CHAPTER V.--ONTOGENY OR INDIVIDUAL DEVELOPMENT.

(§§ 141-152.)

141. _Individual Developmental Stages._--The germinal history of the Radiolaria presents great obstacles to direct observation, and hence is very incompletely known. The fragmentary observations, however (having been made on Radiolaria of very various groups and supplemented by comparative anatomical considerations), allow us to draw a general picture of the essential developmental processes in this great class. It may probably be assumed that in all Radiolaria, after maturation, the central capsule discharges the function of a sporangium, and its contents are broken up into numerous flagellate swarm-spores (zoospores). After these flagellate swarm-spores (resembling _Astasia_) have emerged from the ruptured central capsule, they probably pass over into a _Heliozoan_-stage (_Actinophrys_) and then after the formation of a jelly-veil into the condition of _Sphærastrum_. Afterwards, when a membrane is formed between the outer jelly-veil and the inner nucleated cell-body, an _Actissa_-stage arises, which exhibits in its simplest form the differentiation of the spherical unicellular body into the central capsule and calymma. _Actissa_ thus represents both ontogenetically and phylogenetically the primitive condition of the Radiolarian organism, and may thus be regarded as the point of departure of all other forms.

142. _The Astasia-Stage._--The formation of flagellate zoospores in the mature central capsule is probably to be regarded as the common form of individual development in all Radiolaria; since the whole contents are utilised in the formation of these swarm-spores, and since the extracapsulum takes no share in the process and perishes after they are evacuated, the _central capsule_ may be regarded as a _sporangium_ (see note A, below). The zoospores of the Radiolaria generally arise in the following way:--the nucleus of the unicellular organism, sometimes early, sometimes late (and in several different ways, §§ 63-70) breaks up into numerous small nuclei, and each of these surrounds itself with a small portion of the endoplasm. Very often, perhaps generally, this endoplasm contains one or several fat-granules and sometimes also a small oblong crystal; from the protoplasm {xciv}of the small roundish or ovoid cells protrudes one or more vibratile flagella. The fully developed spores, which commence their vibrations even within the central capsule, emerge when it ruptures, and swim about freely in the surrounding water by means of the flagellum. At this stage of its existence the young Radiolarian represents essentially the simplest form of the Flagellata, such as _Astasia_ or _Euglena_; the unicellular body is for the most part ovoid or subcylindrical, sometimes fusiform or reniform, usually from 0.004 to 0.008 mm. in diameter (Pl. 1, fig. 1_c_; Pl. 129, fig. 11). In the anterior part of the flagellate cell, immediately behind the base of the flagellum, lies a homogeneous, spherical nucleus, whilst in the posterior part are usually several small fat-granules and often also a small oblong crystal (hence the name "crystal-spore," "Krystall-Schwärmer"). The number of vibrating flagella, which are extremely long and fine, seems to be variable, usually one, sometimes two, occasionally perhaps three, or even four or more (see note B).

A. The formation of the motile spores in the central capsule was first observed by J. Müller in _Acanthometra_ (1856, L. N. 10, p. 502), then by A. Schneider in _Thalassicolla_ (1858, L. N. 13, p. 41), and finally by myself in _Sphærozoum_ (1859, L. N. 16, p. 141). These older observations were, however, incomplete, for the origin of the motile corpuscles from the contents of the central capsule was not observed. The first complete and detailed observations upon the formation of spores in the Radiolaria were published in 1871 by Cienkowski (L. N. 22, p. 372, Taf. xxix.); they relate to two different Polycyttaria, _Collosphæra_ and _Collozoum_. These investigations were supplemented by R. Hertwig on _Collozoum_ and _Thalassicolla_ (1876, L. N. 26, pp. 28, 43, &c.); on _Collozoum_ he made the important discovery that the Polycyttaria form two kinds of spores, one with and the other without crystals, and that the latter are divided into macrospores and microspores (compare the chapter on "Reproduction," §§ 212-216). Quite recently Karl Brandt has confirmed these observations, and has extended them to all the genera of Polycyttaria (1881, L. N. 38, p. 393, and 1885, _loc. cit._).

B. The number of flagella, projecting from each spore, is very difficult to determine, owing to their extraordinary length and slenderness. It appeared to me that in the majority of those Radiolaria whose spores I investigated only a single flagellum could be demonstrated with certainty, although sometimes two, springing from a common base, seemed to be present. Compare the chapter on "Reproduction," (§ 215) and the recent work of Karl Brandt on Sphærozoea (1885, L. N. 52, pp. 145-174).

