Family XXXIX. Sphærocapsida, Haeckel (Pl. , figs. 7-11; Pl. , figs. 6-10)

Suborder I. Sphærophracta, Haeckel.

Definition.—Shell spherical, with twenty radial beams of equal size.

Sphærocapsida, Haeckel, 1881, Prodromus, p. 469.

Definition.—Acantharia with simple spherical porous shell, composed of innumerable very small plates, each of which is pierced by one radial porule. Twenty radial spines of equal size meeting in the centre of the shell and disposed according to the Müllerian law of the Icosacantha, sometimes short and enclosed in the shell, at other times long and piercing it (rarely rudimentary or quite absent). Shell pierced therefore either by twenty larger perspinal pores or by eighty smaller aspinal pores. Central capsule spherical, enclosed in the porous shell.

The family Sphærocapsida, founded by me in 1881 for the single genus Sphærocapsa, represents a very peculiar and remarkable group of the Acanthophracta, very different from the five other families of this suborder, and probably derived, independently of them, directly from the Acanthonida. Whilst the lattice-shell of the five other families is composed of the meeting branches of lateral apophyses of the twenty spines, and its meshes are all or partly the intervals between these apophyses, in the Sphærocapsida the spherical shell has quite another structure, and is composed of innumerable small plates (each with one pore) which are secreted on the surface of the spherical calymma, independently of the twenty radial spines, which do not possess true apophyses.

In all Sphærocapsida the structure of the spherical shell is quite peculiar and different from that of all other Radiolaria. It is composed everywhere of innumerable very small plates or aglets, which are connected irregularly like paving-stones, and form a single continuous layer or pavement on the surface of the spherical calymma (Pl. , fig. 11, a; Pl. , figs. 8, 10). The small plates or paving-stones, which we will call "aglets," are connected at their meeting edges by a kind of cement, and form together with it a continuous thick capsule of acanthin. The form of the aglets is commonly more or less irregular, roundish or polygonal, sometimes longish (Pl. , fig. 11, a), more rarely it becomes rather regular, hexagonal, square, or roundish (Pl. , fig. 8). Usually all aglets of one and the same individual are of nearly equal size, between 0.01 and 0.02 in diameter, rarely less or more. The outer face of the aglets is more or less concave, so that the elevated meeting edges of the neighbouring aglets commonly form together a prominent network of crests (Pl. , figs. 8, 10); rarely the meeting edges partly cover one another like squamules (Pl. , fig. 11, a). Each small plate or aglet is pierced in its centre by a single radial canalicule or porule. The dimpled surface, so produced, resembles somewhat the dimpled plates of Ceriaspis, &c. Different from these innumerable very small dimples of the surface are the twenty larger "spinal dimples," or the concave larger plates, which are originally pierced by the twenty radial spines. Before we describe these, we must examine the spines themselves.

The twenty radial spines of all observed Sphærocapsida (sixteen species) agree perfectly with those of the genus Acanthonia (p. ), and especially with Acanthonia tetracopa, Acanthonia denticulata, &c. All twenty spines, regularly disposed according to the Müllerian law of the Icosacantha, are of equal size, constantly four-edged prismatic, of equal breadth throughout their whole length. The prominent four edges are parallel, sometimes smooth, at other times elegantly denticulated. The central bases of the twenty spines are pyramidal, without leaf-cross, and propped one upon another with their triangular faces, as in the majority of the Acanthonida.

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. , figs. 6-10). In the next allied genus, Astrocapsa (Pl. , 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. , 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.