Family XXIII. Spongodiscida, Haeckel (Pl. , fig. 11)

Spongodiscida et Spongocyclida, Haeckel, 1862, Monogr. d. Radiol.,

pp. 452, 460, 469.

Spongodiscida, Haeckel, 1881, Prodromus, p. 461.

Calodictya, Ehrenberg, 1847, Monatsber. d. k. preuss. Akad. d. Wiss.

Berlin, p. 53 (partim).

Definition.—Discoidea without a phacoid shell, with a flat discoidal shell, in which a simple spherical central chamber is surrounded by an irregular spongy framework (sometimes with concentric rings around the central chamber). Surface of the disk quite spongy, without porous sieve-plates.

The family Spongodiscida is the sixth and last family of the Discoidea, and bears to the other families of this group the same relation as the family Spongosphærida does to the other Sphæroidea, or the Spongurida to the other Prunoidea. Its characteristic structure consists in the irregular spongy framework of the disk, and mainly in the rough, irregular shape of its spongy surface, which is never covered with porous plates (neither phacoid shell nor corresponding sieve-plates), as in all other Discoidea. Of course a little spongy structure occurs also in many Porodiscida and even in some Coccodiscida (principally in the peripheral part of the disk or its chambered arms); but both flat (or convex) surfaces of the disk (at least in the central part) remain here constantly as simple lattice-plates, whilst in all Spongodiscida the whole surface of the disk is spongy.

When I constituted the family Spongodiscida in my Monograph (1862, pp. 452, 460) I had separated from them the Spongocyclida, exhibiting in the central part of the disk a more or less distinct concentric arrangement of the spongy chambers, whilst in the former the delicate spongy framework is quite irregular, composed of branched siliceous threads, connected and interwoven in all directions. But in all Spongocyclida the whole surface of the spongy disk is quite as irregularly rough and deprived of smooth sieve-plates as in all true Spongodiscida, and the more or less concentric structure of the central part of the disk in the former (very variable and often scarcely able to be recognised) seems not sufficient to separate both groups; even the single genera cannot be sufficiently separated by this character. I now therefore give up entirely the group of Spongocyclida (as already done in my Prodromus, 1881). Nevertheless the concentric annular structure in the dark central part of some Spongodiscida is very interesting as transition to the Porodiscida; it indicates already that the former are derived from the latter. Even the single genera in both families are corresponding.

In the new system of "Polycystina," which Ehrenberg gave, 1875 (Abhandl. d. k. Akad. d. Wiss. Berlin, p. 157), are enumerated under the Calodictya four genera "with spongy disk," viz., Spongodiscus, Rhopalodictyum, Dictyocoryne, Spongaster. Indeed these four genera, which I here retain, are true Spongodiscida, and must be separated from the other Calodictya, the greater part of which are Porodiscida. The number of species of true Spongodiscida now amounts to sixty-seven, which I dispose in thirteen genera.

The whole family may be divided into three subfamilies. The first of these are the Spongophacida (corresponding to the Trematodiscida among the Porodiscida), in which the circular margin of the spongy disk bears no radial appendages; either the margin is quite simple, spongy (Spongodiscus), or surrounded by a hyaline, solid, or porous equatorial girdle (Spongophacus). The disk is either more lenticular (biconvex) or more flat discoidal (a shortened cylinder), rarely a little biconcave (thicker at the margin than in the centre). The spongy framework of the solid disk is either quite irregular (Spongodisculus), or in the central part with concentric circular rings (Spongocyclia), or in the central part spirally convoluted (Spongospira).

The second subfamily, Spongotrochida, corresponds to the Stylodictyida (among the Porodiscida), and is distinguished by solid radial spines on the margin of the disk, disposed in the equatorial plane either irregularly or regularly (after the same order as in the other families of Discoidea).

The third subfamily, Spongobrachida, correspond perfectly to the Euchitonida (among the Porodiscida), bearing on the disk-margin two, three, or four spongy arms, commonly disposed regularly in the equatorial plane. Here also occurs the peculiar formation of a "patagium," or of an interbrachial spongy framework different from that of the arms, which connects the arms like a web-membrane in the equatorial plane.

The spongy framework exhibits in all these Spongodiscida no remarkable differences, being everywhere composed of fine branched solid siliceous threads, interwoven in all directions, with irregular meshes of very different size.

The central capsule of all Spongodiscida is filled up with the same spongy framework which covers also both its sides. It grows according to the enveloping skeleton, but remains constantly smaller. The form of the central capsule is circular (lenticular or discoidal) in the Spongophacida and Spongotrochida, whilst in the Spongobrachida it enters into the radial spongy arms, developed from the margin of the spongy disk.

Synopsis of the Genera of the Spongodiscida.

III. Subfamily Spongophacida.

Spongy disk without radial appendages.

Spongy disk with simple margin (without peculiar equatorial girdle), 253. Spongodiscus.

Spongy disk with a peculiar (solid or porous) equatorial girdle, 254. Spongophacus.

III. Subfamily Spongotrochida.

Spongy disk with solid radial spines on the margin (in the equatorial plane).

Few (two, three, or four) radial spines regularly disposed. Two opposite spines, 255. Spongolonche.

Three marginal spines, 256. Spongotripus.

Four crossed spines, 257. Spongostaurus.

Numerous (five to ten or more) radial spines, often irregularly disposed. Spines only on the margin (equatorial), 258. Stylotrochus.

Spines on both sides of the disk, 259. Spongotrochus.

III. Subfamily Spongobrachida.

Spongy disk with spongy radial arms on the margin (in the equatorial plane).

Two arms, opposite in one axis. Without a patagium, 260. Spongolena.

With a patagium, 261. Spongobrachium.

Three arms on the margin. Without a patagium, 262. Rhopalodictyum.

With a patagium, 263. Dictyocoryne.

Four arms in cross form. Without a patagium, 264. Spongasteriscus.

With a patagium, 265. Spongaster.

I. Subfamily Spongophacida. Spongy disk without radial appendages.

Spongy disk with simple margin (without peculiar equatorial girdle),

253. Spongodiscus.

Spongy disk with a peculiar (solid or porous) equatorial girdle,

254. Spongophacus.

II. Subfamily Spongotrochida. Spongy disk with solid radial spines on the margin (in the equatorial plane).

Few (two, three, or four) radial spines regularly disposed.

Two opposite spines,

255. Spongolonche.

Three marginal spines,

256. Spongotripus.

Four crossed spines,

257. Spongostaurus.

Numerous (five to ten or more) radial spines, often irregularly disposed.

Spines only on the margin (equatorial),

258. Stylotrochus.

Spines on both sides of the disk,

259. Spongotrochus.

III. Subfamily Spongobrachida. Spongy disk with spongy radial arms on the margin (in the equatorial plane).

Two arms, opposite in one axis.

Without a patagium,

260. Spongolena.

With a patagium,

261. Spongobrachium.

Three arms on the margin.

Without a patagium,

262. Rhopalodictyum.

With a patagium,

263. Dictyocoryne.

Four arms in cross form.

Without a patagium,

264. Spongasteriscus.

With a patagium,

265. Spongaster.