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.
| Cyrtoidea brace |
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| Botryodea brace |
Triradiata | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Pylobotryida | Podocampida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Eradiata | Multiradiata | Spyroidea brace |
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| Lithocampida | Phormocampida | Androspyrida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Lithobotryida | Podocyrtida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Theocyrtida | Phormocyrtida | Tholospyrida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Cannobotryida | Tripocyrtida | Phormospyrida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Sethocyrtida | Anthocyrtida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Tripocalpida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Cyrtocalpida | Phænocalpida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Stephoidea brace |
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| Tympanida | Zygospyrida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| (Spyroidea triradiata) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Tripocalpida (Cyrtioidea triradiata monocyrtida) |
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| Coronida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Semantida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Cyrtellaria | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Cortiniscus | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Stephanida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Cortina | * | long dash | Cortinida | (Plectellaria) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| (Cortina) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Plectaniscus | long dash | Plagoniscus | long dash | brace | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Plectoidea | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Tetraplecta | long dash | Tetraplagia | long dash | Plectanida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Plectophora | long dash | Plagiacantha | long dash | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Plagonida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Triplecta | long dash | Triplagia | long dash | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Nassoidea (Nassellida) |
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| Nassella (Cystidium) |
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| Actissa | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Cyrtoidea brace |
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| Botryodea brace |
Triradiata | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Pylobotryida | Podocampida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Eradiata | Multiradiata | Spyroidea brace |
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| Lithocampida | Phormocampida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Lithobotryida | Podocyrtida | Androspyrida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Theocyrtida | Phormocyrtida | Tholospyrida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Cannobotryida | Tripocyrtida | Phormospyrida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Sethocyrtida | Anthocyrtida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Tripocalpida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Cyrtocalpida | Phænocalpida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Zygospyrida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| (Spyroidea triradiata) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Tripocalpida (Cyrtioidea triradiata monocyrtida) |
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| Stephoidea brace |
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| Tympanida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Cyrtellaria | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Coronida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Semantida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Cortiniscus | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Stephanida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Cortina | * | —— | Cortinida | (Plectellaria) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| (Cortina) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Plectaniscus | —— | Plagoniscus | brace | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Plectoidea | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Tetraplecta | —— | Tetraplagia | Plectanida | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Plectophora | —— | Plagiacantha | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Plagonida | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Triplecta | —— | Triplagia | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Nassoidea (Nassellida) |
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| Nassella (Cystidium) |
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| Actissa | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
181. Plectellaria and Cyrtellaria.—The extensive legion Nassellaria far surpasses the other three legions in the endless variety of its skeletal structures, and owing to the complicated relationships of its numerous families presents no lack of difficult phylogenetic problems. All Nassellaria may be divided first into two main groups or sublegions, Plectellaria and Cyrtellaria; the latter having a complete lattice-shell, the former not. Probably the Cyrtellaria have been polyphyletically developed from several different groups of Plectellaria. These groups are, however, connected in such manifold ways that a monophyletic origin of all the Nassellarian skeletons from one original element is possible. Such a primitive element may have been furnished by any one of three different skeletal parts, the sagittal ring, the basal tripod, and the latticed cephalis (compare pp. 891-895, Bütschli, L. N. 40, 41).
182. Phylogenetic Skeletal Elements of the Nassellaria.—The multiform skeleton of the Nassellaria may be referred in different ways to one of the three above-mentioned structural elements. Each of these (p. 891) may by itself form the skeleton; the sagittal ring in the simplest Stephoidea (Archicircus, Lithocircus), the basal tripod in the simplest Plectoidea (Triplagia, Plagiacantha), the latticed cephalis in the simplest Cyrtoidea (Cyrtocalpis, Archicapsa). In the great majority of the Nassellaria, however, two of these elements, or even all three, are found combined. In most Cyrtellaria, more especially, both the sagittal ring and the basal tripod may be recognised in the lattice-shell, though often only in slight rudiments or scarcely perceptible traces. In the Plectellaria also (which possess no latticed cephalis) there are individual genera with complete development both of the sagittal ring and basal tripod; this important combination is especially well represented in the Cortinida (Cortina, Cortiniscus, Stephanium, Stephaniscus, Tripocoronis, &c.). The greatest difficulty as regards the phylogeny of the Nassellaria lies in the fact that the most various combinations of the three elements are presented by closely related or very similar forms. If, in spite of this, a monophyletic hypothesis as to the origin of the Nassellaria seems essential all sides of the three possible hypotheses must receive full consideration and critical comparison (§§ 183-191).
