The Phacodiscida (Pls. 31-35) perhaps constitute the primitive group of the Phacodiscaria, their lenticular or Phacoid cortical shell being connected by radial bars with one or two concentric spherical medullary shells; they may have originated directly from the Dyosphærida or Triosphærida by flattening of the spheroidal cortical shell. From the Phacodiscida the Cenodiscida (if indeed they be not the primitive stem-form) have been developed by retrogression and loss of those medullary shells. The Coccodiscida (Pls. 36-38), on the other hand, have been developed from the Phacodiscida by the addition of concentric rings of chambers, which may be regarded as incomplete cortical shells, only the equatorial portion of which is developed. Perhaps the Porodiscida, the primitive group of the Cyclodiscaria, have arisen in a similar way; they lack, however, the typical Phacoid shell, the concentric rings of chambers being directly applied to a small spherical medullary shell in the equatorial plane (Pls. 41-46). If those rings from the commencement be interrupted by three interradial gaps (gates) the family Pylodiscida arises (Pl. 38, figs. 6-20). If, on the contrary, the concentric radially divided chambers of the Porodiscida become quite irregular and spongy, they pass over into the Spongodiscida (Pls. 46, 47). It is not, however, impossible that part of the Discoidea (especially the Cenodiscida) have originated directly from skeletonless Collodaria with a lenticular central capsule, such as are found in a subgenus of Actissa (Actidiscus, p. 15).
167. Genealogical Tree of the Larcoidea.—The suborder Larcoidea presents in the structure, composition, and development of its variously formed lattice-shells much more complicated relations than the other Sphærellaria; it is essentially distinguished from them by the characteristic ground-form of its lattice-shells, which is a "lentellipsis" or a triaxial ellipsoid (also the ground-form of the rhombic crystallographic system, the rhombic octahedron). Hence all parts of the body are regularly disposed with respect to three different dimensive axes; all three axes, perpendicular one to another, are isopolar but of different lengths; the longest is the vertical main axis, the mean the horizontal frontal axis, the shortest the horizontal sagittal axis. In the great majority of the Larcoidea the lentelliptical ground-form is indicated in the central capsule, even when it is not at once obvious in the skeleton. Since such lentelliptical central capsules are developed even in Actissa (Actilarcus, p. 16), it is possible that the simplest Larcoidea may have arisen directly from these by deposition of a simple lentelliptical lattice-shell in the sarcodictyum, on the surface of the calymma (Cenolarcus, Pl. 50, fig. 7). It is more probable, however, that these simplest forms (Cenolarcus, Larcarium) have been developed from the simplest Sphæroidea (Cenosphæra), by the spherical body growing unequally in the three dimensions of space. It appears especially likely from a study of the concentrically disposed lattice-shells of some Larcoidea (Coccolarcus, Larcidium, Pl. 50, fig. 8), in which the inner medullary shell is spherical, the outer cortical shell more or less elliptical. In the great majority of Larcoidea the latter arises in quite a peculiar manner, three broad lattice-zones, which are developed in three planes at right angles to each other, growing out from a small spherical or lentelliptical medullary shell, Trizonium, Larnacilla (compare pp. 600, 615, 628, &c.).
The trizonal Larnacilla-shell commences by the formation of a transverse girdle, by the union of two lateral latticed processes, which spring right and left in the equatorial plane from the poles of the frontal axis of a lentelliptical medullary shell (Monozonium, p. 633, Pl. 9, fig. 1). This is followed by a second lateral girdle, which lies in the frontal plane and proceeds from its lateral poles (Dizonium, p. 634, Pl. 9, figs. 2, 3). Finally the sagittal girdle is formed, lying in the sagittal plane and arising from the lateral girdle on the two poles of the main axis (Trizonium, p. 637, Pl. 9, fig. 4). Whilst the gaps between the three zones of this trizonal shell remain open in the Pylonida, in Larnacilla, the important primitive form of the Larnacida, they are closed by lattice-work (Pl. 50, figs. 3-8). From this trizonal Larnacilla-shell the great majority of Larcoid shells may be derived. Such a system of zones may be repeated (Diplozonaria) or even developed a third time (Triplozonaria, p. 632). In most Larcoidea the zones are secondarily connected by lattice-work. In the Tholonida (Pl. 10) each of the two opposite latticed wings of a zone becomes a closed dome. In the Zonarida (Pl. 50, figs. 9-12) these domes are partially or wholly bisected by constrictions or latticed septa which are developed in the three dimensive planes. The Lithelida (Pl. 49, figs. 1-7) are characterised by the fact that one of each pair of opposite latticed processes (or half zones) grows more strongly than the other, and that the larger completely embraces the smaller so as to form a complicated spiral. Whilst in this case the spiral lies in a plane, in the Streblonida (Pl. 49, figs. 8, 9) it becomes turbinoid like a gastropod shell and forms an ascending spiral. Finally, two small families of Larcoidea are characterised by quite irregular growth (a very rare occurrence among the Radiolaria); these are the simple-chambered Phorticida (Pl. 49, figs. 10, 11) and the many chambered Soreumida (Pl. 49, figs. 12, 13). The phylogenetic relationship of these families of Larcoidea is probably very complicated and demands closer investigation (compare pp. 599-604).
