Family XXXVII. Quadrilonchida, Haeckel (Pl. )

Acanthostaurida, Haeckel, 1881, Prodromus, p. 466.

Definition.—Acantharia with twenty radial spines of very unequal size, disposed according to the law of the Icosacantha; four equatorial spines much larger than the sixteen others. No lattice-shell.

The family Quadrilonchida differs from the foregoing ancestral family, the Astrolonchida, in the unequal development of the twenty radial spines. The four equatorial spines are constantly much larger, and often also of another form and shape, than the sixteen other spines; often also among these the eight tropical spines are larger and of another form than the eight polar spines. Therefore the five parallel girdles or zones of every four spines, which in the Astrolonchida are equal, are here distinctly unequal. The whole body is flattened and compressed in the direction of the spineless axis, so that the equatorial plane is larger than any other plane, laid through the centre. In consequence of this flattening the central capsule is also commonly compressed and flattened, lenticular or discoidal, rarely spherical. In the Astrolonchida the "promorph" or the "geometrical fundamental form" is constantly a square double pyramid, the axes of which are of equal length. In the Quadrilonchida it becomes a square double pyramid, the two equatorial axes of which (or the diagonals of the square) are constantly longer than all other axes.

In the simpler forms of Quadrilonchida are found only two different kinds of spines, the four larger equatorial spines being of the same size and form, and the sixteen smaller spines also not differing from one another (Acanthostaurus, &c.). But in the majority of this family there are three different kinds: four larger equatorial spines, eight tropical spines of middle size, and eight smaller polar spines (Belonostaurus, &c.); the latter become sometimes rudimentary, so that only twelve spines are developed (four larger equatorial and eight smaller tropical spines). In this case the development of the flat discoidal body is much stronger in the equatorial plane than in all other planes. These discoidal or lenticular Quadrilonchida exhibit a relation to the spherical Astrolonchida similar to that which the Discoidea exhibit to the Sphæroidea among the Sphærellaria.

A further morphological differentation takes place in the remarkable genera Lonchostaurus and Zygostaurus. In the former (Pl. , figs. 4-6) the four larger equatorial spines becomes differentiated in pairs, so that the opposite equal spines of one pair, c1, c3 (in the longitudinal or hydrotomical axis), are larger, and often also of another form, than the opposite equal spines of the other pair, c2, c4 (in the transverse or geotomical axis). The most peculiar form is the rather common Zygostaurus (Pl. , figs. 7, 8). Here the two opposite spines of one equatorial axis (of the longitudinal axis) become very different, so that the anterior or frontal spine (c1) is very unequal to the posterior or caudal spine (c3), whereas the two opposite spines, of the other equatorial axis (of the transverse axis) remain equal (c2, c4). Therefore the fundamental forms become here "amphithect," as in the Ctenophora.

The numerous Quadrilonchida may be disposed in two different subfamilies: in the Acanthostaurida all twenty radial spines are simple, without apophyses; in the Lithopterida all twenty spines (or only one part of them) bear two opposite apophyses (or lateral transverse processes). The former correspond to the Zygacanthida, the latter to the Phractacanthida among the Astrolonchida. The two opposite apophyses are simple in Quadrilonche (Pl. , fig. 1). In Xiphoptera they are provided on their distal side with lateral branches which are parallel to the spine itself. In Lithoptera the spines bear two to four parallel pairs of transverse apophyses, and these are crossed by perpendicular branches, parallel to the spine itself, so that there arise fenestrated wings or latticed plates, comparable to the sails of a wind-mill. The lattice-work of these plates lies in the same meridian plane with the radial spine itself, and is therefore not comparable to the fenestrated apophyses of Doracantha, of Phatnacantha, and of the Dorataspida; in these the lattice-plates lie in tangential planes, perpendicular to the radial spine.

The apophyses of the Lithopterida may be developed either on all twenty spines equally, or only on twelve spines (four equatorial and eight tropical, whilst the eight polar spines are simple, Pl. , fig. 10), or only on the four equatorial spines (whilst the sixteen others are simple, Pl. , fig. 9).

The Central Capsule of the Quadrilonchida is rarely spherical, commonly more or less compressed from both poles of the spineless axis, lenticular or discoidal, sometimes square. It is enveloped by a voluminous calymma constantly bearing coronals of "Myophrisca" (compare p. ).

Synopsis of the Genera of Quadrilonchida.

II. Subfamily Acanthostaurida.

All twenty spines simple, without lateral apophyses (sometimes forked, but neither branched nor latticed).

Four equatorial spines of equal size and form. Eight tropical and eight polar spines nearly equal, 334. Acanthostaurus.

Eight tropical and eight polar spines very different, 335. Belonostaurus.

Four equatorial spines of very different size or form (the two lateral constantly equal). Two principal spines of equal size and form, 336. Lonchostaurus.

Two principal spines (frontal and caudal) very different, 337. Zyostaurus.

II. Subfamily Lithopterida.

Either all twenty spines or a part of them provided with two opposite lateral branches or apophyses.

Apophyses simple, neither branched nor latticed, 338. Quadrilonche.

Apophyses branched or pinnate, but not latticed, 339. Xiphoptera.

Apophyses latticed, with fenestrated network, 340. Lithoptera.

I. Subfamily Acanthostaurida. All twenty spines simple, without lateral apophyses (sometimes forked, but neither branched nor latticed).

Four equatorial spines of equal size and form.

Eight tropical and eight polar spines nearly equal,

34. Acanthostaurus.

Eight tropical and eight polar spines very different,

35. Belonostaurus.

Four equatorial spines of very different size or form (the two lateral constantly equal).

Two principal spines of equal size and form,

336. Lonchostaurus.

Two principal spines (frontal and caudal) very different,

337. Zyostaurus.

II. Subfamily Lithopterida. Either all twenty spines or a part of them provided with two opposite lateral branches or apophyses.

Apophyses simple, neither branched nor latticed,

338. Quadrilonche.

Apophyses branched or pinnate, but not latticed,

339. Xiphoptera.

Apophyses latticed, with fenestrated network,

340. Lithoptera.