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:--

{cxiv}172. Adelacantha and Icosacantha.--The numerous forms of ACANTHARIA, here disposed in twelve families and sixty-five genera, may be divided phylogenetically into two main groups of very different extent--Adelacantha and Icosacantha. The more primitive group, Adelacantha, have an indefinite and variable number of radial spines, which are always quite simple in form and usually irregularly distributed; this main division includes only the one order #Actinelida#, with six genera, among which is Actinelius, the common stem-form of all the ACANTHARIA. The more recent group, Icosacantha, includes all the other ACANTHARIA (fifty-nine genera), and is very markedly distinguished from the Adelacantha by the fact that the radial spines are always twenty in number, and arranged according to Müller's law (compare pp. 717-725, and § 110). Since this regular disposition (in five alternating zones each of four spines) has been retained by inheritance in the whole of the Icosacantha, it is probable that this large group has been developed monophyletically from a twig of the Adelacantha; Actinastrum (p. 732) and Chiastolus (p. 738) still present connecting links between the former and the latter, between Actinelius and Acanthometron.

173. Acanthonida and Acanthophracta.--The extensive main division Icosacantha (§ 110), which embraces all ACANTHARIA with twenty radial spines, disposed according to Müller's law, may be subdivided into two large groups or orders:--the #Acanthonida# (p. 740, Pls. 130-132) and the #Acanthophracta# (p. 791, Pls. 133-140). The latter possess a complete extracapsular lattice-shell, which the former have not. The more recent #Acanthophracta# may be derived phylogenetically from the more primitive #Acanthonida# simply by the development of this lattice-shell, with which process are usually (perhaps always) connected certain alterations in the malacoma, e.g., degeneration of the myophriscs (§ 96). The most primitive form of all Icosacantha is the genus Acanthometron (p. 324), in which all the twenty acanthin spines are of the simplest constitution and of equal dimensions.

174. Differentiation of the Acanthonida.--The order #Acanthonida#, which embraces all Icosacantha which have no complete lattice-shell, divides early into three main branches, the three families Astrolonchida, Quadrilonchida, and Amphilonchida (p. 727, Pls. 130-132). The first of these constitutes the common stem-group from which the other two as well as the whole group #Acanthophracta# have been developed; the common stem-form of all is Acanthometron (§ 173). All the Astrolonchida (p. 740, Pl. 130) have twenty radial spines of equal size and similar form. On the other hand, in the Quadrilonchida (p. 766, Pl. 131) the four equatorial spines differ from the others in size and sometimes also in form. In the Amphilonchida (p. 781, Pl. 132) two opposite equatorial spines (lying in the hydrotomical axis) are much larger than the other eighteen and of a different shape. Of the three families of the #Acanthonida# the most important is the primitive group Astrolonchida, for from this the various stem-forms of the #Acanthophracta# arise. They are subdivided according to the formation of the spines into three subfamilies: the Zygacanthida, with simple spines without apophyses (or transverse processes); the Phractacanthida, with two opposite apophyses on each radial {cxv}spine, and the Stauracanthida, with four crossed apophyses on each radial spine. The three genera of the Zygacanthida represent the stem-forms of the three families, since the radial spines in Acanthometron (the most primitive form of #Acanthonida#) are cylindrical, in Zygacantha two-edged, and in Acanthonia four-edged (p. 741).

175. Capsophracta and Cladophracta.--The extensive order #Acanthophracta#, which embraces all ACANTHARIA with a complete lattice-shell, is polyphyletic, its main subdivisions have been developed independently from different branches of the #Acanthonida#. The whole order may be divided directly into two main groups, the #Capsophracta# and #Cladophracta# (p. 793), which differ in the structure and the origin of their lattice-shell. The group (or suborder) #Capsophracta# includes only the single family Sphærocapsida (p. 795, Pl. 133, figs. 7-11; Pl. 135, figs. 6-10); the lattice-shell arises independently of the twenty radial spines, being made up like a pavement of innumerable small acanthin plates, united by a kind of cement; each plate being perforated by a fine pore. In addition twenty larger main pores (or groups of four pores each) are present, corresponding to the twenty radial spines; these are always equal, quadrangular prismatic, without transverse processes as in Acanthonia. In the #Cladophracta#, which include the five remaining families of the #Acanthophracta#, the structure and origin of the lattice-shell are quite different; the lattice-shell is here made up of the branches of the transverse processes, which radiate tangentially from the twenty radial spines and are only united secondarily.

