| Diploconida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Phractopeltida | Hexalaspida | Cenocapsida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Phatnaspida | Lychnaspida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Porocapsida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Coleaspida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Ceriaspida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Belonaspida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Phractaspida | Stauraspida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Astrocapsida Sphærocapsida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Diporaspida (Dorataspida dipora) | Tessaraspida (Dorataspida tetrapora) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| [Dorataspida] | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Quadrilonchida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Phractacanthida | Stauracanthida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Amphilonchida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Acanthonia | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Astrolonchida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Litholophida | Chiastolida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Zyganthida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Acanthonida | Actinastrum | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Acanthometron | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Astrolophida | Acanthochiasmida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Acanthometron | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Actinelida Actinelius | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Actissa | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
172. Adelacantha and Icosacantha.—The numerous forms of Acantharia, here disposed in twelve families and sixty-five genera, may be divided phylogenetically into two main groups of very different extent—Adelacantha and Icosacantha. The more primitive group, Adelacantha, have an indefinite and variable number of radial spines, which are always quite simple in form and usually irregularly distributed; this main division includes only the one order Actinelida, with six genera, among which is Actinelius, the common stem-form of all the Acantharia. The more recent group, Icosacantha, includes all the other Acantharia (fifty-nine genera), and is very markedly distinguished from the Adelacantha by the fact that the radial spines are always twenty in number, and arranged according to Müller's law (compare pp. [717]-[725], and § [110]). Since this regular disposition (in five alternating zones each of four spines) has been retained by inheritance in the whole of the Icosacantha, it is probable that this large group has been developed monophyletically from a twig of the Adelacantha; Actinastrum (p. [732]) and Chiastolus (p. [738]) still present connecting links between the former and the latter, between Actinelius and Acanthometron.
173. Acanthonida and Acanthophracta.—The extensive main division Icosacantha (§ [110]), which embraces all Acantharia with twenty radial spines, disposed according to Müller's law, may be subdivided into two large groups or orders:—the Acanthonida (p. [740], Pls. [130]-[132]) and the Acanthophracta (p. [791], Pls. [133]-[140]). The latter possess a complete extracapsular lattice-shell, which the former have not. The more recent Acanthophracta may be derived phylogenetically from the more primitive Acanthonida simply by the development of this lattice-shell, with which process are usually (perhaps always) connected certain alterations in the malacoma, e.g., degeneration of the myophriscs (§ [96]). The most primitive form of all Icosacantha is the genus Acanthometron (p. [324]), in which all the twenty acanthin spines are of the simplest constitution and of equal dimensions.
174. Differentiation of the Acanthonida.—The order Acanthonida, which embraces all Icosacantha which have no complete lattice-shell, divides early into three main branches, the three families Astrolonchida, Quadrilonchida, and Amphilonchida (p. [727], Pls. [130]-[132]). The first of these constitutes the common stem-group from which the other two as well as the whole group Acanthophracta have been developed; the common stem-form of all is Acanthometron (§ [173]). All the Astrolonchida (p. [740], Pl. [130]) have twenty radial spines of equal size and similar form. On the other hand, in the Quadrilonchida (p. [766], Pl. [131]) the four equatorial spines differ from the others in size and sometimes also in form. In the Amphilonchida (p. [781], Pl. [132]) two opposite equatorial spines (lying in the hydrotomical axis) are much larger than the other eighteen and of a different shape. Of the three families of the Acanthonida the most important is the primitive group Astrolonchida, for from this the various stem-forms of the Acanthophracta arise. They are subdivided according to the formation of the spines into three subfamilies: the Zygacanthida, with simple spines without apophyses (or transverse processes); the Phractacanthida, with two opposite apophyses on each radial spine, and the Stauracanthida, with four crossed apophyses on each radial spine. The three genera of the Zygacanthida represent the stem-forms of the three families, since the radial spines in Acanthometron (the most primitive form of Acanthonida) are cylindrical, in Zygacantha two-edged, and in Acanthonia four-edged (p. [741]).
175. Capsophracta and Cladophracta.—The extensive order Acanthophracta, which embraces all Acantharia with a complete lattice-shell, is polyphyletic, its main subdivisions have been developed independently from different branches of the Acanthonida. The whole order may be divided directly into two main groups, the Capsophracta and Cladophracta (p. [793]), which differ in the structure and the origin of their lattice-shell. The group (or suborder) Capsophracta includes only the single family Sphærocapsida (p. [795], Pl. [133], figs. 7-11; Pl. [135], figs. 6-10); the lattice-shell arises independently of the twenty radial spines, being made up like a pavement of innumerable small acanthin plates, united by a kind of cement; each plate being perforated by a fine pore. In addition twenty larger main pores (or groups of four pores each) are present, corresponding to the twenty radial spines; these are always equal, quadrangular prismatic, without transverse processes as in Acanthonia. In the Cladophracta, which include the five remaining families of the Acanthophracta, the structure and origin of the lattice-shell are quite different; the lattice-shell is here made up of the branches of the transverse processes, which radiate tangentially from the twenty radial spines and are only united secondarily.