143. _The Actinophrys-Stage._--The fate of the flagellate zoospores which emerge from the mature central capsule of the Radiolaria has not hitherto been decided by actual observation; all attempts to rear the swarming zoospores have been in vain, for they have soon died. From what we know, however, of the comparative morphology of the Protista, the hypothesis is fully justified, that between the _Astasia_-stage of the flagellate swarm-spores, and the well-known _Actissa_-stage of the simplest Radiolaria, there lies an intermediate developmental stage, which may be regarded as being essentially the simplest Heliozoan form, _Actinophrys_ or _Heterophrys_. The swarm-spore is very probably converted directly in to a simple floating _Heliozoon_ by its elongated or ovoid body {xcv}becoming spherical and by fine pseudopodia protruding all round instead of a single flagellum; the nucleus at the same time assuming a central position.

144. _The Sphærastrum-Stage._--The _Actinophrys_-stage of the young Radiolaria, which proceeds immediately from the flagellate zoospore, is probably connected with the _Actissa_-stage by an intermediate form, which may be regarded as a simple skeletonless _Heliozoon_ with a jelly-veil; a well-known example of such a form is _Sphærastrum_ (in the solitary, not the social condition) and _Heterophrys_. This important intermediate form has arisen from the simple _Actinophrys_-stage by the excretion of an external structureless jelly-veil, such as is formed in many other Protista (_e.g._, in the encystation of many Infusoria). The young Radiolarian in this second _Heliozoon_-stage becomes a simple cell with pseudopodia radiating on all sides; its body consists of three concentric spheres, the central nucleus, the protoplasmic body proper, and the surrounding calymma or jelly-veil. When a firm membrane is developed between the last two spheres this _Sphærastrum_-stage passes over into the _Actissa_.

The gap in our empirical knowledge which still exists between the flagellate stage (§ 142) and the simplest Radiolarian stage (_Actissa_, § 145), can be filled hypothetically only by the assumption of several _Heliozoon_-stages following one upon another. It is possible also that the capsule-membrane is not formed between the endoplasm and exoplasm (as here supposed), but that the membrane was formed first outside the cell and the extracapsulum subsequently secreted around it.

145. _The Actissa-Stage._--The first SPUMELLARIAN genus, _Actissa_, is not only the simplest form actually observed among the Radiolaria, and the true prototype of the whole class, but also the simplest form under which the Radiolarian organisation can be conceived. It is therefore extremely probably that _Actissa_ not only forms the common stem-form of the whole class in a phylogenetic sense, but is also its common ontogenetic or germinal form. Probably in all Radiolaria the _Sphærastrum_-stage develops immediately into the typical _Actissa_-stage, by the formation of a firm membrane between the protoplasmic body of the spherical Heliozoan cell and its jelly-veil. Thus arises the characteristic central capsule, which is wanting in the nearly related Heliozoa. It is further probable that all Radiolaria in their early stage will so far conform to the state of things in _Actissa_ as to have the capsule-membrane of the spherical skeletonless cell perforated everywhere by fine pores. This structure is retained in all SPUMELLARIA, whilst in the other three legions those structural relations of the capsule which are characteristic of each develop from the _Actissa_-stage.

146. _The Ontogeny of the Spumellaria._--In the simplest case the individual development in the SPUMELLARIA ceases with the _Actissa_-stage. In all other genera of this legion diverging forms proceed from this, of which the different growth of the three dimensive {xcvi}axes on the one hand (§§ 44, 45), and the differentiation of the various parts of the unicellular organism with the formation of the skeleton on the other, are of pre-eminent significance. Even in the varying growth of the central capsule in the different dimensions of space in the skeletonless #Colloidea#, four different modes may be distinguished, which further, in the corresponding development of the skeleton, furnish the basis for the origin of the four orders of #Sphærellaria#. The most primitive and simplest form of growth, equal extension in all directions, is found in the spherical central capsule and the concentric spherical skeletons (_Procyttarium_, #Sphæroidea#). When the growth of the central capsule proceeds more rapidly in the direction of the vertical main axis than in any other direction, the ellipsoidal or cylindrical central capsule (_Actiprunum_) arises, and the vertically elongated skeleton of the #Prunoidea#, which is derived from it. When, on the contrary, the growth of the central capsule and lattice-shell is less in the direction of the vertical main axis than in any other direction, the lenticular or discoid central capsule (_Actidiscus_) arises, and the corresponding lenticular shell of the #Discoidea#. Finally, even quite early in many SPUMELLARIA, the growth of the central capsule and of the corresponding lattice-shell in the three dimensive axes is different, and hence arise the lentelliptical forms whose geometrical type is the triaxial ellipsoid or the rhombic octahedron (_Actilarcus_, #Larcoidea#). Thus the origin of the four orders of #Sphærellaria# is simply explained by a varying growth in the different dimensive axes. The _primary_ (innermost) lattice-shell is in this legion always _simultaneously_ developed (suddenly excreted at the moment of lorication from the sarcodictyum). The _secondary_ lattice-shells, on the other hand, which surround the former concentrically, and are united with it by radial bars, arise _successively_ from within outwards.

