Report on the Radiolaria Collected by H.M.S. Challenger During the Years 1873-1876, First Part: Porulosa (Spumellaria and Acantharia)

Chapter V: Ontogeny or Individual Development (2)

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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.

{cxiii}171. _Hypothetical Genealogical Tree of the Acantharia_:--

{cxiv}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 {cxv}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 {cxvi}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.

{cxvii}180. _Hypothetical Genealogical Tree of the Nassellaria._

CYRTOIDEA.
~~~~~~~~~~~~~~~~~~~~~~~~~
BOTRYODEA _Triradiata_
~~~~~~~~~~~~~
Pylobotryida Podocampida
| |
| _Eradiata_ | _Multiradiata_ SPYROIDEA
| | | ~~~~~~~~~~~~~~~~~
| Lithocampida | Phormocampida Androspyrida
| | | | |
Lithobotryida | Podocyrtida | |
| | | | |
| Theocyrtida | Phormocyrtida |
Tholospyrida
| | | | | |
Cannobotryida | Tripocyrtida | Phormospyrida |
| | | | | |
| Sethocyrtida | Anthocyrtida | |
| | | | | |
| | Tripocalpida | | |
| | | | | |
| Cyrtocalpida | Phænocalpida +---+----+
| | | | |
STEPHOIDEA | +---------+------------+ Zygospyrida
~~~~~~~~~~~~ | | (Spyroidea
triradiata)
Tympanida | Tripocalpida |
| | (Cyrtoidea triradiata monocyrtida) |
| | | |
Coronida | +------------------+-----------------------------+
| | |
| Semantida |
| | |
+--+---+ Cyrtellaria
| |
| Cortiniscus |
| | |
Stephanida | |
| | |
+-----+------+ |
| |
Cortina *--------- Cortinida (PLECTELLARIA)
(Cortina)
|
Plectaniscus ---- Plagoniscus ---- }
| }
Tetraplecta ---- Tetraplagia ---- } PLECTOIDEA
| } Plectanida
Plectophora ---- Plagiacantha ---- } |
| } |
Triplecta ---- Triplagia ---- } Plagonida
| }
Nassoidea
(Nassellida)
|
Nassella
(Cystidium)
|
Actissa

{cxviii}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#. {cxix}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 {cxx}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_ {cxxi}(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 {cxxii}(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._, {cxxiii}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.

192. _Phylogeny of the Phæodaria._--The legion PHÆODARIA or CANNOPYLEA is so clearly marked off from other Radiolaria by the double membrane of the central capsule and the astropyle at its oral pole, as well as by the extracapsular phæodium, that it must be regarded phylogenetically as an independent stem (§ 9). This stem is only connected at its root by _Phæodina_ with the stem-form of the SPUMELLARIA, _Actissa_. The stem itself is monophyletic, inasmuch it its members may be derived without violence from the skeletonless Phæodinida (_Phæodina_, _Phæocolla_). On the other hand, the formation of the skeleton of the PHÆODARIA is undoubtedly polyphyletic, different Phæodinida having independently commenced the formation of a skeleton and having carried it out in very different ways.

193. _Origin of the Phæodaria._--The Phæodinida (p. 1544, Pl. 101), which may naturally be regarded as the common stem-group of the PHÆODARIA, have their nearest relations among other Radiolaria in the Thalassicollida (p. 10); and since this family is to be regarded as the primitive group of all Radiolaria, they may be directly derived from them phylogenetically. The essential modifications by which the primitive Phæodinida have arisen from the more archaic Thalassicollida are of three kinds; (1) the doubling of the membrane of the central capsule; (2) the reduction of the numerous fine pores in the membrane and the formation of an osculum, and of an astropyle closing it, at the oral pole of the main axis; (3) the production of an extracapsular phæodium. This last may, perhaps, be regarded as a unilateral hypertrophy of the voluminous pigment masses which are deposited in the sarcomatrix of certain Thalassicollida. Of the two genera of Phæodinida hitherto known, probably _Phæodina_ (Pl. 101, fig. 2) approaches the original stem of the PHÆODARIA more nearly than _Phæocolla_ (Pl. 101, fig. 1), for the latter exhibits only the large main opening of the central capsule (astropyle), whilst the former possesses also a pair of accessory openings (parapylæ). The hypothetical stem-form (_Phæometra_) presumably had a larger number of small parapylæ (like many Circoporida and Tuscarorida), and the astropyle was probably but little differentiated from them.

