoidea there often develop in definite places characteristic processes or apophyses, whose branches combine to form a loose tissue or an incomplete lattice-shell. This becomes complete in the Cyrtellaria, the majority of which retain more or less distinct traces of the sagittal ring. Hence the skeletons of all Nassellaria may be derived monophyletically (Hypothesis A, p. [893]) from a simple sagittal ring (Archicircus and Lithocircus, Pl. [81]). This theory, however, encounters the great difficulty that in many Stephoidea (Cortina, Cortiniscus, &c.) it is combined in a remarkable manner with the basal tripod of the Plectoidea, whilst in these latter it is entirely wanting (compare p. [894]).
125. The Plectoid Skeleton.—Those forms are distinguished as Plectoid in which three, four, or more radial siliceous spines proceed from a common point, which lies excentrically outside the central capsule and at the basal pole of its vertical allopolar main axis. This peculiar type of skeletal formation only occurs in the legion Nassellaria, and is specially characteristic of the order Plectoidea (p. [898], Pl. [91]). But since the essential elements of this remarkable skeleton also occur in many other Nassellaria, sometimes combined with the Circoid, sometimes with the Cyrtoid skeleton, it perhaps has a fundamental significance in this legion; at all events it is possible to derive monophyletically all the other forms of this legion from it (Hypothesis B, p. [893]). The simplest form of the Plectoid skeleton is a tripod, the three feet of which either lie in a horizontal plane (Triplagia, Pl. [91], fig. 2), or correspond to the three edges of a low pyramid (Plagiacantha). A fourth ray is sometimes added, which stands vertically upon the summit of the pyramid (Plagoniscus, Plagiocarpa, Pl. [91], figs. 4, 5). In other Plectoidea three secondary rays are intercalated between the three primary (Hexaplagida, &c.); seldom the number is greatly increased (Polyplagida, &c.). The rays are rarely simple, but usually branched; in the Plagonida (Pl. [91], figs. 2-6) the branches remain free; in the Plectanida (Pl. [91], figs. 7-13) they are united to form a loose wicker-work. From such a web a perfect Cyrtoid shell may arise. Several forms of Plagonida may also be readily confounded with the isolated triradiate or quadriradiate spicula of many Beloid skeletons (Sphærozoum, Lampoxanthium, &c.).
126. The Spongoid Skeleton.—From the simple lattice-skeleton which the majority of Radiolaria possess, some of them develop a spongy shell; the trabeculæ of the lattice-work, situated in one plane in the former, are developed in the latter in different planes and cross irregularly in all directions; thus arises a kind of wicker-work of more or less spongy structure, usually with very thin trabeculæ and irregular meshes. Such Spongoid shells are most common among the Spumellaria, especially in the Sphæroidea (Spongosphærida, Pl. [18]) and Discoidea (Spongodiscida, Pls. [41]-[47]), more rarely in the Prunoidea and Larcoidea. Lattice-work of similar spongy structure occurs very seldom among the Nassellaria, e.g., in some Plectoidea (Pl. [91]) and Cyrtoidea (Spongocyrtis, Spongopyramis, Spongomelissa, &c., Pl. [56], fig. 10; Pl. [64], figs. 5-10, &c.). Among the Phæodaria spongy skeletons are very rare; they are to be seen in some Phæosphæria (Oroplegma, Pl. [107], fig. 1; Sagoplegma, Pl. [108], fig. 2; Auloplegma, Pl. [111], fig. 8). No Spongoid skeletons are known among the Acantharia.
127. The Cannoid Skeleton.—Cannoid or tubular skeletons are those which are composed of hollow tubes; they occur exclusively in the Phæodaria or Cannopylea. Tubular processes, nevertheless, occur in some other Radiolaria, as, for example, among the Spumellaria in a portion of the Collosphærida (Siphonosphæra, Caminosphæra, Pls. [6], [7]), and of the Prunoidea (Pipetta, Cannartus, &c., Pl. [39], figs. 6-10, &c.), also among the Nassellaria in Theosyringium (Pl. [68], figs. 4-6), Cannobotrys (Pl. [96], figs. 3, 4, 8-11, 20-22), &c. In all these cases, however, the tubes are direct processes of the cavity of the shell, the trabeculæ of the lattice-work being solid. Only in the Cannopylea are the lattice-bars themselves, the radial spines and appendicular organs, generally tubular (hence the designation "Pansolenia"). The lumen of the thin-walled siliceous tubes is filled with jelly, and hence the specific gravity of the relatively large skeleton is considerably diminished. This peculiarity is not found in all Cannopylea; it is wanting in all Sagosphærida and Concharida, as well as in a part of the Orosphærida and Castanellida; in the latter there are found intermediate stages between hollow and solid skeletal rods. Very often a fine siliceous thread runs in the axis of the tubes, which is connected with its wall by lateral branches (Pl. [110], figs. 4, 6; Pl. [115], figs. 6, 7). More seldom the tubes are divided by horizontal septa into a series of chambers (Medusettida, Pls. [118]-[120]). The two families Aulosphærida (Pls. [109]-[111]) and Cannosphærida (Pl. [112]) are distinguished from all other Phæodaria by the fact that their tubes are separated by astral septa in the nodal points of the lattice-shell (§§ [112], [134]).