147. _The Ontogeny of the Acantharia._--The individual development of the ACANTHARIA in the simplest case (_Actinelius_) stops at a point which differs from the _Actissa_-stage only in the change of radial axial threads into acanthin spines. In the small group #Actinelida#, their number remains variable and usually indeterminate (Adelacantha), whilst in the great majority of the legion (#Acanthonida# and #Acanthophracta#) the number is constantly twenty, and those spines are regularly arranged according to the Müllerian law in five parallel circles, each containing four crossed spines (Icosacantha). The simplest form among these latter is _Acanthometron_, which may be regarded both ontogenetically and phylogenetically as the common starting-point of all the Icosacantha. Within this extensive group variations in the length of the dimensive axes appear, similar to those observed in the SPUMELLARIA. In the Astrolonchida and #Sphærophracta# the central capsule remains spherical, extending equally in all directions; and correspondingly the lattice-shell, which is excreted on the surface of the spherical calymma, remains spherical. In the Belonaspida (just as in the #Prunoidea#) {xcvii}this form passes over into an ellipsoid by prolongation of one axis; on the contrary, in the Hexalaspida (as in the #Discoidea#) the discoidal or lenticular form arises by shortening of an axis. Finally, in the Diploconida, and in some Hexalaspida in which the growth is different in all three dimensive axes (as in the #Larcoidea#), both the central capsule and the shell assume the lentelliptical form. The lattice-shell of the #Acanthophracta# is usually successive in its development, since from each of the twenty radial spines two or four tangential apophyses proceed, whose branches subsequently unite and combine to form the lattice-shell. Only in the peculiar Sphærocapsida can the pavement-like shell arise simultaneously or in a moment of lorication.

148. _The Ontogeny of the Nassellaria._--The individual development of the NASSELLARIA in the simplest instance remains stationary at the skeletonless Nasselid stage (_Cystidium_, _Nassella_), which can be immediately derived from the foregoing _Actissa_-stage by the disappearance of the pores in the upper (apical) hemisphere of the central capsule, whilst in the lower (basal) portion they are modified to form a porochora; the podoconus is developed within the endoplasm upon this latter. Usually the spherical form of the central capsule passes over into an ovoid or ellipsoidal one, the vertical axis which passes through the centre of the porochora being elongated. From the skeletonless Nassellida the other NASSELLARIA may be derived both ontogenetically and phylogenetically by the excretion of an extracapsular siliceous skeleton. Unfortunately, the earliest stages in the formation of this skeleton are unknown, and hence no answer can at present be given to the important question, in what order the three primary skeletal elements of the NASSELLARIA (the basal tripod, sagittal ring, and latticed cephalis) appear (compare §§ 111 and 182). If, for example, in _Cortina_ and _Tripospyris_ the basal tripod were to appear first in the ontogeny, and the sagittal ring were developed from this, then the #Plectoidea# would be rightly considered to be the oldest forms in the phylogeny of the skeleton-forming NASSELLARIA; and in the contrary case the #Stephoidea# would be so regarded. The relations of growth in the three dimensive axes are very variable in the NASSELLARIA; the three most important factors in this respect (partly separately and partly in combination) are; (1) the development of the basal tripod to a triradial stauraxon form (the ground-form being a three-sided pyramid); (2) the development of the sagittal ring in the median plane of the body (the vertical axis having the poles different); (3) the development of the latticed cephalis outside the central capsule (the poles of the vertical axis being again different). Since the development both of the skeleton and of the malacoma is characterised in most NASSELLARIA by the stronger growth of the vertical axis and the differentiation of the two poles, the allopolar monaxon ground-form acquires a predominant significance in this legion (§ 32); the starting point of most of the further modifications is the basal pole of the vertical main axis. Next to this the sagittal axis is usually the most important determining factor (its dorsal and ventral poles being {xcviii}usually different), more rarely the frontal axis (with equal right and left poles). In the zygothalamous #Spyroidea# (as in the #Stephoidea#) the formation of the shell proceeds from the sagittal ring, whilst in the polythalamous #Cyrtoidea# the latticed cephalis is always the starting point, from which a series of joints (thorax, abdomen, and in the Stichocyrtida, the numerous post-abdominal joints) successively arise (unipolar growth).

