Family XIX. Phacodiscida, Haeckel (Pls. [31]-[35]).
Phacodiscida, Haeckel, 1881, Prodromus, p. 456.
Definition.—Discoidea with simple extracapsular phacoid shell (or lenticular latticed cortical shell), connected by radial beams with an intracapsular, simple or double, concentric medullary shell, without chambered equatorial girdles.
The family Phacodiscida comprises a large number of splendid forms (about a hundred species), which agree with the preceding Cenodiscida in the possession of the characteristic extracapsular "phacoid shell," but differ from them in having one or two intracapsular "medullary shells"; these concentric spherical medullary shells are connected with the lenticular cortical shell or phacoid shell by means of radial beams perforating the central capsule. The Phacodiscida bear therefore the same relation to the Cenodiscida that the Disphærida and Trisphærida do to the Monosphærida.
Formerly several species belonging to the family were described by Ehrenberg and Johannes Müller, but not distinguished from the Sphæroidea, genus Haliomma (e.g., Haliomma sol et Haliomma humboldtii of the former, Haliomma amphidiscus of the latter). For these oldest known species I constituted in 1862 my genus Heliodiscus (Monogr. d. Radiol., p. 436). Some other genera were afterwards (1875) figured by Ehrenberg as Periphæna and Chilomma. The rich material of the Challenger revealed this family as very polymorphic and widely distributed, so that in my Prodromus (1881, p. 457) I could enumerate eighteen different genera of Phacodiscida. This number is here reduced to fifteen, uniting several of them into one genus as "subgenera."
The Medullary Shell of the Phacodiscida, or the intracapsular latticed shell, is either simple and spherical, or double, composed of two concentric spheres, which are united by a variable number of radial beams. We could distinguish therefore as two subfamilies the Carpodiscida (with simple medullary shell) and the Thecodiscida (with double concentric medullary shell); the former corresponding to the Carposphærida (or Dyosphæria), the latter to the Thecosphærida (or Triosphæria). But as this difference seems not to be so important as the different shape of the disk margin, we prefer this latter as a character for the separation of subfamilies. The form of the medullary shells is commonly quite spherical (as also in the above mentioned Sphæroidea); sometimes it is a little compressed in the same direction as the enclosing lenticular phacoid shell. The diameter of the latter is commonly three to four times as large as the diameter of the medullary shell; if this be double (in the Thecodiscida) then the diameter of the outer medullary shell is commonly three to four times as large as that of the inner. The radial beams connecting the two seem to be very variable in number and disposition (compare Pl. [31], fig. 8; Pl. [32], figs. 3, 4a, 7, 8a; Pl. [33], figs. 2, 3; Pl. [35], figs. 4, 8, 9, &c.).
The Radial Beams, which connect the medullary shell with the cortical or "phacoid shell," and which pierce the membrane of the lenticular central capsule, are commonly aggregated into two polar bunches around the shortened axis of the disk (compare Pl. [31], fig. 8; Pl. [32], figs. 3, 8; Pl. [33], figs. 2, 3, &c.). Their number seems to be usually between ten and thirty. More rarely piercing radial beams lie also in the equatorial plane, and then commonly as inner prolongations of the outer marginal spines; so we find two opposite in one axis, in Heliostylus (Pl. [34], figs. 1, 2), or four opposite in pairs in two crossed axes, in Phacostaurus (Pl. [31], figs. 1, 2, 7). Often the thickness of the beams increases from the centre towards the periphery.
The Phacoid Shell, or the lenticular extracapsular cortical shell, exhibits in the Phacodiscida quite the same general character as in the Cenodiscida, above described (p. [410]). Commonly the equatorial diameter of the lens is twice to three times as large as the vertical diameter or the shortened "main axis." The convexity of both opposite faces is either quite even to the sharp margin (Pl. [31], fig. 2; Pl. [33], figs. 2, 5), or the central part of the lens is more strongly vaulted, and often the margin is thickened or truncated (Pl. [31], fig. 10; Pl. [35], fig. 7). The surface of the lens is commonly smooth, but sometimes also covered with bristle-shaped radial spines (Pl. [34], figs. 3, 5; Pl. [32], figs. 2, 3); rarely these spines are prolonged and branched (Pl. [35], figs. 3, 5). The pores of the phacoid shell are usually quite regular, circular, and regularly arranged, either in more concentric or in more radial rows; the latter are sometimes separated by radial crests arising towards the margin (Pl. [35], fig. 6). If the wall of the phacoid shell be much thickened, the pores in its central part are shorter and cylindrical, in its marginal part longer and conical (Pl. [31], fig. 7; Pl. [32], fig. 1).
The Margin of the Lens of the Phacodiscida is very polymorphic, and serves mainly for the separation of genera. In the first subfamily, Sethodiscida, it is either quite simple (Pl. [35], figs. 6-8) or surrounded by a thin solid equatorial girdle, the basal part of which is often radially striped (Pl. [32], figs. 7, 8). In the second subfamily, Heliosestrida, we find on the margin a small number of radial spines in the equatorial plane regularly disposed, either two opposite in one axis (Pl. [31], figs. 9-12) or four opposite in pairs in two crossed axes (Pl. [31], figs. 1-8), or eight opposite in pairs in four axes, crossed at angles of 45° (Pl. [34], figs. 3, 6); in the latter case we can sometimes distinguish (as in many Medusæ) four larger perradial spines alternating with four smaller interradial spines. Often in one and the same species occur abnormalities in number and disposition of the radial spines, three or five spines instead of four, or also seven or nine spines instead of eight; often both halves of the disk become asymmetrical. If the number of the marginal spines exceed eight to ten, they commonly become very variable in size and irregular in disposition; these variations characterise the third subfamily, Heliodiscida. Commonly also here all spines lie in the equatorial plane; but sometimes they become crowded in several parallel circles on both sides of the margin (Pl. [32], fig. 1). The form of the marginal spines is commonly conical or flattened triangular, often also pyramidal or deeply furrowed (Pl. [31], figs. 6-9). Very rarely the spines are fenestrated (Pl. [35], fig. 1); only in one genus (Heliodrymus) they are all or partly branched (Pl. [33], fig. 9; Pl. [35], figs. 3, 5).
The peculiar development of the phacoid shell has been already described by J. Müller (compare my Monograph, 1862, pp. 156, 438).