176. Ascent of the Dorataspida.--The group #Cladophracta#, or those ACANTHARIA whose lattice-shell arises by the union of transverse processes of the twenty radial spines, includes five different families, whose stem-group is the family Dorataspida, with a simple spherical lattice-shell. This family itself is, however, diphyletic in origin, being composed of two essentially and originally different subfamilies--Diporaspida and Tessaraspida (p. 803). The Diporaspida (p. 808, Pls. 137, 138) have been developed from the Phractacanthida, and as each radial spine of the latter bears two opposite apophyses, so the lattice-shell of the former has forty primary aspinal pores (two on the base of each spine). On the other hand, the Tessaraspida (p. 830, Pls. 135, 136) have been developed from the Stauracanthida, and as each radial spine of the latter bears four crossed apophyses, so the lattice-shell of the former has eighty primary aspinal pores (four at the base of each spine).

177. Descent of the Diporaspida.--Whilst the Tessaraspida (§ 176) have given rise to no new groups which could take rank as independent families, no less than four separate families of ACANTHARIA have arisen from the Diporaspida. The Phractopeltida (Pl. 133, figs. 1-6) are distinguished from all other ACANTHARIA by the possession of two concentric spherical lattice-shells, and have probably been developed from the {cxvi}Diporaspida in the same way as the Dyosphærida from the Monosphærida among the #Sphæroidea#; in that case the smaller inner lattice-sphere (medullary shell) would be the primary, and the larger outer sphere (cortical shell) the secondary; this latter shows forty primary aspinal pores like those of the Diporaspida. The possibility is not excluded, however, that the small inner lattice-sphere of the Phractopeltida is a secondary product. The three remaining families, which must be regarded as descendants of the Diporaspida, form together a single phylogenetic series, and are separated from the primitive group mainly by the fact that the original spherical form of the lattice-shell has been modified into one distinguished by an elongated equatorial axis (the hydrotomical axis); hence the #Prunophracta# (pp. 794-859). The ellipsoidal Belonaspida have arisen directly by hypertrophy of the two opposite equatorial spines of this hydrotomical axis (p. 859, Pl. 136, figs. 6-9; Pl. 139, figs. 8, 9; perhaps they have also arisen directly from the Amphilonchida). In the lentelliptical Hexalaspida (Pl. 139) all six spines which lie in the hydrotomical meridian plane (two equatorial and four polar) are very strongly developed, the remaining fourteen being rudimentary. Finally, in the Diploconida the two conical sheaths of the two opposite hydrotomical equatorial spines are so predominant that they take the chief part in the formation of the hour-glass-shaped shell.

178. Phylogeny of the Nassellaria.--The legion NASSELLARIA or MONOPYLEA is so clearly characterised by the peculiar porochora, which closes the osculum at the oral pole of the monaxon central capsule, and by the podoconus connected with it, that there can be no doubt that phylogenetically it represents an independent stem (§ 8). This stem is only connected at its base by means of Cystidium and Nassella with Actissa and Thalassicolla, the stem-forms of the SPUMELLARIA. This stem is monophyletic, inasmuch as all its members may be derived without violence from the skeletonless Nassellida (Nassella, Cystidium, p. 896, Pl. 91, fig. 1).

179. Origin of the Nassellaria.--The Nassellida (p. 896), which may naturally be considered as the common stem-group of the NASSELLARIA, are most nearly related among other Radiolaria to the Thalassicollida, and in both these skeletonless families the simplest forms, Cystidium and Actissa correspond; on the other hand, those which have arisen from them by the formation of alveoles in the calymma (Nassella and Thalassicolla) also correspond. The origin of the simplest Nassellida from these primitive Thalassicollida may be explained by supposing that the numerous (formerly evenly distributed) pores of the capsule membrane became obliterated in the upper (apical) half of the central capsule, whilst in the lower (basal) half they became correspondingly more strongly developed; hence the porochora was formed at the oral pole of the vertical main axis, and a differentiation of the endoplasm proceeding from this gave rise to the characteristic podoconus. Both these organs still at present exhibit very various degrees of progressive development.

{cxvii}180. Hypothetical Genealogical Tree of the Nassellaria.

