157. Palæontological Development.—The palæontology of the Radiolaria already offers very considerable material for study; but in consequence of its incompleteness this is of little value for the study of the phylogeny of the class. By far the larger portion of the fossil Radiolaria belong to the Tertiary period; only quite recently have numerous well-preserved fossil Radiolaria been described from the Mesozoic period, and especially from the Jura. Of Palæozoic Radiolaria (from the coal measures) only slight traces are known. Moreover, the fossil Radiolaria hitherto known have been found only in very circumscribed and widely separated localities. The majority of all the species belong to the small island of Barbados. Although our palæontological acquaintance with the Radiolaria must necessarily be incomplete for this reason, it is still more so since at least thirty out of the eighty-five families (that is more than a third) could not possibly leave any fossil remains, either because they possess no skeleton, or because of its chemical composition.
Of the four legions of the Radiolaria, the Acantharia (on account of the solubility of their astroid acanthin skeletons) have entirely vanished and have never been found fossil. Of the Phæodaria, whose silicate skeleton is not as a rule capable of fossilisation, only one section (Dictyochida) of a single family (Cannorrhaphida) has been observed fossil. Hence the fossil remains of the Radiolaria belong almost exclusively to the two legions, Spumellaria and Nassellaria, which were formerly united under the term "Polycystina." Among these, however, the skeletonless Thalassicollida, Collozoida, and Nassellida could leave no traces. Hence there only remain fifty-five families of which we might expect to find fossil siliceous skeletons. Even of these, however, scarcely the half are certainly known in the fossil condition, whilst of the remainder nothing certain is known; for example, of the large order Larcoidea (among the Spumellaria) and of the Stephoidea (among the Nassellaria) with a few isolated exceptions, no fossils are known. The great majority of fossil Radiolaria belong to the two Nassellarian orders Cyrtoidea and Spyroidea (two relatively very highly developed groups); next to these follow the orders Discoidea and Sphæroidea among the Spumellaria. From these palæontological facts it is obvious that our present very incomplete acquaintance with the fossil Radiolaria is quite insufficient to warrant us in drawing any conclusions from it regarding the phylogenetic development or palæontological succession of the individual groups.
158. Origin of the Four Legions.—The agreement of all Radiolaria in those constant and essential characters of the unicellular body, which distinguish them from all other Protista (especially the differentiation of the malacoma into a central capsule and extracapsulum), justifies the conclusion that all members of this class have been developed from a common undifferentiated stem-form. Only the simplest form of the Spumellaria, a skeletonless spherical cell with concentric spherical nucleus and calymma, can be regarded as such. The simplest form of the Thalassicollida which is now extant (Actissa, Procyttarium, p. 12), corresponds so exactly to the morphological idea of that hypothetical stem-form that it may unhesitatingly be regarded in a natural system as the common point of origin of the whole class. On the other hand, Actissa is so closely related to the simple Heliozoa (Actinophrys, Actinosphærium, Heterophrys, Sphærastrum, &c.) that its origin from this group of Rhizopoda is exceedingly probable. The three legions Acantharia, Nassellaria, and Phæodaria are to be regarded as three main diverging branches of the genealogical tree, which have been developed in different directions and are only connected by their simplest stem-forms (Actinelius, Nassella, Phæodina) with the stem-form of the Spumellaria, the primordial Actissa.
159. Phylogeny of the Spumellaria.—The legion Spumellaria or Peripylea is to be regarded as the common stem-group of the Radiolaria, and its simplest form, Actissa, as the primitive genus or radical form of the whole class; for it possesses in the simplest and most undifferentiated form all those characters by which the Radiolaria are distinguished from other Protista; all the other genera of the class may be derived from it by successive modifications. Considered as a legion the whole group Spumellaria is undoubtedly monophyletic, for all its members possess those essential characters by which it is distinctively marked off from the other three legions, more especially a simple capsule-membrane, which is everywhere evenly perforated by innumerable small pores; the nucleus lies originally in the centre of the spherical central capsule. Furthermore, all Spumellaria lack those positive characters which distinguish the three remaining legions—the centrogenous acanthin skeleton of the Acantharia, the basal porochora and the monaxon podoconus of the Nassellaria, the astropyle and phæodium of the Phæodaria.