149. _The Ontogeny of the Phæodaria._--The individual development of the PHÆODARIA in the simplest case stops with the skeletonless condition of the Phæodinida (_Phæodina_, _Phæocolla_), which can be immediately derived from the foregoing _Actissa_-stage by the disappearance of the pores in the greater part of the central capsule, the characteristic astropyle being developed at the basal pole (§ 60). Since this particular form and structure of the spheroidal central capsule remains the same in all PHÆODARIA, whilst the formation of their skeleton follows very different directions, it follows that further common paths of development are excluded both ontogenetically and phylogenetically. What will be laid down in this respect as regards the phylogeny of the different groups of PHÆODARIA (§§ 194-199) holds true also of their ontogeny. The relations of growth in the three dimensive axes are hence very different in the skeletons of the various groups of PHÆODARIA. This difference is best marked in the #Phæoconchia#, whose bivalved lattice-shells have as their ground-form the rhomboid pyramid of Ctenophora. In most #Phæogromia# the monaxon lattice-shell may develop simultaneously by sudden excretion at a particular moment of lorication; this is also the case with the polyaxon lattice-shells of the #Phæosphæria#. In their future growth the development of basal or radial apophyses is of special importance. In the majority of the PHÆODARIA these apophyses are tubes of silicate filled with jelly (often provided with an axial siliceous thread); thus their development is distinguished by complications which are absent in the case of the other three legions.

150. _Growth._--The growth of the body in the Radiolaria, as in all other organisms, is the fundamental function of individual development (see note A). All structural relations which this richest class of the Protista exhibits may be referred to different forms of growth, either of the unicellular malacoma or of the skeleton which it produces. In general the special development of the skeleton is dependent upon that of the central capsule, and of the sarcodictyum on the surface of the calymma; in the further growth, however, the conditions are reversed, and the condition of the skeleton already formed directly determines the further development of the central capsule and of the calymma with its sarcodictyum. The four legions of Radiolaria show, speaking generally, certain characteristic differences in growth, which are due in great part to the different structure and ground-form of their central capsule. In the two legions of the Porulosa (SPUMELLARIA and ACANTHARIA), in which the central capsule is originally spherical and {xcix}the ground-form of the skeleton either polyaxon or isopolar monaxon, two fundamental and variously combined directions of growth are recognisable; firstly, the _concentric_ growth (equal increase of volume in all directions), and secondly, multipolar or _diametral_ growth (hypertrophy of certain parts in the direction of definite pairs of radii). A different state of things obtains, however, for the most part, in the two legions of the Osculosa (NASSELLARIA and PHÆODARIA), in which the central capsule possesses a vertical main axis with different poles, and the structure of the skeleton is determined by this allopolar monaxon ground-form. The two fundamental directions of growth here combined in the most various ways are, firstly, _unipolar_ growth (starting from the basal pole of the vertical main axis), and secondly, radial or _pyramidal_ growth (characterised by the different development of separate parts in the direction of definite radii). Whilst the growth of the _malacoma_ is dependent on intussusception (as in most organic structures capable of imbibing), the growth of the _skeleton_ in all Radiolaria takes place by apposition (see note B).

A. The earliest investigations into the modes of growth in the Radiolaria are due to J. Müller (L. N. 12, pp. 21-33). More detailed communications I gave myself in my Monograph (L. N. 16, pp. 150-159). The relations there sketched have now, in consequence of the examination of the Challenger collection, undergone many important additions, and in some divisions, important modifications; these are for the most part treated of in the general account of the separate families.

B. The view here maintained, that the skeleton of all Radiolaria grows only by apposition, appeared formerly to have certain exceptions. I thought I had shown that in _Coelodendrum_ the thin-walled tubes grew not only in length but also in thickness, with continuous increase in the lumen (L. N. 16, pp. 152, 360). Further K. Brandt concluded, from the varying size of the median bars in the twin-spicules of _Sphærozoum_, that these siliceous structures grow by intussusception (L. N. 38, p. 401). Both suppositions have been proved erroneous and I have come to the opinion that in all Radiolaria the skeleton grows by apposition.