183. Ascent of the Nassellaria from the Plectoidea.—The monophyletic hypothesis (No. 2, p. 893) which regards the basal tripod as the common origin of the skeleton of all Nassellaria, starts from the simplest forms of the Plectoidea (Triplagia, Plagoniscus, Triplecta, Plectaniscus, &c., Pl. 91). All Plectoidea may be immediately derived as diverging twigs of these, as well as all triradial and multiradial forms of Cyrtoidea and Spyroidea; for in all these cases the distinctive triradial (or the derived multiradial) form of skeleton appears directly derivable from the simple basal tripod of the former. The same is perhaps also true of many Botryodea. Furthermore, certain important forms of Stephoidea (Cortina, Cortiniscus, Stephanium, Stephaniscus, &c.), which have a characteristic combination of the sagittal ring and basal tripod, may be immediately derived from such forms of Plectoidea as Plagoniscus cortinaris, Plagiocarpa procortina, Plectaniscus cortiniscus, &c. On the contrary, those Stephoidea and Cyrtoidea in which the basal tripod is wanting can only be derived from the Plectoidea by the assumption that this structure has disappeared in consequence of phylogenetic degeneration. The monophyletic derivation of the Nassellaria from the Plectoidea has more internal probability than that from the Stephoidea, since it is easier to suppose that the Cortinida (Cortina, Stephanium, &c.) have been derived from the Plectoidea (Plagoniscus, Plagiocarpa) than the converse. This view is the basis of the hypothetical tree shown in § 180.
184. Ascent of the Nassellaria from the Stephoidea.—The monophyletic hypothesis (No. 1, p. 893) which regards the primary sagittal ring as the common starting point of the skeleton in all Nassellaria, starts from the simplest forms of Stephoidea (Archicircus, Lithocircus, &c., Pl. 81). All Stephoidea and Spyroidea may be immediately derived from these, as also the majority of the Cyrtoidea and probably of the Botryodea. Those numerous forms of the last two groups, however, which possess no trace of a sagittal ring, can only be derived from the former by the supposition that the latter has completely disappeared in in consequence of gradual phylogenetic degeneration. The same holds true also of the Plectoidea, although certain forms (e.g., Plagiocarpa procortina, Pl. 91, fig. 5; Plectaniscus cortiniscus, Pl. 91, fig. 9) appear to indicate the commencing formation of the sagittal ring by the concrescence of two branches, which approach each other from the upper part of the apical rod and the ventral part of the basal rod. In any case, it is a fact of great phylogenetic significance, that the primary sagittal ring in the cephalis of the Cyrtoidea shows all conceivable stages of degeneration (compare Bütschli, L. N. 40, 41, as well as the general account of and critical comparison of the Nassellaria, pp. 889-895, &c.).
185. Ascent of the Nassellaria from the Cyrtoidea.—The monophyletic hypothesis (No. 3, p. 894) which regards the latticed cephalis as the common point of origin of all the skeletons of the Nassellaria, starts from the simplest forms of the Cyrtoidea, that is, from the Cyrtocalpida or eradial Monocyrtida (Archicorida, Archicapsida, Pls. 51, 52, 98). All Cyrtoidea and Botryodea may be regarded as divergent forms of these monothalamous Cyrtoidea; the polythalamous simply by the addition of fresh joints at the basal pole, the triradiate and multiradiate by the development of three or more apophyses. The origin of the sagittal ring (which presents every stage of development and degeneration in the Cyrtoidea) may be regarded as a mechanical thickening of the latticed plate in the sagittal circumference of the cephalis. By stronger development of this ring and coincident sagittal constriction of the cephalis the order Spyroidea may be derived from the Cyrtoidea. On the other hand, the Plectellaria, which possess no cephalis, and indeed no complete lattice-shell whatever, may be derived from the Monocyrtida by the assumption of a degeneration of this structure; the sagittal ring having been preserved in the Stephoidea, and the tripod of the Tripocalpida in the Plectoidea. Although such a monophyletic derivation of the Nassellaria from the Cyrtocalpida is possible, and though here, too, the Cortinida play an important part as connecting links, this hypothesis has less internal probability than that of the derivation from the Stephoidea (§ 184) or Plectoidea (§ 183).
