Already the central bases, by which the twenty spines are united in the centre of the sphere, exhibit certain differences in the five zones. Commonly these bases are small pyramids, all meeting with their apex in the centre, and the triangular faces of the neighbouring pyramids are supported one upon another. The four equatorial pyramids are commonly six-sided, the other sixteen five-sided; but sometimes there are eight six-sided and twelve five-sided basal pyramids; two opposite polar spines on each pole having a six-sided base (like the four equatorial), the other two polar spines on each pole having a five-sided base (like the eight tropical). Rarely the central bases are perfectly grown together, forming a single spherical central piece of acanthin.
The three different fundamental forms of radial spines, which are found in all Acantharia, the cylindrical, the two-edged, and the four-edged (spines with circular, with elliptical, and with square transverse section respectively) occur also in the different groups of Dorataspida; but commonly the two-edged or compressed form is prevalent in the Diporaspida, the four-edged or quadrangular form in the Tessaraspida. In the majority of species the spines are thickened in the shell-face, where the apophyses arise, and thinner towards the two ends. Usually the outer or distal part of the spine (outside the shell) is longer than the inner or proximal part (inside the shell). The distal apex is commonly simple, conical or pyramidal, rarely bifid or truncate. The edges of the spines are commonly smooth, rarely denticulate or serrate.
The apophyses, or the lateral transverse processes of the radial spines, in the Dorataspida assume the greatest variety and complexity in form, size, mode of ramification, and in composition of the shell. An expert and practised observer may determine easily the range of each spine, whether it be an equatorial (c), or a tropical (b, d), or a polar spine (a, e, Pls. [133]-[138]). The two opposite apophyses of the Diporaspida, as well as the four crossed apophyses of the Tessaraspida, lie constantly in certain meridian planes of the spine, which have a legitimate signification for each of the five zones. The comparative morphology of this regular disposition of the apophyses and the regular meeting of their branches is of the greatest interest, and necessary for the complete understanding of the complicated structure of these wonderful shells.
The pores or meshes of the spherical shell, offering the most varied forms, may generally be divided into two different groups, into sutural and parmal meshes. The sutural pores are bordered by the meeting branches of the apophyses of two, three, or four neighbouring spines, and therefore also by the sutures in which they meet. The parmal pores on the other hand are bordered only by the united branches of the apophyses of a single spine and pierce the shield or lattice-plate formed by them. Therefore the shell-meshes of the Cladophracta are all sutural pores (Pl. [137], figs. 1-8; rarely and only in a part of the spines parmal pores also: Zonaspis, Dodecaspis); whereas the shell-meshes of the Peltophracta, piercing the shields or lattice-plates of all twenty spines, are always partly sutural, partly parmal pores (Pls. [135], [136], [138]). The parmal pores again may be divided into two different groups—aspinal and coronal pores. Aspinal pores ("ad spinam") are those which lie immediately on the sides of the radial spine and are bordered by the primary branches of its apophyses; therefore constantly only two in the Diporaspida, four in the Tessaraspida. Coronal pores on the contrary are those which lie in the periphery of the lattice-plates, surrounding in a circle or crown the aspinal pores and not touching the spine itself. In Dorataspis, Ceriaspis, Tessaraspis, Lychnaspis, &c., all parmal meshes are only aspinal pores (Pl. [135], figs. 2-5; Pl. [136]); whilst in Coscinaspis, Acontaspis, Icosaspis, Hylaspis, &c., one part of the parmal pores is aspinal, one part coronal (Pl. [136]). The number, form, and size of the coronal pores is very variable and often very large (sometimes more than a hundred in one plate).
The Cladophracta exhibit a comparatively simple shell-formation; either all twenty spines or at least a part of them not forming lattice-plates. The most primitive form among these is Phractaspis (Pl. [137], figs. 1, 2). The forty apophyses of its twenty spines are simply forked, and their eighty fork-branches united by forty sutures, enclosing twenty-two sutural meshes: two square polar meshes (between the four polar spines on the poles of the spineless axis, a a a a and e e e e); eight triangular circumpolar meshes (each between two polar and one tropical spine, a b a and e d e); eight quadrangular tropical meshes (each between one polar, one equatorial, and two tropical spines, a b c b and e d c d); and four rhomboidal equatorial meshes (between two tropical and two equatorial spines, c b c d). If the fork-branches be again forked (Phractaspidium, Pl. [137], fig. 3), the number of the sutures and sutural meshes is doubled, and the same is the case in Stauraspis, the most simple form of the Tessaraspida.
A peculiar small group, and an interesting transition for the Cladophracta to the Peltophracta, is presented by the Zonaspida among the Tessaraspida (Zonaspis and Dodecaspis). Here only one part of the radial spines bears lattice-plates, the other part not. In Zonaspis the four equatorial spines bear lattice-plates, the sixteen other only free branches of the apophyses. In Dodecaspis twelve spines are provided with lattice-plates (four equatorial and eight polar spines), whilst the eight other (tropical) spines are devoid of them.
The Peltophracta exhibit a great variety in the form and composition of their twenty lattice-plates or fenestrated shields. In the most simple case (a part of Dorataspis and Diporaspis) the shell is composed of four (equatorial) hexagonal plates, and sixteen pentagonal plates (four tropical and four polar); in this case the four polar plates meet on each pole in one common point. More commonly, however, the shell seems to be composed of eight hexagonal plates (four equatorial and the four polar spines of the hydrotomical plane) and twelve pentagonal plates (eight tropical and the four polar spines of the geotomical plane); in this case only two (hexagonal) polar plates meet on each pole in a suture which separates the two other (pentagonal) polar plates (Pl. [138], fig. 4). In the majority of the Dorataspida the composition of the shell is much more complicated and often very difficult to understand. Often the surface of the plates is covered with a network of elevated crests, by which concave blind dimples are separated (Ceriaspis, Hystrichaspis, Pl. [138]); and sometimes these dimples become pierced by coronal pores (Pl. [138], fig. 11, &c.).
Peculiar by-spines or "accessory spines" cover the outer surface of the shell in a great part of Dorataspida, and commonly these most characteristic by-spines are not placed radially, but parallel to the radial spine, from the lattice-plate of which they arise (Pl. [135], figs. 1, 5; Pl. [137], figs. 4-8; Monogr. d. Radiol., 1862, Taf. xxi. figs. 8, 9). They are commonly placed perpendicular to the sutural condyles, or the branch-ends of the apophyses; so that close to each suture arises a pair of divergent by-spines, belonging to the meeting apophyses of the two neighbouring spines, which meet in the suture (Pl. [137], fig. 4). Rarely these thin, bristle-shaped by-spines are quite simple and straight, commonly they undulate or are zigzag and often armed with recurved hooks. Sometimes they are also forked or arborescent (Pl. [138], fig. 7).
The Central Capsule of the Dorataspida is constantly spherical and about one-third smaller than the enclosing shell, from which it is separated by the spherical calymma. The membrane of the central capsule is commonly rather thin, and pierced by the twenty radial spines, meeting in the centre of the capsule. Between these lie innumerable small pores for the radiating pseudopodia; however, in many cases (and perhaps constantly) these pores exhibit a certain regular disposition. In many species the central capsule encloses Xanthellæ or symbiotic yellow unicellular Algæ. The nucleus becomes cleft very early in the majority of Dorataspida.