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

{cxviii}181. Plectellaria and Cyrtellaria.--The extensive legion NASSELLARIA far surpasses the other three legions in the endless variety of its skeletal structures, and owing to the complicated relationships of its numerous families presents no lack of difficult phylogenetic problems. All NASSELLARIA may be divided first into two main groups or sublegions, #Plectellaria# and #Cyrtellaria#; the latter having a complete lattice-shell, the former not. Probably the #Cyrtellaria# have been polyphyletically developed from several different groups of #Plectellaria#. These groups are, however, connected in such manifold ways that a monophyletic origin of all the NASSELLARIAN skeletons from one original element is possible. Such a primitive element may have been furnished by any one of three different skeletal parts, the sagittal ring, the basal tripod, and the latticed cephalis (compare pp. 891-895, Bütschli, L. N. 40, 41).

182. Phylogenetic Skeletal Elements of the Nassellaria.--The multiform skeleton of the NASSELLARIA may be referred in different ways to one of the three above-mentioned structural elements. Each of these (p. 891) may by itself form the skeleton; the sagittal ring in the simplest #Stephoidea# (Archicircus, Lithocircus), the basal tripod in the simplest #Plectoidea# (Triplagia, Plagiacantha), the latticed cephalis in the simplest #Cyrtoidea# (Cyrtocalpis, Archicapsa). In the great majority of the NASSELLARIA, however, two of these elements, or even all three, are found combined. In most #Cyrtellaria#, more especially, both the sagittal ring and the basal tripod may be recognised in the lattice-shell, though often only in slight rudiments or scarcely perceptible traces. In the #Plectellaria# also (which possess no latticed cephalis) there are individual genera with complete development both of the sagittal ring and basal tripod; this important combination is especially well represented in the Cortinida (Cortina, Cortiniscus, Stephanium, Stephaniscus, Tripocoronis, &c.). The greatest difficulty as regards the phylogeny of the NASSELLARIA lies in the fact that the most various combinations of the three elements are presented by closely related or very similar forms. If, in spite of this, a monophyletic hypothesis as to the origin of the NASSELLARIA seems essential all sides of the three possible hypotheses must receive full consideration and critical comparison (§§ 183-191).

183. Ascent of the Nassellaria from the Plectoidea.--The monophyletic hypothesis (No. 2, p. 893) which regards the basal tripod as the common origin of the skeleton of all NASSELLARIA, starts from the simplest forms of the #Plectoidea# (Triplagia, Plagoniscus, Triplecta, Plectaniscus, &c., Pl. 91). All #Plectoidea# may be immediately derived as diverging twigs of these, as well as all triradial and multiradial forms of #Cyrtoidea# and #Spyroidea#; for in all these cases the distinctive triradial (or the derived multiradial) form of skeleton appears directly derivable from the simple basal tripod of the former. The same is perhaps also true of many #Botryodea#. {cxix}Furthermore, certain important forms of #Stephoidea# (Cortina, Cortiniscus, Stephanium, Stephaniscus, &c.), which have a characteristic combination of the sagittal ring and basal tripod, may be immediately derived from such forms of #Plectoidea# as Plagoniscus cortinaris, Plagiocarpa procortina, Plectaniscus cortiniscus, &c. On the contrary, those #Stephoidea# and #Cyrtoidea# in which the basal tripod is wanting can only be derived from the #Plectoidea# by the assumption that this structure has disappeared in consequence of phylogenetic degeneration. The monophyletic derivation of the NASSELLARIA from the #Plectoidea# has more internal probability than that from the #Stephoidea#, since it is easier to suppose that the Cortinida (Cortina, Stephanium, &c.) have been derived from the #Plectoidea# (Plagoniscus, Plagiocarpa) than the converse. This view is the basis of the hypothetical tree shown in § 180.

184. Ascent of the Nassellaria from the Stephoidea.--The monophyletic hypothesis (No. 1, p. 893) which regards the primary sagittal ring as the common starting point of the skeleton in all NASSELLARIA, starts from the simplest forms of #Stephoidea# (Archicircus, Lithocircus, &c., Pl. 81). All #Stephoidea# and #Spyroidea# may be immediately derived from these, as also the majority of the #Cyrtoidea# and probably of the #Botryodea#. Those numerous forms of the last two groups, however, which possess no trace of a sagittal ring, can only be derived from the former by the supposition that the latter has completely disappeared in in consequence of gradual phylogenetic degeneration. The same holds true also of the #Plectoidea#, although certain forms (e.g., Plagiocarpa procortina, Pl. 91, fig. 5; Plectaniscus cortiniscus, Pl. 91, fig. 9) appear to indicate the commencing formation of the sagittal ring by the concrescence of two branches, which approach each other from the upper part of the apical rod and the ventral part of the basal rod. In any case, it is a fact of great phylogenetic significance, that the primary sagittal ring in the cephalis of the #Cyrtoidea# shows all conceivable stages of degeneration (compare Bütschli, L. N. 40, 41, as well as the general account of and critical comparison of the NASSELLARIA, pp. 889-895, &c.).