186. Genealogical Tree of the Plectoidea.—The order Plectoidea includes those Nassellaria whose rudimentary skeleton does not contain the characteristic sagittal ring of the Stephoidea, but consists of several (at least three) radial spines, which proceed from a point in the centre of the porochora. The branches of these radial spines remain free in the Plagonida, whilst in the Plectanida they unite with each other to form a loose meshwork (not, however, a complete lattice-shell). The number and arrangement of the radial spines, which serve for generic distinctions, are the same in both families, so that each genus of the Plectanida has arisen from a corresponding genus of the Plagonida. The simplest Plagonida, which possess a basal tripod (Triplagia or Plagiacantha with three rays, Tetraplagia with four rays) are probably to be regarded as forming the common origin of the whole order. These agree with certain three- and four-rayed skeletal pieces of the Beloidea (Thalassosphærida and Sphærozoida); and also the four and six-rayed twinned pieces of the latter (spicula bigemina and trigemina) repeat in the same fashion the skeleton of the former (Plagonidium, Plagonium). This similarity, however, is a mere analogy and possesses no phylogenetic significance. On the other hand, certain Plagonida (Plagoniscus, Plagiocarpa), and the corresponding genera of Plectanida (Plectaniscus, Periplecta) seem to have important phylogenetic relations to certain Stephoidea (Cortina, Cortiniscus, &c.); the sagittal ring of the latter having perhaps arisen by the vertical apical spine of the former having been connected with their horizontal basal rod by two ventral apophyses growing out opposite to each other (compare pp. 902, 914, Plagiocarpa procortina, Pl. 91, fig. 5). In this case the Plectanida would belong to the simplest stem-forms of the Nassellaria.
187. Genealogical Tree of the Stephoidea.—The order Stephoidea includes all those Nassellaria whose skeleton does not form a complete lattice-shell, but consists of one or more rings, and often of a loose meshwork which arises by the union of branches of the rings. A vertical sagittal ring is constantly present, embracing the central capsule in the median sagittal plane, and forming at its basal pole various processes, the starting point for other skeletal forms. The most important of these is the tripodal Cortina (p. 950, § 182). The Stephanida are the most archaic family among the Stephoidea (p. 937, Pl. 81), perhaps indeed among all the Nassellaria (§ 184); in them the sagittal ring and its processes alone constitute the skeleton; secondary rings and meshes are wanting. Two diverging families, the Semantida and Coronida, have been developed from the Stephanida, and from one of them the family Tympanida has arisen.
The Semantida (p. 953, Pl. 92) develop a horizontal basal ring at the oral side of the vertical sagittal ring; the basal meshes or lattice gates, which remain between the former and the latter, are the important cortinar pores (one pair jugular, one pair cardinal, p. 954); they usually appear inherited in the cortinar septum of the Cyrtellaria. In the Coronida (p. 967, Pls. 82, 94) a second vertical ring (the frontal ring) appears in addition to the sagittal ring; it lies in the frontal plane at right angles to the latter. Finally the Tympanida (p. 987, Pls. 93, 94) have probably arisen from the Semantida by the formation of a second horizontal ring (mitral ring) parallel to the basal and attached to the upper portion of the sagittal ring.
188. Genealogical Tree of the Spyroidea.—The extensive order Spyroidea is of especial interest in connection with the phylogeny of the Nassellaria, since all its members show two well-developed skeletal elements in combination, the sagittal ring of the Stephoidea and the latticed cephalis of the Cyrtoidea; the majority possess also the basal tripod of the Plectoidea (or a radial skeleton derived from it). Hence there is a possibility of deriving the stem-forms of the Spyroidea from each of these three groups. The four families of this order exhibit similar relationships to those of the four families of Cyrtoidea; the common stem-group is the family Zygospyrida; from this the Tholospyrida have arisen by the development of a galea on the apical pole, the Phormospyrida by the addition of a thorax on the basal pole. The Androspyrida may be derived either from the Tholospyrida by the formation of a basal thorax, or from the Phormospyrida by the development of an apical galea. Some groups, however, such as the peculiar Nephrospyrida (Pl. 90) have probably been developed directly from the Stephoidea.
