The relation of the twenty radial spines to the spherical shell exhibits in the five genera described very peculiar and important differences. In the first described genus, in Sphærocapsa, the spines are exactly as long as the shell-radius, and therefore are not prominent over the surface of the shell, with which they are firmly connected; the truncated distal end of the spine lies therefore here in the surface of the shell itself, and is connected with it by its four edges, between which four open aspinal pores remain, as in Tessaraspis, &c. (Pl. [135], figs. 6-10). In the next allied genus, Astrocapsa (Pl. [133], figs. 9, 10), the spines are longer than the shell-radius, and therefore more or less prominent over its surface; the piercing part of each spine is also surrounded by four aspinal pores. In the two following genera, Porocapsa and Cannocapsa (Pl. [133], figs. 7, 8), the radial spines are shorter than the shell-radius and therefore quite hidden and withdrawn inside the shell, which they do not reach. But in the ideal prolongation of each spine the shell is pierced by a single large opening, the "perspinal pore" or "perspinal hole," composed of the four united aspinal pores. Whilst in Porocapsa the perspinal pores are simple, they are prolonged in Cannocapsa into cylindrical tubes, open at both ends. The twenty perspinal holes of these Porocapsida are therefore derived by confluence of the eighty original aspinal pores of the Astrocapsida and preserve the same regular disposition, according to the Müllerian law of the Icosacantha. Finally, the same law as is valid also in the last genus is found in Cenocapsa; here the radial spines have completely disappeared, and the whole skeleton is a simple sphere, but of the same structure, and with the same twenty perspinal pores as in Porocapsa. It is very interesting that this spineless Cenocapsa among the Acantharia exhibits the same shell (a simple hollow sphere) as a last reduced form, which Cenosphæra among the Sphærellaria produces as a primitive ancestral form of numerous genera.
The Central Capsule of the Sphærocapsida is spherical, constantly smaller than the enclosing concentric shell, and separated from it by the calymma. Its structure seems to be the same as in the Acanthonida, and specially in the Astrolonchida. The pseudopodia (not yet observed) are probably protruded only through the twenty perspinal holes or the eighty aspinal pores.
Synopsis of the Genera of Sphærocapsida.
III. Subfamily Astrocapsida. Radial spines connected with the porous shell, as long or longer than its radius. Eighty aspinal pores. |  | Spines as long as the radius, without external prolongation, | 344. Sphærocapsa. |
| Spines longer than the radius, with external prolongation, | 345. Astrocapsa. | ||
III. Subfamily Porocapsida. Radial spines not connected with the porous shell, shorter than its radius. Twenty perspinal pores. | | Perspinal holes of the shell simple, without external prolongation, | 346. Porocapsa. |
| Perspinal holes of the shell prolonged into radial centrifugal tubuli, | 347. Cannocapsa. | ||
III. Subfamily Cenocapsida. Radial spines disappeared. |  | Twenty perspinal holes of the shell simple, without tubular prolongation, | 348. Cenocapsa. |
| I. Subfamily Astrocapsida. Radial spines connected with the porous shell, as long or longer than its radius. Eighty aspinal pores. | ||||
| Spines as long as the radius, without external prolongation, | ||||
| 344. Sphærocapsa. | ||||
| Spines longer than the radius, with external prolongation, | ||||
| 345. Astrocapsa. | ||||
| II. Subfamily Porocapsida. Radial spines not connected with the porous shell, shorter than its radius. Twenty perspinal pores. | ||||
| Perspinal holes of the shell simple, without external prolongation, | ||||
| 346. Porocapsa. | ||||
| Perspinal holes of the shell prolonged into radial centrifugal tubuli, | ||||
| 347. Cannocapsa. | ||||
| III. Subfamily Cenocapsida. Radial spines disappeared. | ||||
| Twenty perspinal holes of the shell simple, without tubular prolongation, | ||||
| 348. Cenocapsa. | ||||
Subfamily 1. Astrocapsida, Haeckel.
Definition.—Radial spines connected with the porous shell, as long as or longer than its radius. Therefore the shell pierced by eighty aspinal pores (four around each spine).
Genus 344. Sphærocapsa,[[384]] Haeckel, 1881, Prodromus, p. 469.
Definition.—Sphærocapsida with twenty radial spines as long as the radius of the shell, without external prolongation; therefore their distal ends inserted in the perspinal holes, each of which is composed of four aspinal pores.
The genus Sphærocapsa is the most common form of the Sphærocapsida, and comprises those species in which the radial spines are as long as the radius of the shell, and therefore are connected with the margin of its aspinal holes, but not prolonged beyond its surface.