160. Origin of the Spumellaria.—The genus Actissa (p. 12, Pl. 1, fig. 1) presents the Radiolarian type in its simplest and most primitive form—a spherical central capsule, which encloses in its middle a spherical nucleus, and which is surrounded by a spherical calymma. The whole unicellular body consists, therefore, of three concentric spheres, and possesses neither skeleton nor alveoles, nor other differentiated parts. The innumerable fine pseudopodia, which issue from the central capsule through the evenly distributed pores in its membrane, radiate in all directions through the calymma and pass out over its surface. Actissa can, therefore, be directly derived phylogenetically from the simplest skeletonless Heliozoa (Actinophrys, Heterophrys, Actinosphærium, Sphærastrum). The only essential difference between the two consists in the development of the central capsule, which in Actissa separates as a distinct membrane the endoplasm from the exoplasm. This differentiation which we regard is the most important distinguishing character of the Radiolaria, has been transmitted by inheritance, along with the formation of flagellate spores in the central capsule, from Actissa, the primitive parent to all the other Radiolaria.
161. Hypothetical Genealogical Tree of the Spumellaria:—
| Larcoidea brace |
Discoidea brace |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Streblonida | Phacodiscaria | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Tholonida | Coccodiscida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Prunoidea brace |
Soreumida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Zygartida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Zonarida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Lithelida | Sphæroidea | Cyclodiscaria | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| brace | Spongodiscida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Stylosphærida | Pylodiscida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Phorticida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Panartida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Artiscida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Phacodiscida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Spongodruppida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Staurosphærida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Spongel- lipsida |
Pylonida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Cyphinida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Porodiscida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Astrosphærida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Larnacida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Cubosphærida | Cenodiscida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Druppulida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Collosphærida | Archidiscida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Larnacilla (Trizonium) |
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| Spongurida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Ellipsida (Cenellipsis) |
Larcarida (Cenolarcus) |
Liosphærida (Cenosphæra) |
Cenodiscida (Cenodiscus) |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| [Actiprunum?] | [Actilarcus?] | [Procyttarium] | [Actidiscus?] | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Cenosphæra (Common stem-form of all Sphærellaria?) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Polycyttaria | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| brace | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Collosphærida | Collozoida | Sphærozoida | brace | Beloidea | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Thalassosphærida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Ethmosphærida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Colloidea | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Thalassicollida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Actissa | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Larcoidea brace |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Streblonida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Tholonida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Prunoidea brace |
Soreumida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Zygartida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Zonarida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Lithelida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Phorticida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Panartida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Artiscida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Spongodruppida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Spongel- lipsida |
Pylonida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Cyphinida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Larnacida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Druppulida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Larnacilla (Trizonium) |
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| Spongurida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Ellipsida (Cenellipsis) |
Larcarida (Cenolarcus) |
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| [Actiprunum?] | [Actilarcus?] | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| ≈ | ≈ | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Discoidea brace |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Phacodiscaria | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Coccodiscida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Sphæroidea | Cyclodiscaria | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| brace | Spongodiscida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Stylosphærida | Pylodiscida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Phacodiscida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Staurosphærida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Porodiscida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Astrosphærida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Cubosphærida | Cenodiscida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Collosphærida | Archidiscida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| ≈ | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Liosphærida (Cenosphæra) |
Cenodiscida (Cenodiscus) |
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Actiprunum Actilarcus |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| [Procyttarium] | [Actidiscus?] | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Cenosphæra (Common stem-form of all Sphærellaria?) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Polycyttaria | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| brace | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Collosphærida | Collozoida | Sphærozoida | brace | Beloidea | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Thalassosphærida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Ethmosphærida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Colloidea | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Thalassicollida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Actissa | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
162. Collodaria and Sphærellaria.—Whilst in all Spumellaria the malacoma agrees in possessing the characteristic features of the legion, and thus justifies its derivation monophyletically from the common stem-form Actissa, the different forms of skeleton, on the other hand, cannot all be referred to the same fundamental form. More especially the spherical lattice-shell, from which all the numerous skeletal forms of the Sphærellaria may be derived, cannot have arisen from the incomplete Beloid skeleton which characterises the Beloidea among the Collodaria. It is probable rather that the formation of the skeleton has taken place independently in those two groups of Spumellaria. From the skeletonless Colloidea, as the common stem-group of the Spumellaria, two different main groups have diverged, on the one hand the Beloidea, whose skeleton consists of separate spicules scattered in the extracapsulum, and on the other hand, the Sphærellaria, which have formed a simple lattice-sphere around the central capsule; from this the manifold forms of the remaining Spumellaria may be derived.