{cxxiv}194. _Hypothetical Genealogical tree of the Phæodaria:_--

{cxxv}195. _Phæocystina and Phæocoscina._--Whilst the malacoma of all PHÆODARIA possesses the characteristics of the legion, and hence justifies the assumption of a monophyletic origin, the skeleton, on the other hand, shows in the different groups such manifold and fundamental variations that a polyphyletic origin of the latter is indubitable. Different Phæodinida have commenced the formation of the skeleton independently, and it has progressed in different directions. In the #Phæocystina# it remained incomplete and led to the formation of various Beloid skeletons, whilst the #Phæocoscina# developed complete lattice-shells. Both of these divisions too are to be regarded as polyphyletic, since the skeletal forms of the different groups cannot be derived without violence from a common primitive form.

196. _Phæocystina with a Beloid Skeleton._--The order #Phæocystina# includes all PHÆODARIA which have no complete lattice-shell; it contains, firstly, the skeletonless Phæodinida (the common stem-group of the legion), and secondly, the Phæacanthida, or PHÆODARIA with a Beloid skeleton (§ 115). The latter are divisible into several very different groups (at least two or three) which are probably different in origin. The Aulacanthida (Pls. 102-105) form radial tubes which perforate the calymma, their proximal end resting upon the surface of the central capsule, whilst the distal extremity projects freely outwards. The skeleton of the Cannorrhaphida, on the other hand, is composed of many separate portions which are never radially arranged but are either placed tangentially to the surface of the calymma or scattered irregularly in its gelatinous mass. Furthermore, in the three subfamilies of which this family is composed, the individual skeletal portions are so different that they have probably arisen independently of each other; in the Cannobelida they form cylindrical tangential tubes (Pl. 101, figs. 3-5), in the Catinulida flat basin or cap-like structures (Pl. 117, fig. 8), in the Dictyochida hollow rings, from which small pyramids are developed by unilateral formation of lattice-work (Pl. 101, figs. 9-14; Pl. 114, figs. 7-12).

197. _Phæosphæria with a Sphæroid Skeleton._--The order #Phæosphæria# includes those PHÆODARIA which possess a spherical (sometimes slightly modified) lattice-shell without the characteristic aperture of the #Phæogromia#. They have probably arisen independently of these, though they may have been derived from the Castanellida by loss of the shell-aperture, which was present originally. The four families which we have distinguished among the #Phæosphæria#, are so different in the structure of their lattice-shell that their phylogenetic connection is doubtful. In the Orosphærida (Pls. 106, 107) and the Sagosphærida (Pl. 108) the whole lattice-shell consists of a single piece and is unjointed (without astral septa); in the former it is very firm and massive, with thick laminated trabeculæ and polygonal meshes; in the latter it is very delicate and brittle, with filiform trabeculæ and large {cxxvi}triangular meshes. On the other hand, the voluminous shell of the Aulosphærida (Pls. 109-111), and of the Cannosphærida (Pl. 112), is characterised by a very peculiar system of joints; it is composed of numerous separate cylindrical tubes, which are placed tangentially and united at the nodes by stellate partitions or astral septa. The Cannosphærida possess further a simple central Cyrtoid shell, connected with the outer jointed shell by hollow radial trabeculæ. Since many Aulosphærida possess rudiments of such centripetal trabeculæ it is possible that these latter have been derived from the former by the loss of the central Cyrtoid shell; the formation of this monaxon shell perhaps indicates descent from the #Phæogromia# (Castanellida).