185. Ascent of the Nassellaria from the Cyrtoidea.--The monophyletic hypothesis (No. 3, p. 894) which regards the latticed cephalis as the common point of origin of all the skeletons of the NASSELLARIA, starts from the simplest forms of the #Cyrtoidea#, that is, from the Cyrtocalpida or eradial Monocyrtida (Archicorida, Archicapsida, Pls. 51, 52, 98). All #Cyrtoidea# and #Botryodea# may be regarded as divergent forms of these monothalamous #Cyrtoidea#; the polythalamous simply by the addition of fresh joints at the basal pole, the triradiate and multiradiate by the development of three or more apophyses. The origin of the sagittal ring (which presents every stage of development and degeneration in the #Cyrtoidea#) may be regarded as a mechanical thickening of the latticed plate in the sagittal circumference of the cephalis. By stronger {cxx}development of this ring and coincident sagittal constriction of the cephalis the order #Spyroidea# may be derived from the #Cyrtoidea#. On the other hand, the #Plectellaria#, which possess no cephalis, and indeed no complete lattice-shell whatever, may be derived from the Monocyrtida by the assumption of a degeneration of this structure; the sagittal ring having been preserved in the #Stephoidea#, and the tripod of the Tripocalpida in the #Plectoidea#. Although such a monophyletic derivation of the NASSELLARIA from the Cyrtocalpida is possible, and though here, too, the Cortinida play an important part as connecting links, this hypothesis has less internal probability than that of the derivation from the #Stephoidea# (§ 184) or #Plectoidea# (§ 183).

186. Genealogical Tree of the Plectoidea.--The order #Plectoidea# includes those NASSELLARIA whose rudimentary skeleton does not contain the characteristic sagittal ring of the #Stephoidea#, but consists of several (at least three) radial spines, which proceed from a point in the centre of the porochora. The branches of these radial spines remain free in the Plagonida, whilst in the Plectanida they unite with each other to form a loose meshwork (not, however, a complete lattice-shell). The number and arrangement of the radial spines, which serve for generic distinctions, are the same in both families, so that each genus of the Plectanida has arisen from a corresponding genus of the Plagonida. The simplest Plagonida, which possess a basal tripod (Triplagia or Plagiacantha with three rays, Tetraplagia with four rays) are probably to be regarded as forming the common origin of the whole order. These agree with certain three- and four-rayed skeletal pieces of the #Beloidea# (Thalassosphærida and Sphærozoida); and also the four and six-rayed twinned pieces of the latter (spicula bigemina and trigemina) repeat in the same fashion the skeleton of the former (Plagonidium, Plagonium). This similarity, however, is a mere analogy and possesses no phylogenetic significance. On the other hand, certain Plagonida (Plagoniscus, Plagiocarpa), and the corresponding genera of Plectanida (Plectaniscus, Periplecta) seem to have important phylogenetic relations to certain #Stephoidea# (Cortina, Cortiniscus, &c.); the sagittal ring of the latter having perhaps arisen by the vertical apical spine of the former having been connected with their horizontal basal rod by two ventral apophyses growing out opposite to each other (compare pp. 902, 914, Plagiocarpa procortina, Pl. 91, fig. 5). In this case the Plectanida would belong to the simplest stem-forms of the NASSELLARIA.

187. Genealogical Tree of the Stephoidea.--The order #Stephoidea# includes all those NASSELLARIA whose skeleton does not form a complete lattice-shell, but consists of one or more rings, and often of a loose meshwork which arises by the union of branches of the rings. A vertical sagittal ring is constantly present, embracing the central capsule in the median sagittal plane, and forming at its basal pole various processes, the starting point for other skeletal forms. The most important of these is the tripodal Cortina {cxxi}(p. 950, § 182). The Stephanida are the most archaic family among the #Stephoidea# (p. 937, Pl. 81), perhaps indeed among all the NASSELLARIA (§ 184); in them the sagittal ring and its processes alone constitute the skeleton; secondary rings and meshes are wanting. Two diverging families, the Semantida and Coronida, have been developed from the Stephanida, and from one of them the family Tympanida has arisen.