163. Descent of the Sphærellaria.—The extensive order Sphærellaria, which includes all Spumellaria with a complete lattice-shell, develops an extraordinary variety of skeletal structures; these may, nevertheless, all be derived without violence from a common stem-form, or simple spherical lattice-shell, Cenosphæra. The main stem of the order, the extensive suborder Sphæroidea (Pls. 5-30), is derived immediately from Cenosphæra (p. 61, Pl. 12); three diverging branches of it being represented by the other three suborders, the Prunoidea (Pls. 16, 17, 39, 40) being developed by elongation, and the Discoidea (Pls. 31-48) by shortening of the vertical main axis, whilst the Larcoidea (Pls. 9, 10, 49, 50) have originated by the modification of the spherical lattice-shell into a lentelliptical or triaxial ellipsoidal one. Although the monophyletic derivation of all Sphærellaria from Cenosphæra is exceedingly probable, the possibility of a polyphyletic origin for the group is by no means excluded. For even in the skeletonless primitive genus of all the Spumellaria, Actissa (as well as in the social Collozoum), there are found, in addition to the usual spherical types, other species (or subgenera, p. 12) whose central capsule is not spherical but a modification of the sphere; in Actiprunum ellipsoidal; in Actidiscus lenticular; in Actilarcus lentelliptical; if such modified forms of Actissa were to develop their lattice-shells independently, then their form would correspond to that of the central capsule; and such simple ellipsoidal, discoidal, and lentelliptical lattice-shells might have been the primitive forms of the Prunoidea, Discoidea and Larcoidea.
164. Genealogical Tree of the Sphæroidea.—Cenosphæra, the simplest form of the spherical lattice-shell, may be unhesitatingly regarded as the common stem-form of all the Sphæroidea (pp. 50-284, Pls. 5-30). Cenosphæra (p. 61, Pl. 12) arose directly from Actissa simply by the silicification of the spherical exoplasmatic network of the sarcodictyum around the central capsule, on the surface of the concentric calymma. From this simple siliceous extracapsular lattice-sphere all other forms of Sphæroidea have arisen, in the main by the manifold combination of two simple processes, first by the formation of radial spines on the surface of the lattice-sphere, and second, the addition of concentric spherical lattice-shells. Both processes may be utilised as the foundation for a systematic treatment of the Sphæroidea (compare pp. 52-58).
If in the Sphæroidea the characteristic number and disposition of the radial spines be regarded as the most important heritable peculiarity of the different families, then we have the following natural arrangement:—(1) Liosphærida, without radial spines; (2) Cubosphærida, with six radial spines (opposite in pairs in three axes perpendicular to each other); (3) Staurosphærida, with four radial spines (in two axes crossed at right angles); (4) Stylosphærida, with two opposite radial spines (in the vertical main axis); and (5) Astrosphærida, with numerous regularly or irregularly distributed radial spines (eight to twenty or more). If, on the contrary, more stress be laid upon the number of the concentric lattice-shells, then we have the following artificial grouping:—(1) Monosphærida, with one simple lattice-sphere; (2) Dyosphærida, with two concentric lattice-spheres; (3) Triosphærida, with three; (4) Tetrasphærida, with four; (5) Polysphærida, with numerous (five to twenty or more) concentric lattice-shells; (6) Spongosphærida, with a spongy spherical shell. In general the former arrangement appears more natural than the latter, since the number of primary radial spines, which grow out from the primary lattice-sphere, determines their ground-form from the outset, whatever may be the number of secondarily added shells. Strictly speaking, according to the view adopted, these Liosphærida which have several shells, on the outer surface of which there are no radial spines, ought to be classified according to the number and arrangement of their internal radial connecting beams and distributed among the other families. The practical application of this correct principle meets, however, with great difficulties. Also in many cases the phylogenetic relations of the different Sphæroidea are more complicated than would appear from both these classificatory principles. In general their phylogeny will quite correspond with their ontogeny, since from the innermost first formed lattice-shell (primary medullary shell) a number of radial spines arises, and upon these the secondary shells are formed from within outwards.