151. _Regeneration._--Whilst the general course of individual development (perhaps without any exception in the Radiolaria), begins with the formation of zoospores in the central capsule, there yet occurs in some groups a different form of ontogeny, introduced by simple division of the unicellular organism, and coming under the term "regeneration" in its wider sense. This spontaneous division occurs quite commonly in the Polycyttaria (or social SPUMELLARIA), and produces their colonies (compare the chapter on Reproduction, § 213). On the contrary, it has not been observed in the solitary SPUMELLARIA, nor in the ACANTHARIA and NASSELLARIA; possibly, however, the peculiar ACANTHARIAN family, Litholophida, has arisen by the division of Acanthonida (compare p. 734). Among the PHÆODARIA division is commonly observed in the order #Phæocystina# (which have an incomplete Beloid skeleton or none), and also in the #Phæoconchia#. In all these cases the increase by division is nothing else than an ordinary case of cell-division, in which bisection of the nucleus precedes that of the central capsule. The regeneration by {c}which each of the two daughter-cells develops to a complete mother-cell depends upon simple growth. Another form of regeneration, different from this, has been observed in _Thalassicolla_. If the central capsule be extracted artificially from the large concentric calymma, the enucleated central capsule produces a new extracapsulum, with sarcomatrix, pseudopodia, and calymma. This experiment may be repeated several times with the same result. (Compare A. Schneider, 1867, L. N. 20.)

152. _The Formation of Colonies._--The individual development of colonies takes place in all three families of the Polycyttaria (Collozoida, Sphærozoida, Collosphærida) in the same simple way, by the repeated division of a single monozootic SPUMELLARIAN. Since these divisions only affect the central capsule and not the extracapsulum, the sister-cells, which arise by repeated division of the mother, remain enclosed in a common rapidly growing calymma. Probably in all Polycyttaria the commencement of the formation of colonies immediately follows the _Actissa_-stage of the monozootic mother-cell (or takes place in the _Thalassicolla_-stage, which arises from the former by the development of alveoles in the calymma). The simple central nucleus separates (by direct nuclear division) into two halves, and the central capsule follows this process of bisection, becoming constricted in the middle between the two daughter nuclei (Pl. 3, fig. 12). In the further growth of the colony the process of division proceeds in the older, now multinucleate, central capsules, in which an oil-globule has taken the place of the original nucleus; then the division of the oil-globules precedes that of the central capsule (Pl. 5, fig. 1). Another mode of growth of the colonies is the multiplication of the central capsules by gemmulation, or the formation of the so-called "extracapsular bodies" (Gemmulæ, § 214). The characteristic skeletal structure of the different species appears at a later stage. Whether ripe central capsules can emerge from the social bond of a coenobium, and, having become isolated, establish the formation of a new colony, is very doubtful. The various forms which the coenobium assumes in the different species of Polycyttaria, are due partly to simple growth, partly to the development of large vacuoles in the calymma.

The _form and size_ of the coenobia appear in many fully developed Polycyttaria to exhibit specific differences, which require further investigation; in the young stage, on the contrary, they are simple spheres or ellipsoids, often cylindrical or sausage-shaped (Pl. 3, figs. 1, 4, 6, 11). In some species the cylindrical gelatinous bodies become moniliform, and separated by transverse constrictions into many segments, each of which encloses a large alveole (Pl. 3, fig. 10). The rare ring-shape (Pl. 4, fig. 1) which I figured in 1862 in the case of _Collozoum_ (L. N. 16, p. 522, Taf. xxxv. fig. 1), I have recently observed in different species of Polycyttaria; it is capable of a very simple mechanical explanation, both ends of a sausage-shaped colony having been accidentally brought into contact by a wave and having united by agglutination. Quite recently Brandt has given a very complete account of the development, form, and growth of Polycyttarian colonies in his work on the colonial Radiolaria of the Bay of Naples (1885, L. N. 52, pp. 71-85).

{ci}CHAPTER VI.--PHYLOGENY OR GENEALOGICAL DEVELOPMENT.

(§§ 153-200.)

