Chapter V: Ontogeny or Individual Development (1)

(§§ 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.