The family Astrolonchida, the first and oldest of the Acanthonida, is no doubt the ancestral stock not only of this suborder but also all Acanthophracta, i.e., of all Icosacantha or all Acantharia in which twenty radial spines are regularly disposed according to the Müllerian law, forming five zones each of four alternating spines (compare above, p. [717]). The Astrolonchida differ from the Acanthophracta in the absence of a complete lattice-shell, from the other two families of Acanthonida (the Quadrilonchida and Amphilonchida) in the equal size and similar form of all the spines. Probably this equality is nowhere quite perfect, since in all Icosacantha the central bases of the twenty spines exhibit originally certain slight differences of form and junction, effected by the regular disposition itself. But setting aside this slight difference, only recognisable by means of a very accurate investigation of the central junction (and in thinner spines often not at all recognisable), the twenty spines of the Astrolonchida appear perfectly equal. Therefore the four equatorial spines are not distinguished from the sixteen other spines, as is constantly the case in the two following families.

The number of genera (eleven) and of species (seventy-six) in the Astrolonchida is far larger than in the five other families of Acanthometra, and requires a distinction into three different subfamilies. (A) In the Zygacanthida the form of the radial spines is quite simple, without apophyses or transverse processes; (B) in the Phractacanthida each spine bears two opposite apophyses (rarely two longitudinal rows of these opposite apophyses); (C) in the Stauracanthida each spine bears a cross of four apophyses, opposite in pairs (rarely four longitudinal crossed rows of apophyses, opposite in pairs). The Phractacanthida and Stauracanthida appear as two divergent branches of the pedigree, derived independently from the common ancestral stock of Zygacanthida.

In the Zygacanthida, constantly devoid of apophyses, we can distinguish only three genera, characterised by the different fundamental form of the radial spines; these are:—(1) Acanthometron, with cylindrical or conical spines (without edges); (2) Zygacantha, with compressed and two-edged spines; (3) Acanthonia, with four-edged, prismatic or pyramidal spines. The transverse section of the spines is in the first case circular, in the second elliptical or lanceolate, in the third square. All the different forms of spines, which we find in the numerous Acantharia, may be reduced to these three forms, and among these the second and third are derived from the first.

The development of apophyses or of lateral transverse processes (wanting in the Zygacanthida) is of the greatest value for the further differentiation of the Acantharia. For from the Phractacanthida (with two opposite apophyses on each spine) we must derive the Phrastaspida, the common ancestral stock of the Diporaspida (and therefore also the Belonaspida, Hexalaspida, Diploconida, and Phractopeltida). On the other hand the Stauracanthida (with four crossed apophyses on each spine) have produced the Stauraspida, or the ancestral group of the Tessaraspida and Sphærocapsida. From all these Acanthophracta, possessing a complete lattice-shell, the Astrolonchida differ in the absence of such a complete shell. Also in the few cases in which the apophyses become latticed (Doracantha among the Phractacanthida, and Phatnacantha among the Stauracanthida), the lattice-plates of the neighbouring spines never meet with their edges, as is the case in all Acanthophracta. But in a phylogenetic as well as in an ontogenetic sense the former are the ancestral stock of the latter.

The Central Capsule in the Astrolonchida is commonly spherical, sometimes with twenty roundish elevations or conical papillæ, extending radially to the basal half of the radial spines. The calymma is voluminous, and forms around the radial spines conical or cylindrical "jelly-sheaths," which are connected with the spines by coronas of Myophrisca (or of the bodies formerly called "Gallert-cilien," afterwards recognised as "contractile Filamente").

Synopsis of the Genera of Astrolonchida.

I. Subfamily Zygacanthida.

Twenty radial spines simple, without apophyses or lateral transverse processes.

Spines cylindrical, with circular transverse section,323. Acanthometron.
Spines compressed, two-edged or lamellar, with elliptical or rhomboidal transverse section,324. Zygacantha.
Spines quadrangular (prismatic or pyramidal), with four edges, with square transverse section,325. Acanthonia.

II. Subfamily Phractacanthida.

Twenty radial spines provided each with two opposite apophyses (or two longitudinal rows of apophyses).

Two apophyses opposite on each spine.Apophyses simple,326. Lithophyllium.
Apophyses branched,327. Phractacantha.
Apophyses latticed,328. Doracantha.
Two opposite longitudinal rows of apophyses (four to eight or more apophyses on each spine, opposite in pairs),329. Astrolonche.

III. Subfamily Stauracanthida.

Twenty radial spines provided each with four crossed apophyses (or four crossed longitudinal rows of apophyses).

Four apophyses (in cross form) opposite in pairs on each spine.Apophyses simple,330. Xiphacantha.
Apophyses branched,331. Stauracantha.
Apophyses latticed,332. Phatnacantha.
Four longitudinal rows of apophyses, opposite in pairs in cross form on each spine,333. Pristacantha.
I. Subfamily Zygacanthida. Twenty radial spines simple, without apophyses or lateral transverse processes.
Spines cylindrical, with circular transverse section,
323. Acanthometron.
Spines compressed, two-edged or lamellar, with elliptical or rhomboidal transverse section,
324. Zygacantha.
Spines quadrangular (prismatic or pyramidal), with four edges, with square transverse section,
325. Acanthonia.
II. Subfamily Phractacanthida. Twenty radial spines provided each with two opposite apophyses (or two longitudinal rows of apophyses).
Two apophyses opposite on each spine.
Apophyses simple,
326. Lithophyllium.
Apophyses branched,
327. Phractacantha.
Apophyses latticed,
328. Doracantha.
Two opposite longitudinal rows of apophyses (four to eight or more apophyses on each spine, opposite in pairs),
329. Astrolonche.
III. Subfamily Stauracanthida. Twenty radial spines provided each with four crossed apophyses (or four crossed longitudinal rows of apophyses).
Four apophyses (in cross form) opposite in pairs on each spine.
Apophyses simple,
330. Xiphacantha.
Apophyses branched,
331. Stauracantha.
Apophyses latticed,
332. Phatnacantha.
Four longitudinal rows of apophyses, opposite in pairs in cross form on each spine,
333. Pristacantha.

Subfamily 1. Zygacanthida, Haeckel.

Definition.—Astrolonchida with twenty simple radial spines, without apophyses or lateral transverse processes.