The Semantida (p. 953, Pl. 92) develop a horizontal basal ring at the oral side of the vertical sagittal ring; the basal meshes or lattice gates, which remain between the former and the latter, are the important cortinar pores (one pair jugular, one pair cardinal, p. 954); they usually appear inherited in the cortinar septum of the #Cyrtellaria#. In the Coronida (p. 967, Pls. 82, 94) a second vertical ring (the frontal ring) appears in addition to the sagittal ring; it lies in the frontal plane at right angles to the latter. Finally the Tympanida (p. 987, Pls. 93, 94) have probably arisen from the Semantida by the formation of a second horizontal ring (mitral ring) parallel to the basal and attached to the upper portion of the sagittal ring.

188. Genealogical Tree of the Spyroidea.--The extensive order #Spyroidea# is of especial interest in connection with the phylogeny of the NASSELLARIA, since all its members show two well-developed skeletal elements in combination, the sagittal ring of the #Stephoidea# and the latticed cephalis of the #Cyrtoidea#; the majority possess also the basal tripod of the #Plectoidea# (or a radial skeleton derived from it). Hence there is a possibility of deriving the stem-forms of the #Spyroidea# from each of these three groups. The four families of this order exhibit similar relationships to those of the four families of #Cyrtoidea#; the common stem-group is the family Zygospyrida; from this the Tholospyrida have arisen by the development of a galea on the apical pole, the Phormospyrida by the addition of a thorax on the basal pole. The Androspyrida may be derived either from the Tholospyrida by the formation of a basal thorax, or from the Phormospyrida by the development of an apical galea. Some groups, however, such as the peculiar Nephrospyrida (Pl. 90) have probably been developed directly from the #Stephoidea#.

189. Genealogical Tree of the Botryodea.--The peculiar order #Botryodea# (p. 1103), which is both difficult to investigate and insufficiently known, presents great phylogenetic difficulties both as to its ascent and descent. Probably the different genera of this order have been polyphyletically developed from different groups of #Cyrtoidea# (perhaps also to some extent of #Spyroidea#) by the formation of lobes in the cephalis. The three families of #Botryodea# are related to each other in the same way as are the three first families of the #Cyrtoidea#. From the single-jointed Cannobotryida (corresponding to the Monocyrtida), the two-jointed Lithobotryida (corresponding to the Dicyrtida), may be derived by the development of a basal thorax, and from the latter the three-jointed Pylobotryida (like the Tricyrtida) by the addition of an abdomen. In the last two families the forms with an open basal mouth {cxxii}(Botryopylida and Botryocyrtida) are to be regarded as primitive: the Botryocellida and Botryocampida have arisen by the closure of this mouth with a basal lattice-plate.

190. Genealogical Tree of the Cyrtoidea.--The multiform and extensive group #Cyrtoidea# presents the greatest difficulties to be found in the phylogeny of the NASSELLARIA, because their morphological relations are most complicated, and because similar forms very often appear to be of quite different origin. The great majority of the #Cyrtoidea# show more or less clearly a combination of the three structural elements: sagittal ring, basal tripod, and latticed cephalis (p. 891). There are also, however, numerous #Cyrtoidea#, whose skeleton no longer shows any trace of the sagittal ring. Many of these show as the basis of the skeleton a strong basal tripod with an apical spine, around which the cephalis has obviously been secondarily developed, e.g., the remarkable Euscenida (p. 1146, Pls. 53, 97) and the interesting Callimitrida (p. 1217, Pls. 63, 64). These may have been derived immediately from the #Plectoidea# without any relation to the #Stephoidea#. There are also numerous true Monocyrtida, whose shell consists of a simple latticed cephalis without a trace of the sagittal ring or basal tripod (Cyrtocalpida, Pl. 51, figs. 9-13; Pl. 98, fig. 13); these may have been developed directly from the skeletonless Nassellida by the formation of a simple ovoid Gromia-like shell, and may have no relation either to the #Stephoidea# or #Plectoidea#. On these grounds, as well as from the complicated relationships of the many smaller groups of #Cyrtoidea#, it is probable that the whole order has been developed polyphyletically from different divisions of the #Plectellaria#.