153. _Sources of Phylogenetic Knowledge._--For the purpose of constructing a hypothetical genealogical tree of the Radiolaria, as of all other organisms, three sources of information are open to us, viz., palæontology, comparative ontogeny, and comparative anatomy. In the present case, however, these three sources are of very different value; the first two are at present only very inadequately known and have only been partially investigated, hence they can only be utilised to a very slight extent. The comparative anatomy of the Radiolaria, on the other hand, is so completely known, and affords such certain glimpses into the morphological relations of the related groups, that by its aid we are in a position at all events to lay down the general features of their phylogeny with some probability, and to lay the foundation of a natural system.

154. _Natural and Artificial Systems._--Although in the classification of the Radiolaria, as in the case of all other organisms, the natural system must be regarded as the goal of systematic classification, our phylogenetic knowledge of the Radiolaria is too fragmentary and inadequate to admit of the systematic arrangement here adopted being regarded as a thoroughly consistent natural system, that is, as representing the true genealogical tree of the class. Owing, however, to the extraordinary variety of form of the Radiolaria, and the complicated relationships of the larger and smaller groups, a synoptical grouping of the different categories and the erection of a complete, even if to some extent artificial, system, becomes a logical necessity. Under these circumstances, and regard being had to both these conditions, the following systematic treatment of the Radiolaria will appear as a _compromise between the natural and artificial systems_, like all other zoological and botanical classificatory attempts. On the one hand, the attempt is made to arrange the larger and smaller groups as nearly as possible according to their phylogenetic relationships, whilst, on the other hand, the practice of circumscribing each by a definition as clear and logical as possible has been carried out. Since these two efforts naturally often come into contact, the insufficiency of many parts of the arrangement is obvious, hence its hypothetical and provisional character is emphatically stated.

155. _Systematic Categories._--The categories or different orders of divisions have in the Radiolaria, as in all other organisms, no _absolute_ significance, but only a _relative_ value. In itself it is quite unimportant whether the whole group be regarded, as at first, as a _family_ (Ehrenberg, 1847), or as an _order_ (J. Müller, 1858), or as a _class_ (Haeckel, {cii}1881). These different views are regulated, on the one hand, by the known extent of the group and by the amount of our acquaintance with it, and on the other, by comparison with related groups and by reference to their conventional disposition. When, therefore, the whole class, Radiolaria, is here divided into two subclasses, four legions, eight orders, eighty-five families, &c., these artificial categories are drawn up only in the conviction that by this means the easiest survey and most thorough insight into the system as a whole may be attained; this latter will indeed approach as far as possible the ideal of a natural system, but must on numerous practical grounds always remain more or less artificial. Since it is to be expected that with the progress of our systematic knowledge the rank of the various categories will rise, it is possible that in the future the arrangement of the group may be somewhat as follows:--_Phylum_, RADIOLARIA; _Four Classes_, SPUMELLARIA, NASSELLARIA, ACANTHARIA, PHÆODARIA; _Eight Legions_ (Nos. I.-VIII. in the following Table); _Twenty Orders_ (Nos. 1-20 in the Table), &c.

Four Legions. Eight Sublegions. Twenty Orders. Typical Families. { {1. Colloidea, { 1a. Thalassicollida. {I. COLLODARIA { { 1b. Collozoida. { (Spumellaria { { palliata) {2. Beloidea, { 2a. Thalassosphærida. { { { 2b. Sphærozoida. I. Legion { (or Subclass){ { { 3a. Ethmosphærida. SPUMELLARIA { {3. Sphæroidea, { 3b. Collosphærida. (PERIPYLEA) { { { {4. Prunoidea, { 4a. Ellipsida. [Porulosa {II. SPHÆRELLARIA { { 4b. Zygartida. peripylea.] { (Spumellaria { { loricata) {5. Discoidea, { 5a. Phacodiscida. { { { 5b. Porodiscida. { { { {6. Larcoidea, { 6a. Larnacida. { { { 6b. Pylonida.

