Family XX. Coccodiscida, Haeckel (Pls. -)

Coccodiscida, Haeckel, 1862, Monogr. d. Radiol., p. 485.

Coccodiscida, Haeckel, 1881, Prodromus, p. 458.

Lithocyclidina, Ehrenberg, 1847, Monatsber. d. k. preuss. Akad.

d. Wiss. Berlin, p. 214 (partim).

Definition.—Discoidea with extracapsular phacoid shell (or lenticular latticed cortical shell), connected by radial beams with an intracapsular, simple or double, concentric medullary shell, and surrounded by one or more concentric chambered equatorial girdles on the margin.

The family Coccodiscida was founded by me in 1862 for those Discoidea which agree with the Phacodiscida in the formation of the lenticular "phacoid shell" (including a simple or double medullary shell), but differ from them in the development of peculiar concentric chambered rings or girdles around the equatorial margin of the disk, similar to those of the Porodiscida.

The Coccodiscida represent a polymorphic family, in which we here distinguish sixteen genera with fifty-seven species; it comprises the greater part of those Discoidea which Ehrenberg united in his group Lithocyclidina (1875, represented by four genera and eight species); several of these, however, appertain to quite different families, as his Astromma entomocora, Lithocyclia amphitrites, &c. His knowledge of the structure was very imperfect. The peculiar differentiation of the genera and species exhibits the greatest analogy to that of the following family, Porodiscida, though the structure of the central disk in both families is quite different.

The Phacoid Shell, or the circular, lenticular cortical shell exhibits in the Coccodiscida quite the same structure and composition as in the Phacodiscida, described above (p. ), so that there can be no doubt as to the phylogenetic origin of the former from the latter. Quite in the same way in both families, the lenticular "phacoid shell" is connected by numerous, short, radial beams with the intracapsular, simple or double, medullary shell; and also here these beams are commonly disposed in two groups around the poles of the shortened main axis of the lens, so that their distal ends are implanted in both circumpolar areas (Pl. , figs. 2-6; Pl. , figs. 3, 7; Pl. , figs. 2, 7). The medullary shell is commonly simple, spherical, sometimes a little lenticularly compressed; more rarely it is double, composed of two concentric lattice-shells, which are connected by radial beams; in this case either both concentric medullary shells are spherical, or the inner is spherical, and the outer lenticular, very rarely the inner is lenticular also. In average size and structure they agree perfectly with those of the Phacodiscida.

The Chamber Girdles or "chambered rings" around the equatorial margin of the disk, which constitute the only difference between the Coccodiscida and the Phacodiscida, seem to exhibit a considerable degree of difference of structure in the numerous species of this family; but I regret that I cannot explain them here satisfactorily. The study of these structures is extremely difficult because of the thickness and darkness of the massive opaque shells; to get a perfect knowledge of them, it is indispensable to compare slides made in different directions (horizontal slides through the equatorial and parallel planes, vertical slides through radial and parallel planes, oblique slides in different directions). But this requires a long time and a most careful study of the slides, which are very difficult to get in satisfactory condition. Therefore the following remarks can have only a provisional value.

In all Coccodiscida we can distinguish on the equatorial chamber-girdle of the lens-margin (even on superficial inspection) three different elements of structure, viz.:—(A) concentric circular rings in the equatorial plane; (B) numerous radial beams piercing the former and dividing them into imperfect chambers; (C) porous plates or sieve-plates on both convex faces of the disk. The probable morphological significance of these three elements is the following:—Each ring or girdle corresponds to an outer lenticular cortical shell, which is only developed on the marginal part, whilst its central part is represented by the phacoid shell. Therefore the radial beams (separating the imperfect chambers) are the same as in the concentric Polysphærida, and the sieve-plates of the surface are the porous walls of the cortical shell itself. The correctness of this explanation seems to be proved by such forms as figured in Pl. , figs. 2, 4, where the whole surface of the phacoid shell is covered by a concentric chamber-work, as a central continuation of the marginal concentric rings. If we imagine a system of perfect concentric lenticular phacoid shells, compressed strongly from both poles of the shortened main axis, we get the same figure.

Rarely one single girdle only is developed on the equatorial margin of the lenticular disk (Pl. , figs. 2, 3, 5). Commonly the number of concentric girdles amounts to three to six, often to ten to twelve or more. Some of these largest Coccodiscida reach a considerable size. Commonly all girdles are of the same breadth, which is about equal to the radius or to the diameter of the inner medullary shell. Rarely the first (or innermost) girdle differs by its greater breadth from the succeeding ones (Pl. , fig. 8).

Only in few Coccodiscida the girdle-building remains restricted to the equatorial planes, so that all the chambers lie in it. Commonly on both sides of this plane become developed several layers, and often the number of these (three to six or more) increases towards the periphery; in other cases not their number, but their height increases. Therefore very often the margin of the discoidal shell is much thickened, as thick as the centre of the lenticular phacoid shell (or even more); whilst between the latter and the former (on the proximal girdles) the disk is considerably thinner (Pl. , figs. 2, 4; Pl. , figs. 7, 8; Pl. , figs. 2, 4). The stratified layers communicate by large openings between their chambers. The radial beams are commonly more or less regular and piercing, but also frequently irregular and interrupted; often their number increases towards the margin by intercalation of new beams.