191. Systematic Arrangement of the Cyrtoidea.--Although for the reasons just given no systematic arrangement of the #Cyrtoidea# can at present, or for a long time in the future, be regarded as other than artificial, yet some general principles of classification for this extensive group can be laid down, which may serve as starting points for some future natural disposition. This is especially true of the relations which in an artificial system (p. 1129) were primarily utilised for the distinction of twelve families and twenty-four subfamilies; the number of segments in the shell, the number of radial apophyses (and parameres), and the constitution of the basal aperture of the shell.

As regards the number of segments, separated by transverse constrictions, of which the shell is composed, it is dependent upon the secondary addition of new joints at the basal pole of the main axis. Hence all many-jointed #Cyrtoidea# are to be derived from single-jointed ones, and the four sections thus distinguished (Monocyrtida, Dicyrtida, Tricyrtida, Stichocyrtida) form a phylogenetic series. Very often, however, the primary cephalis disappears owing to retrograde metamorphosis; and in such cases the single joint of the apparent Monocyrtida is formed of the thorax (e.g., {cxxiii}Pls. 52, 54, &c.). As regards the number of radial apophyses, three sections of #Cyrtoidea# may be distinguished; the Pilocyrtida with three, the Astrocyrtida with numerous apophyses, and the Corocyrtida with none (p. 1129). The last two may in general be regarded as two divergent branches from the first, for the eradiate Corocyrtida have probably been formed from the triradial Pilocyrtida by entire loss of the radial apophyses, whilst on the other hand the multiradiate Astrocyrtida have arisen from them by additions to the primary apophyses (interpolation of interradial between the perradial ones). As regards the constitution of the shell-aperture, the #Cyrtoidea# may be divided into Cyrtaperta and Cyrtoclausa (p. 1129); in general the Cyrtoclausa (with latticed shell-aperture) have arisen from the Cyrtaperta (with simple open mouth); in many Monocyrtida the converse may be supposed, the simple basal mouth having been formed by degeneration of a basal lattice.

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

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

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

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

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

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

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

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

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

{cxxviii}PHYSIOLOGICAL SECTION.

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VII. VEGETATIVE FUNCTIONS

(§§ 201-217.)

201. Mechanism of the Functions.--The vital phenomena of the Radiolaria are dependent upon the mechanical functions of their unicellular body, and like those of all other organisms, are to be referred to physical and chemical natural laws. All processes which appear in the life of the Radiolaria are, therefore, ultimately to be explained by the attraction and repulsion of the smallest particles, which compose the different portions of their unicellular body; and the sensation of pleasure or the opposite is in its turn the exciting cause of these elementary movements. Many adaptive arrangements in the Radiolarian organism may produce the appearance of being the premeditated result of causes working towards an end ("zweckthätig," causæ finales), but as opposed to this deceptive appearance it must here be expressly stated that these may be recognised in accordance with the developmental theory as the necessary consequence of mechanical causes (causæ efficientes).

Our physiological acquaintance with the Radiolaria has by no means progressed so far as our morphological, so that the incomplete communications which are placed here for the sake of completeness must be regarded merely as preliminary fragments, not as fully elaborated results. Since my recent investigations have been mainly in the direction of morphology, I can add but little to the physiological conclusions, which I stated at length in my monograph twenty-four years ago (L. N. 16, pp. 127-165). Recently the vegetative physiology of the Radiolaria has been much advanced by the recognition of the symbiosis with the Xanthellæ (§ 205, L. N. 22, 39, 42). In addition Karl Brandt has recently (1885) published several important contributions to the physiology of the Polycyttaria or Sphaerozoea (L. N. 52).

202. Distribution of Functions.--The distribution of the functions among the various parts of the unicellular organism of the Radiolaria corresponds directly to their anatomical composition, so that physiologically as well as morphologically the central capsule and the extracapsulum appear as the two coordinated main components. On the one hand the central capsule with its endoplasm and enclosed nucleus is the central organ of the "cell-soul" (Zellseele), the unit regulating its animal and vegetative functions, and the special organ of reproduction and inheritance. The extracapsulum forms, on the other hand, by its calymma the protective envelope of the central {cxxix}capsule, the support of the soft pseudopodia and the substratum of the skeleton; the calymma acts also as a hydrostatic apparatus, whilst the radiating pseudopodia are of the greatest importance both as organs of nutrition and adaptation, as well as of motion and sensation (§ 15). If, however, the vital functions as a whole be divided in accordance with the usual convention into the two great groups of vegetative (nutrition and reproduction) and animal (motion and sensation), then the central capsule would be mainly the organ of reproduction and sensation, and the extracapsulum the organ of nutrition and motion.