176. Ascent of the Dorataspida.—The group Cladophracta, or those Acantharia whose lattice-shell arises by the union of transverse processes of the twenty radial spines, includes five different families, whose stem-group is the family Dorataspida, with a simple spherical lattice-shell. This family itself is, however, diphyletic in origin, being composed of two essentially and originally different subfamilies—Diporaspida and Tessaraspida (p. [803]). The Diporaspida (p. [808], Pls. [137], [138]) have been developed from the Phractacanthida, and as each radial spine of the latter bears two opposite apophyses, so the lattice-shell of the former has forty primary aspinal pores (two on the base of each spine). On the other hand, the Tessaraspida (p. [830], Pls. [135], [136]) have been developed from the Stauracanthida, and as each radial spine of the latter bears four crossed apophyses, so the lattice-shell of the former has eighty primary aspinal pores (four at the base of each spine).
177. Descent of the Diporaspida.—Whilst the Tessaraspida (§ [176]) have given rise to no new groups which could take rank as independent families, no less than four separate families of Acantharia have arisen from the Diporaspida. The Phractopeltida (Pl. [133], figs. 1-6) are distinguished from all other Acantharia by the possession of two concentric spherical lattice-shells, and have probably been developed from the Diporaspida in the same way as the Dyosphærida from the Monosphærida among the Sphæroidea; in that case the smaller inner lattice-sphere (medullary shell) would be the primary, and the larger outer sphere (cortical shell) the secondary; this latter shows forty primary aspinal pores like those of the Diporaspida. The possibility is not excluded, however, that the small inner lattice-sphere of the Phractopeltida is a secondary product. The three remaining families, which must be regarded as descendants of the Diporaspida, form together a single phylogenetic series, and are separated from the primitive group mainly by the fact that the original spherical form of the lattice-shell has been modified into one distinguished by an elongated equatorial axis (the hydrotomical axis); hence the Prunophracta (pp. [794]-[859]). The ellipsoidal Belonaspida have arisen directly by hypertrophy of the two opposite equatorial spines of this hydrotomical axis (p. [859], Pl. [136], figs. 6-9; Pl. [139], figs. 8, 9; perhaps they have also arisen directly from the Amphilonchida). In the lentelliptical Hexalaspida (Pl. [139]) all six spines which lie in the hydrotomical meridian plane (two equatorial and four polar) are very strongly developed, the remaining fourteen being rudimentary. Finally, in the Diploconida the two conical sheaths of the two opposite hydrotomical equatorial spines are so predominant that they take the chief part in the formation of the hour-glass-shaped shell.
178. Phylogeny of the Nassellaria.—The legion Nassellaria or Monopylea is so clearly characterised by the peculiar porochora, which closes the osculum at the oral pole of the monaxon central capsule, and by the podoconus connected with it, that there can be no doubt that phylogenetically it represents an independent stem (§ [8]). This stem is only connected at its base by means of Cystidium and Nassella with Actissa and Thalassicolla, the stem-forms of the Spumellaria. This stem is monophyletic, inasmuch as all its members may be derived without violence from the skeletonless Nassellida (Nassella, Cystidium, p. [896], Pl. [91], fig. 1).
179. Origin of the Nassellaria.—The Nassellida (p. [896]), which may naturally be considered as the common stem-group of the Nassellaria, are most nearly related among other Radiolaria to the Thalassicollida, and in both these skeletonless families the simplest forms, Cystidium and Actissa correspond; on the other hand, those which have arisen from them by the formation of alveoles in the calymma (Nassella and Thalassicolla) also correspond. The origin of the simplest Nassellida from these primitive Thalassicollida may be explained by supposing that the numerous (formerly evenly distributed) pores of the capsule membrane became obliterated in the upper (apical) half of the central capsule, whilst in the lower (basal) half they became correspondingly more strongly developed; hence the porochora was formed at the oral pole of the vertical main axis, and a differentiation of the endoplasm proceeding from this gave rise to the characteristic podoconus. Both these organs still at present exhibit very various degrees of progressive development.
180. Hypothetical Genealogical Tree of the Nassellaria.