{ {7. Actinelida, { 7a. Astrolophida. { { { 7b. Litholophida. {III. ACANTHOMETRA { { 7c. Chiastolida. { (Acantharia { II. Legion { palliata) { { 8a. Astrolonchida. (or Subclass){ {8. Acanthonida, { 8b. Quadrilonchida. ACANTHARIA { { { 8c. Amphilonchida. (ACTIPYLEA) { { { { 9a. Sphærocapsida. [Porulosa { {9. Sphærophracta, { 9b. Dorataspida. actipylea.] {IV. ACANTHOPHRACTA { { 9c. Phractopeltida. { (Acantharia { { loricata) { {10a. Belonaspida. { {10. Prunophracta, {10b. Hexalaspida. { { {10c. Diploconida. { {11. Nassoidea, 11. Nassellida. { { {V. PLECTELLARIA {12. Plectoidea, {12a. Plagonida. { (Nassellaria { {12b. Plectanida. { palliata) { { {13. Stephoidea, {13a. Stephanida. { { {13b. Tympanida. { III. Legion { {14. Spyroidea, {14a. Zygospyrida. (or Subclass){ { {14b. Androspyrida. Nassellaria { { (MONOPYLEA) { { {15a. Cannobotryida. {VI. CYRTELLARIA {15. Botryodea, {15b. Lithobotryida. [Osculosa { (Nassellaria { {15c. Pylobotryida. monopylea.] { loricata) { { { {16a. Monocyrtida. { {16. Cyrtoidea, {16b. Dicyrtida. { { {16c. Tricyrtida. { { {16d. Stichocyrtida.

{ { {17a. Phæodinida. { {17. Phæocystina, {17b. Cannorrhaphida. { { {17c. Aulacanthida. {VII. PHÆOCYSTINA { IV. Legion { (Phæodaria { {18a. Orosphærida. (or Subclass){ palliata) {18. Phæosphæria, {18b. Aulosphærida. Phæodaria { { {18c. Cannosphærida. (CANNOPYLEA).{ { { {19a. Challengerida. [Osculosa { {19. Phæogromia, {19b. Castanellida. cannopylea.]{VIII. PHÆOCOSCINA { {19c. Circoporida. { (Phæodaria { { loricata) { {20a. Concharida. { {20. Phæoconchia, {20b. Coelodendrida. { { {20c. Coelographida.

156. _Formation of Species._--The totality of similar forms, which we unite in one species, and which in the earlier dogmatic systems was regarded as a category of absolute value, possesses only a _relative value_ like all other systematic categories (§ 155). According to the individual views of the systematist and the general survey which he has attained of the smaller and larger systematic groups, the conception of a species adopted in his practical work will be wider or narrower. In the present systematic arrangement a medium extent has been adopted. It is shown that in the Radiolaria, as in all other extensive groups of organisms, the constancy of the species is very variable in the different groups. Many families of Radiolaria are very rich in "bad species," _i.e._, very _variable_ forms, in which the process of the formation of species is seen in progress; such, for example, are--among the SPUMELLARIA, the Sphærozoida, Stylosphærida, Phacodiscida and Pylonida; among the ACANTHARIA, the Amphilonchida and Phractopeltida; among the NASSELLARIA, the #Stephoidea# and #Botryodea#; and among the PHÆODARIA, the Aulacanthida, Sagosphærida, Castanellida and Concharida. On the {civ}other hand, in some families numerous "good species" may be distinguished, since the intermediate connecting forms are no longer present and the forms have become _relatively constant_. As instances of such families may be mentioned, among the SPUMELLARIA, the Astrosphærida, Cyphinida, Porodiscida and Tholonida; among the ACANTHARIA the Quadrilonchida and Dorataspida; among the NASSELLARIA, the #Spyroidea# and #Cyrtoidea#; among the PHÆODARIA, the Challengerida, Medusettida, Circoporida and Coelographida. The more carefully the different groups are studied, the more numerous the individuals of each species under comparison, the greater becomes the number of "bad" species among the Radiolaria, and the smaller the number of good ones. Originally, no doubt, all "species bonæ" were "malæ." There may be observed in the manifold skeletal forms of the Radiolaria, on the one hand, the utmost accuracy of configuration, and on the other, the greatest variability, and hence a careful comparative study of them leads to a firm conviction of the gradual "Transformation of Species," and of the truth of the "Theory of Descent."

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 {cv}#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, {cvi}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.