The numerous separate vital phenomena, which by accurate physiological investigation may be distinguished in the unicellular Radiolarian organism, may be distributed in the above indicated conventional fashion into a few larger and several smaller groups; it must always be borne in mind, however, that these overlap in many respects, and that the division of labour among the different organs in these Protista is somewhat complicated, notwithstanding the apparent simplicity of their unicellular organization. A general classification of the groups of functions is difficult, because each individual organ discharges several different functions. Thus the central capsule is pre-eminently the organ of reproduction and inheritance, but not less (though less conspicuous) is its importance as the psychical central organ, the unit regulating the processes of sensation, motion, and also nutrition. In this last respect it is comparable to the nerve-centres of the Metazoa, whilst the peripheral nervous system of the latter (including the organs of sense and the muscles) are in the present instance represented by the pseudopodia, which are at the same time the most important organs of nutrition and adaptation. In the calymma also in similar fashion several different physiological functions are united.

203. Metastasis.--The functions of metastasis and nutrition have in all Radiolaria a purely animal character, so that these Rhizopoda from the physiological standpoint are to be regarded as truly unicellular animals, or Protozoa ("Urthiere"). Since they do not possess, like plants, the power of forming synthetically the compounds (protoplasm, carbohydrates, &c.) necessary for their sustenance, they are compelled to obtain them ready-formed from other organisms. Like other true animals they evolve carbon dioxide by the partial oxidation of those products, and hence they successively take up the oxygen necessary to their existence from their environment.

The question whether the Radiolaria are to be regarded as true animals I discussed fully from various points of view in 1862, and finally answered in the affirmative (L. N. 16, pp. 159-165). Afterwards, when in my Generelle Morphologie (1866) I sought to establish the kingdom Protista, I removed the Radiolaria along with the other Rhizopoda from the animal kingdom proper and placed them in the kingdom Protista (Bd. i. pp. 215-220; Bd. ii. p. xxix). Compare also my Protistenreich (L. N. 32) and my Natürliche Schöpfungsgeschichte (vii. Aufl., 1879, p. 364). Both these steps appear fully justified when considered in the light of our present increased knowledge. From the physiological standpoint the Radiolaria appear as unicellular animals, for in this respect the animal character of their metastasis (that proper to an oxidising organism) furnishes the sole {cxxx}criterion. On the other hand, from the morphological standpoint, they are to be classed as neutral Protista, for in this respect their unicellular character is the prominent feature, and distinguishes them from all true multicellular animals (Metazoa). Compare my Gastræa Theorie (1873, Jena. Zeitschr. für Naturwiss., Bd. viii. pp. 29, 53).

204. Nutrition.--The nutritive materials which the Radiolaria require for their sustenance, especially albuminates (plasma) and carbohydrates (starch, &c.), they obtain partly from foreign organisms which they capture and digest, and partly directly from the Xanthellæ or Philozoa, the unicellular Algæ, with which they live in symbiosis (§ 205). Zooxanthella intracapsularis, found in the ACANTHARIA (§ 76), is probably of the same significance in this respect as Zooxanthella extracapsularis of the SPUMELLARIA and NASSELLARIA (§ 90); and perhaps the same is true also of Phæodella extracapsularis (or Zoochlorella phæodaris?) of the PHÆODARIA (§ 89). The considerable quantity of starch or amyloid bodies, elaborated by these inquiline symbiontes, as well as their protoplasm and nucleus, are available, on their death, for the nutrition of the Radiolaria which harbour them. Nutrition by means of other particles obtained by the pseudopodia from the surrounding medium is by no means excluded; indeed it may be regarded as certain that numerous Radiolaria (especially such as contain no symbiotic Algoid cells) are nourished for the most part or exclusively by this means. Diatoms, Infusoria, Thalamophora (Foraminifera) as well as decaying particles of animal and vegetable tissues can be seized directly by the pseudopodia and conveyed either to the sarcodictyum (on the surface of the calymma) or to the sarcomatrix (on the surface of the central capsule) in order to undergo digestion there. The indigestible constituents (siliceous shells of Diatoms and Tintinnoidea, calcareous shells of small Monothalamia and Polythalamia, &c.) are here collected often in large numbers and removed by the streaming of the protoplasm.