189. Genealogical Tree of the Botryodea.—The peculiar order Botryodea (p. 1103), which is both difficult to investigate and insufficiently known, presents great phylogenetic difficulties both as to its ascent and descent. Probably the different genera of this order have been polyphyletically developed from different groups of Cyrtoidea (perhaps also to some extent of Spyroidea) by the formation of lobes in the cephalis. The three families of Botryodea are related to each other in the same way as are the three first families of the Cyrtoidea. From the single-jointed Cannobotryida (corresponding to the Monocyrtida), the two-jointed Lithobotryida (corresponding to the Dicyrtida), may be derived by the development of a basal thorax, and from the latter the three-jointed Pylobotryida (like the Tricyrtida) by the addition of an abdomen. In the last two families the forms with an open basal mouth (Botryopylida and Botryocyrtida) are to be regarded as primitive: the Botryocellida and Botryocampida have arisen by the closure of this mouth with a basal lattice-plate.
190. Genealogical Tree of the Cyrtoidea.—The multiform and extensive group Cyrtoidea presents the greatest difficulties to be found in the phylogeny of the Nassellaria, because their morphological relations are most complicated, and because similar forms very often appear to be of quite different origin. The great majority of the Cyrtoidea show more or less clearly a combination of the three structural elements: sagittal ring, basal tripod, and latticed cephalis (p. 891). There are also, however, numerous Cyrtoidea, whose skeleton no longer shows any trace of the sagittal ring. Many of these show as the basis of the skeleton a strong basal tripod with an apical spine, around which the cephalis has obviously been secondarily developed, e.g., the remarkable Euscenida (p. 1146, Pls. 53, 97) and the interesting Callimitrida (p. 1217, Pls. 63, 64). These may have been derived immediately from the Plectoidea without any relation to the Stephoidea. There are also numerous true Monocyrtida, whose shell consists of a simple latticed cephalis without a trace of the sagittal ring or basal tripod (Cyrtocalpida, Pl. 51, figs. 9-13; Pl. 98, fig. 13); these may have been developed directly from the skeletonless Nassellida by the formation of a simple ovoid Gromia-like shell, and may have no relation either to the Stephoidea or Plectoidea. On these grounds, as well as from the complicated relationships of the many smaller groups of Cyrtoidea, it is probable that the whole order has been developed polyphyletically from different divisions of the Plectellaria.
191. Systematic Arrangement of the Cyrtoidea.—Although for the reasons just given no systematic arrangement of the Cyrtoidea can at present, or for a long time in the future, be regarded as other than artificial, yet some general principles of classification for this extensive group can be laid down, which may serve as starting points for some future natural disposition. This is especially true of the relations which in an artificial system (p. 1129) were primarily utilised for the distinction of twelve families and twenty-four subfamilies; the number of segments in the shell, the number of radial apophyses (and parameres), and the constitution of the basal aperture of the shell.
As regards the number of segments, separated by transverse constrictions, of which the shell is composed, it is dependent upon the secondary addition of new joints at the basal pole of the main axis. Hence all many-jointed Cyrtoidea are to be derived from single-jointed ones, and the four sections thus distinguished (Monocyrtida, Dicyrtida, Tricyrtida, Stichocyrtida) form a phylogenetic series. Very often, however, the primary cephalis disappears owing to retrograde metamorphosis; and in such cases the single joint of the apparent Monocyrtida is formed of the thorax (e.g., Pls. 52, 54, &c.). As regards the number of radial apophyses, three sections of Cyrtoidea may be distinguished; the Pilocyrtida with three, the Astrocyrtida with numerous apophyses, and the Corocyrtida with none (p. 1129). The last two may in general be regarded as two divergent branches from the first, for the eradiate Corocyrtida have probably been formed from the triradial Pilocyrtida by entire loss of the radial apophyses, whilst on the other hand the multiradiate Astrocyrtida have arisen from them by additions to the primary apophyses (interpolation of interradial between the perradial ones). As regards the constitution of the shell-aperture, the Cyrtoidea may be divided into Cyrtaperta and Cyrtoclausa (p. 1129); in general the Cyrtoclausa (with latticed shell-aperture) have arisen from the Cyrtaperta (with simple open mouth); in many Monocyrtida the converse may be supposed, the simple basal mouth having been formed by degeneration of a basal lattice.