{cvii}161. _Hypothetical Genealogical Tree of the Spumellaria_:--

LARCOIDEA DISCOIDEA ~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~ Streblonida PHACODISCARIA | Coccodiscida | THOLONIDA | | | | PRUNOIDEA | | | ~~~~~~~~~~~~~~~~~~ Soreumida| | | | | | | Zygartida | | | | | | | | | | | |Zonarida| | | |Lithelida| | SPHÆROIDEA |CYCLODISCARIA | | | | |~~~~~~~~~~~~~ | Spongodiscida | | | | |Stylosphærida |Pylodiscida| | Phorticida| | | | | | | | | | | | | | | | Panartida | | | | | | | | | Artiscida| +----+---+ | | | | | | | | | | | | | | | | | Phacodiscida| | |Spongodruppida | | | | | | | | | | | | Staurosphærida | +-----+ | | | | Pylonida | | | | | | | | | | | Cyphinida | | +---------+ | | | | | | | | | Porodiscida | | | | Astrosphærida| | | +-----+ | | | | | | | | | | | Cenodiscida | |Spongellipsida| Larnacida | | | | | | | | | | | Archidiscida | | | | |Cubosphærida | | Druppulida | | | | | +--------+ | | | | | | | | | | | | |Collosphærida | | | | | | | | | | | | LARNACILLA +-----+-----+ | | | | (Trizonium) | | | Spongurida | | | | +------+-------+ | | | | | | | Ellipsida Larcarida Liosphærida Cenodiscida (CENELLIPSIS) (CENOLARCUS) (CENOSPHÆRA) (CENODISCUS) | | | | [Actiprunum?] [Actilarcus?] [Procyttarium] [Actidiscus?] | | | | +---------------+------+-------+--------------+ | CENOSPHÆRA (Common stem-form of all Sphærellaria?) | | | | POLYCYTTARIA | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |Collosphærida Collozoida Sphærozoida } | | | | } BELOIDEA +-----+ | Thalassosphærida } | | | Ethmosphærida | | | | | +----------------+----------+ | COLLOIDEA | Thalassicollida | ACTISSA

{cviii}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 {cix}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 {cx}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).

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 {cxi}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 {cxii}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.

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

Diploconida | | Phractopeltida Hexalaspida Cenocapsida | | | | Phatnaspida | Lychnaspida | | | | | | | | | | Porocapsida | | Coleaspida | | | | | | | | Ceriaspida +-----+-----+ | | | | | | | +--+--+ | | | | Belonaspida | | | | | | Phractaspida | Stauraspida | | | | | | | | Astrocapsida | | | Sphærocapsida +-------+----------+ ------+------ | | | | Diporaspida Tessaraspida | (Dorataspida dipora) (Dorataspida tetrapora) | | | | | | | +--------+--------+ | | | | | | | [Dorataspida] | | | | | | | | | Quadrilonchida | | | | | | Phractacanthida | Stauracanthida | | | | | | Amphilonchida | +--+--+ | | | | | | | Acanthonia | | | | +---------+--------+--+------------------+ | Astrolonchida | Litholophida | Chiastolida | | | | Zygacanthida | | Acanthonida Actinastrum | Acanthometron | Astrolophida | | | Acanthochiasmida | | | | | | | | | Acanthometron | | | | | +----------+-----------------+-------------------+---------+ | Actinelida Actinelius | | Actissa

{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:_--

PHÆOCONCHIA ~~~~~~~~~~~~~~~~~~~~ PHÆOSPHÆRIA Coeloplegmida PHÆOGROMIA ~~~~~~~~~~~~~~~~~~~ | ~~~~~~~~~~~~~~~~~~ Aularida | Tuscarorida | | | |Aulonida | Coelodrymida | | | | | | +---+ Coelotholida | | | Coelographida | | Haeckelinida | | | | | | Conchopsida | Coelodorida | | Aulosphærida | | Coelodendrida |Circogonida| | | +----+---+ | | | | Conchasmida | | | | | Concharida | | | | | Sagmarida +-----------+ | +--+--+ | | | | | Cannosphærida | |Castanellida | Circoporida | | | | | | | |Oroscenida Concharida| +------+-+ | | | | | | | Sagenida | | | | | Sagophærida | | |Gazellettida | | | | | | | | | | Oronida | | |Pharyngellida| | |Orosphærida | | | | | | | | | | | | | +--------+-----+ | | | | | | | | | | | +--------------+-+ | | | | | |Euphysettida| | | |Medusettida | | | | | | | | | |Lithogromida | | | |Challengerida| Phæodinida | | | | | | | | | PHÆOCYSTINA | +-----+-+----+ | ~~~~~~~~~~~~~~~~~~~~ | | | Aulacanthida | | | |Cannobelida Catinulida | +------+----+ | | | | | | | Dictyochida | | | | | | | | | | +-----+-------+ | | | | | Phæodinida | Cannorrhaphida | Phæodinida | | | | | | +----------+------------+----+------+----------+ | Phæodina | (Phæometra) | Actissa

{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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