The Pores of the sieve-plates, which cover both sides of the chambered disk, appear on the margin of the phacoid shell as direct continuations of the pores of the latter, and sometimes they are so regularly disposed that one single circular pore is situated on the surface of each chamber (Pl. , fig. 7; Pl. , fig. 1). But commonly the pores are of variable size and number, two to three on each chamber, and often quite irregularly scattered.

The Margin of the chambered disk exhibits many differences, which afford characters for the distinction of genera. In the first subfamily, the Lithocyclida, the margin is quite simple without radial appendages. In the second subfamily, the Stylocyclida, it is armed with solid radial spines lying in the equatorial plane, and often regularly disposed in the same manner as in the Phacodiscida (compare above, p. ). In the third subfamily, the Astracturida, the margin bears two or more (commonly three or four) chambered arms, also situated in the plane of the disk, and of the same structure as the circular chambered girdles (Pl. ). In some cases even the whole system of chamber-girdles is represented only by the radial arms, which are inserted immediately on the margin of the phacoid shell. We may regard therefore these formations as imperfect chambered disks, which are developed only in the direction of certain rays (perradii), and reduced in the direction of the alternating rays (interradii). In some Astracturida the chambered arms exhibit a structure different from the more irregular chamber-work between them, so that we can distinguish the latter, connecting the arms like a web-membrane, as a peculiar "patagium" (as in many Porodiscida) (Pl. , figs. 8, 9). Often the distal ends of the chambered arms are armed with a radial spine (Pl. , figs. 5, 6, 9).

The Central Capsule of the Coccodiscida is originally always of the same form and shape as in their ancestral group, the Phacodiscida; a circular lenticular disk, which envelops the simple or double medullary shell and is enclosed by the cortical phacoid shell. But whilst in the Phacodiscida the phacoid shell envelops the central capsule perfectly, in the Coccodiscida it envelops only the capsule from the two flat sides (by the upper and lower sieve-plates); the marginal part of the lenticular capsule overgrows the margin of the phacoid shell by peripheral extension, and fills out the chambered cavity of the concentric girdles. In the Lithocyclida and the Stylocyclida, where there are no chambered arms, the central capsule remains a simple circular lens or disk; in the Astracturida, where chambered radial arms surround the margin of the circular central disk, the capsule enters also into these arms and fills out the greatest part of their chambered cavities.

Synopsis of the Genera of Coccodiscida.

I. Subfamily Lithocyclida.

Margin of the disk simple, circular, without radial appendages.

Neither radial spines nor chambered arms on the circular margin. Medullary shell simple, 196. Lithocyclia.

Medullary shell double, 197. Coccodiscus.

II. Subfamily Stylocyclida.

Margin of the disk armed with solid radial spines.

Two opposite spines. Medullary shell simple, 198. Stylocyclia.

Medullary shell double, 199. Amphicyclia.

Three radial spines. Medullary shell simple, 200. Trigonocyclia.

Four crossed spines. Medullary shell simple, 201. Staurocyclia.

Five to ten or more radial spines. Medullary shell simple, 202. Astrocyclia.

Medullary shell double, 203. Coccocyclia.

III. Subfamily Astracturida.

Margin of the disk with two to five or more (commonly three or four) hollow radial chambered arms (with or without a connecting patagium). (Medullary shell commonly simple.)

Two arms, opposite in one axis. Without patagium, 204. Diplactura.

With patagium, 205. Amphiactura.

Three radial arms (at equal distances). Without patagium, 206. Trigonactura.

With patagium, 207. Hymenactura.

Four arms (in two crossed diameters). Without patagium, 208. Astractura.

With patagium, 209. Stauractura.

Five radial arms (at variable distances). Without patagium, 210. Pentactura.

With patagium, 211. Echinactura.

I. Subfamily Lithocyclida. Margin of the disk simple, circular, without radial appendages.

Neither radial spines nor chambered arms on the circular margin.

Medullary shell simple,

196. Lithocyclia.

Medullary shell double,

197. Coccodiscus.

II. Subfamily Stylocyclida. Margin of the disk armed with solid radial spines.

Two opposite spines.

Medullary shell simple,

198. Stylocyclia.

Medullary shell double,

199. Amphicyclia.

Three radial spines.

Medullary shell simple,

200. Trigonocyclia.

Four crossed spines.

Medullary shell simple,

201. Staurocyclia.

Five to ten or more radial spines.

Medullary shell simple,

202. Astrocyclia.

Medullary shell double,

203. Coccocyclia.

III. Subfamily Astracturida. Margin of the disk with two to five or more (commonly three or four) hollow radial chambered arms (with or without a connecting patagium). (Medullary shell commonly simple.)

Two arms, opposite in one axis.

Without patagium,

204. Diplactura.

With patagium,

205. Amphiactura.

Three radial arms (at equal distances).

Without patagium,

206. Trigonactura.

With patagium,

207. Hymenactura.

Four arms (in two crossed diameters).

Without patagium,

208. Astractura.

With patagium,

209. Stauractura.

Five radial arms (at variable distances).

Without patagium,

210. Pentactura.

With patagium,

211. Echinactura.