168. Descent of the Polycyttaria.—The polyzootic or colonial Radiolaria, which we unite in the group Polycyttaria (sometimes known as "Sphærozoea"), belong without doubt to the legion Spumellaria, for they possess all the peculiarities by which these Peripylea are distinguished from the other legions of the Radiolaria. Only the morphological position of the Polycyttaria in that legion, and their phylogenetic relation to the monozootic or solitary Spumellaria, can be variously interpreted. The three families which we distinguish among the Polycyttaria are so closely related to three different families of the Monocyttaria, that they may be directly derived from them by the formation of colonies. According to this triphyletic hypothesis the social skeletonless Collozoida (Pl. 3) would be descended from the solitary Thalassicollida (Pl. 1), the polyzootic Sphærozoida with a Beloid skeleton (Pl. 4) from the monozootic Thalassosphærida (Pl. 2), and the colonial Collosphærida with a Sphæroid skeleton (Pls. 5-8) from the solitary Ethmosphærida (Pl. 12, &c.). Many species of monozootic and polyzootic forms in all three groups are so alike that they can only be distinguished by the fact that the one series are colonial, the others solitary. On the other hand, there are some reasons which would justify a monophyletic hypothesis for the Polycyttaria, e.g., the precocious nuclear division; in this case it would be most natural to hold that the Sphærozoida and Collosphærida have arisen as two diverging branches from the Collozoida, whilst the latter are nothing else than colonial Thalassicollida.
169. Phylogeny of the Acantharia.—The legion Acantharia or Actipylea is distinguished by its peculiar acanthin skeleton, which develops centrogenously, as well as by the disposition in groups of the pores in its central capsule, and its excentric usually precocious nucleus; it is thus so different from all other Radiolaria as undoubtedly to furnish, phylogenetically considered, an independent stem (§ 7). This stem is only connected at the root by Actinelius with the primitive form of the Spumellaria, Actissa. The stem is monophyletic, since all the forms belonging to it may be derived without violence from Actinelius as a common primitive form.
170. Origin of the Acantharia.—The genus Actinelius (p. 730, Pl. 129, fig. 1), which may naturally be regarded as the common primitive form of all Acantharia, possesses a spherical central capsule, which in consequence of the early division of the nucleus (§ 63), encloses numerous small nuclei; from its centre arise many simple radial spines of equal size, which penetrate the central capsule. A large number of radial pseudopodia issue between the spines from the sarcomatrix which surrounds the capsule. Actinelius may have been directly derived from Actissa, the common stem-form of all Radiolaria, by the division of the pseudopodia into two groups, myxopodia, which remained soft, and axopodia, which became firm (§ 95A). As the latter became changed into strong acanthin rods, and touched each other in the centre, they forced the nucleus from its originally central position and brought about its early division. Actinelius is also of all Radiolaria the form which, next to Actissa, most nearly approaches the Heliozoa. If the stiff axial threads of Actinosphærium be conceived of as partially converted into acanthin spines, and its nucleated medullary substance as separated from the alveolar cortical layer by a membrane (central capsule), then Actinelius would be produced.
171. Hypothetical Genealogical Tree of the Acantharia:—
| Diploconida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Phractopeltida | Hexalaspida | Cenocapsida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Phatnaspida | Lychnaspida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Porocapsida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Coleaspida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Ceriaspida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Belonaspida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Phractaspida | Stauraspida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Astrocapsida Sphærocapsida |
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| Diporaspida (Dorataspida dipora) |
Tessaraspida (Dorataspida tetrapora) |
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| [Dorataspida] | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Quadrilonchida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Phractacanthida | Stauracanthida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Amphilonchida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Acanthonia | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Astrolonchida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Litholophida | Chiastolida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Zyganthida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Acanthonida | Actinastrum | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Acanthometron | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Astrolophida | Acanthochiasmida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Acanthometron | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Actinelida Actinelius |
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| Actissa | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Diploconida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Phractopeltida | Hexalaspida | Cenocapsida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Phatnaspida | Lychnaspida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Porocapsida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Coleaspida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Ceriaspida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Belonaspida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Phractaspida | Stauraspida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Astrocapsida Sphærocapsida |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Diporaspida (Dorataspida dipora) |
Tessaraspida (Dorataspida tetrapora) |
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| [Dorataspida] | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| |
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| Quadrilonchida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Phractacanthida | Stauracanthida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Amphilonchida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Acanthonia | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Astrolonchida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Litholophida | Chiastolida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Zyganthida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Acanthonida | Actinastrum | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Acanthometron | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Astrolophida | Acanthochiasmida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Acanthometron | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Actinelida Actinelius |
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| 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).