165. Genealogical Tree of the Prunoidea.—The suborder Prunoidea is very closely related to the Sphæroidea, and is distinguished from it by the elongation of one axis; from the simple lattice-sphere (Cenosphæra) is developed a latticed ellipsoid (Cenellipsis, Pl. 39, fig. 1). The development of this vertical isopolar main axis is foreshadowed even among the Sphæroidea, in that family in which two opposite radial spines grow out of the primary lattice-sphere at the two poles of the vertical main axis (Stylosphærida, Pls. 13, 14). These latter pass over without any sharp boundary into those forms of Prunoidea whose ellipsoidal lattice-shell bears two opposite main-spines (Stylatractida, Pls. 15, 16). Other very intimate relationships between the Sphæroidea and Prunoidea are indicated in certain of the latter by the fact that of the two concentric lattice-shells the inner (medullary) shell is spherical, the outer (cortical) shell ellipsoidal (Pl. 39, figs. 3, 7, 8, 14, 19); often three concentric lattice-shells are present, of which the two inner are spherical intracapsular medullary shells, whilst the outer is an extracapsular cortical shell, ellipsoidal or cylindrical in form (Pl. 39, figs. 4, 12, 17, 18). Owing to the manifold nature of these phylogenetical relations and the variety of their combinations, the derivation of the individual Prunoidea from the Sphæroidea is rendered very difficult; in addition to which it is possible that the simplest Prunoidea (Cenellipsis, Ellipsidium) have been directly developed from the skeletonless Actiprunum (a form of Actissa with ellipsoidal central capsule, p. 14) by the excretion of a simple ellipsoidal lattice-shell on the surface of their calymma.
The phylogeny of the Prunoidea is especially complicated by the formation of peculiar transverse constrictions, perpendicular to the longitudinal axis. They are wanting only in the Monoprunida (Ellipsida, Druppulida, and Spongurida); the Dyoprunida (Artiscida and Cyphinida, Pl. 39, figs. 9-19) possess only one such constriction (in the equatorial plane); the Polyprunida, on the other hand, have three, five, or more parallel constrictions (Panartida and Zygartida, Pl. 40). The chambers, which are separated off by these constrictions, may be regarded as polar sections of incomplete cortical shells.
166. Genealogical Tree of the Discoidea.—The suborder Discoidea is closely related to the Sphæroidea, but separated from it by shortening of one axis; from a simple lattice-sphere (Cenosphæra) a latticed lens or flattened spheroid is developed, whose circular equatorial plane is larger than any other section (Cenodiscus, Pl. 48, fig. 1). The formation of this horizontal equatorial plane is perhaps indicated in that family of Sphæroidea in which four crossed radial spines, lying in one plane, are developed (Staurosphærida, Pls. 15, 31, 42). The morphological and phylogenetical relations of the Discoidea to the Sphæroidea are precisely the converse of those of the Prunoidea; in the latter the vertical axis appears longer, in the former shorter than any other axis of the body. The Discoidea are probably polyphyletic, having originated from several different groups of Sphæroidea; at least two essentially different main groups may be distinguished among them; of these the one is characterised by the formation of a large extracapsular lenticular cortical shell (Phacodiscaria), whilst in the other this typical "Phacoid shell" or lattice-lens is wanting (Cyclodiscaria, compare pp. 403-409).