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. | ||||||
Subfamily 1. Spongophacida, Haeckel, 1881, Prodromus, p. 461.
Definition.—Spongodiscida with a simple circular disk, without radial appendages on the margin (neither solid spines nor chambered arms).
Genus 253. Spongodiscus,[[292]] Ehrenberg, 1854, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 237.
Definition.—Spongodiscida with a simple circular disk, without radial appendages and without an equatorial girdle on the margin.
The genus Spongodiscus represents the most simple and primitive form of the Spongodiscida, or of those Discoidea in which the central disk is more or less spongy, composed of an irregular fine framework. In my Monograph (1862, pp. 452, 460, 469) I had separated the true Spongodiscus (first described by Ehrenberg, loc. cit.) and the Spongocyclia; the former being characterised by the irregular spongy framework of the whole disk, whilst in the latter this framework includes in the central part some concentric circular rings (approaching Porodiscus). In the same way afterwards Stöhr (1880, loc. cit.) separated the genus Spongospira as spongy disks, which include in the central part some spiral convolutions. But as these differences are rather inconstant and not sharply discernible, I think it now better to regard these three forms as subgenera of Spongodiscus. All three have the common simple circular disk, without any marginal appendages.