198. _Phæogromia with a Cyrtoid Skeleton._--That order of the PHÆODARIA which we designate #Phæogromia#, contains many very different forms, all agreeing in the possession of a Cyrtoid skeleton, or a monaxon lattice-shell, which has a large aperture at one pole of its vertical main axis (§ 123). This Cyrtoid skeleton is sometimes ovoid or conical, sometimes lentiform or helmet-shaped, sometimes polyhedral or almost spherical. Although the principle of its structure is simple and often like that of the Monocyrtida among the NASSELLARIA, yet the structure of the wall and of the apophyses is so different in the various groups of the #Phæogromia#, that the order is probably polyphyletic, and its Cyrtoid shells have arisen independently of each other. Only in the Castanellida (Pl. 113) does the shell-wall usually consist of simple lattice-work; in the Challengerida, on the other hand (Pl. 99), it has an extremely fine Diatom-like structure; in the Medusettida (Pls. 118-128) a peculiar alveolar structure, and in the Circoporida (Pls. 114-117) and Tuscarorida (Pl. 100) it possesses a characteristic porcellanous constitution (with tangential spicules in a porous cement-mass); in the latter of these groups the surface is smooth, in the former peculiarly tabulate; the two families have also different stem-forms.

199. _Phæoconchia with a Conchoid Shell._--The order #Phæoconchia# (Pls. 121-128) is separated not only from all other PHÆODARIA, but also from all other Radiolaria, by the possession of a bivalved shell resembling that of a Lamellibranch; the two valves of this Conchoid skeleton are to be interpreted as dorsal and ventral (§ 128). Probably these bivalved shells are independent products, but possibly they may have been formed by the bisection of a simple spherical lattice-shell; in the former case the #Phæoconchia# would be directly descended from the Phæodinida, in the latter from the Castanellida. The three families which we have distinguished among the #Phæoconchia#, probably constitute a connected stem, the most primitive group of which are the Concharida (Pls. 123-125). From these the Coelodendrida (Pls. 121, 122) have next arisen by the formation of a "galea" upon the apex of each valve, and the growth of hollow tubes from this helmet-like structure. Finally, the Coelographida {cxxvii}(Pls. 120-128) have been developed from the Coelodendrida by the formation of a basal nasal tube (rhinocanna) from each galea, and the formation of a median or paired frenulum, which connects the opening of the nasal tube with the apex of the galea. In the Coelodendrida, as well as in the Coelographida, there are two different subfamilies, of which the more primitive (Coelodorida, Coelotholida) have free branches from the hollow radial tubes, whilst the more recent (Coelodrymida, Coeloplegmida) form an outer bivalved shell by anastomosis of the branches of the tubes.

200. _The Fundamental Biogenetic Law._--The causal connection between ontogeny and phylogeny, which finds its most precise statement in the fundamental biogenetic law, holds in general for the Radiolaria as for all other organisms. In order to furnish direct proof of this, however, a complete empirical knowledge both of individual and of palæontological development would be necessary. In both these directions, as has been shown in the foregoing chapters, our knowledge of the Radiolaria is very incomplete and fragmentary, but still we are able to convince ourselves indirectly of the validity of the law as applied to Radiolaria by the aid of comparative anatomy. This is now so fully known to us (§§ 1-140) that we are able not only to draw a complete and satisfactory picture of their morphology, but also to arrive at most important conclusions regarding the ontogeny and phylogeny of the individual groups. As regards the formation of the multiform skeleton of the Radiolaria, most of the ontogenetic series of forms, with which we have become acquainted by comparative anatomy, are of _palingenetic_ nature; that is, they are primarily due to inheritance and thus of direct phylogenetic significance. On the other hand, among the ontogenetic phenomena of the Radiolaria, as far as they have yet been investigated, only very few are _cenogenetic_, that is, brought about by adaptive modification and without direct significance as regards phylogeny.

{cxxviii}PHYSIOLOGICAL SECTION.

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Report on the Radiolaria Collected by H.M.S. Challenger During the Years 1873-1876, First Part: Porulosa (Spumellaria and Acantharia)Chapter V: Ontogeny or Individual Development (2)

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