The inception and digestion of nutriment, as it usually appears to take place by the pseudopodia, has already been so fully treated in my Monograph (L. N. 16, pp. 135-140), and since then in my paper on the sarcode body of the Rhizopoda (L. N. 19, p. 342), that I have nothing of importance to add. Quite recently Karl Brandt has expressed a doubt as to whether the taking up of formed particles by the pseudopodia and their aggregation in the calymma be really connected with the process of nutrition. He is disposed rather to believe that these foreign bodies are usually only accidentally and mechanically brought into the calymma, and that the nourishment of the Radiolaria is derived exclusively or pre-eminently from the symbiotic Xanthellæ (L. N. 52, pp. 88-93). I must, however, maintain my former opinion, which I have only modified insomuch that I now regard the sarcodictyum (on the outer surface of the calymma, § 94) rather than the sarcomatrix (on the outer surface of the central capsule, § 92) as the principal seat of true digestion and assimilation. From the sarcodictyum the dissolved and assimilated nutritive matters may pass by the intracalymmar pseudopodia (or sarcoplegma, § 93) into the sarcomatrix, and hence may reach the endoplasm through the openings in the central capsule. To what an extent the Radiolaria are capable of taking up even large formed bodies into the calymma, is shown by the {cxxxi}striking instance of Thalassicolla sanguinolenta, which becomes so deformed by the inception of numerous coccospheres and coccoliths, that I described it as a special genus under the name Myxobrachia (compare pp. 23, 30; also L. N. 21, p. 519, Taf. xviii., and L. N. 33, p. 37).

205. Symbiosis.--Very many Radiolaria, but by no means all members of this class, live in a definite commensal relation with yellow unicellular Algæ of the group Xanthellæ. In the ACANTHARIA they live within the central capsule (Zooxanthella intracapsularis, § 76), in the SPUMELLARIA and NASSELLARIA, on the other hand, within the calymma but outside the central capsule (Zooxanthella extracapsularis, § 90); in the PHÆODARIA a special form of these symbiotic unicellular Algæ appears to inhabit the phæodium in the extracapsulum, and to compose a considerable portion of the phæodellæ (Zooxanthella phæodaris, § 90, or better perhaps Zoochlorella phæodaris, § 89). Undoubtedly this commensal life is in very many cases of the greatest physiological significance for both the symbiontes, for the animal Radiolarian cells furnish the inquiline Xanthellæ not only with shelter and protection, but also with carbon dioxide and other products of decomposition for their nutriment; whilst on the other hand the vegetable cells of the Xanthellæ yield the Radiolarian host its most important supply of nutriment, protoplasm and starch, as well as oxygen for respiration. Hence it is not only theoretically possible, but has been experimentally proved, that Radiolaria which contain numerous Xanthellæ can exist without extraneous nutriment for a long period in closed vessels of filtered sea-water, kept exposed to the sunlight; the two symbiontes furnish each other mutually with nourishment, and are physiologically supplementary to each other by reason of the opposite nature of their metastasis. This symbiosis is not necessary, however, for the existence of the Radiolaria; for in many species the number of Xanthellæ is very variable and in many others they are entirely wanting.

The symbiosis of the Radiolaria and Xanthellæ, or "yellow cells" (§§ 76, 90) was first discovered by Cienkowski in 1871 (L. N. 22). Ten years later this important and often doubted fact was established by extended observations and experiments almost simultaneously by Karl Brandt (L. N. 38, 39) and Patrick Geddes (L. N. 42, 43). This commensal life may be compared with that of the lichens, in which an organism with vegetable metastasis (the Algoid gonidia) and an organism with animal metastasis (the Fungoid hyphæ) are intimately united for mutual benefit. But the symbiosis of the Xanthellæ and Radiolaria is not as in the lichens a phenomenon essential for their development, but has more or less the character of an accidental association. The number of the inquiline Xanthellæ is so variable even in one and the same species of Radiolaria, that they do not appear to be exactly essential to its welfare; and in many species they are entirely wanting. Their significance is questionable in the case of those numerous deep-sea Radiolaria which live in complete darkness, and in which, therefore, the Xanthellæ, even if present, could excrete no oxygen on account of the want of light. Nevertheless it is possible that the phæodellæ of the PHÆODARIA (usually green, olive, or brown in colour), which are true cells, represent vegetable symbiontes, {cxxxii}which in the absence of sunlight are able to evolve oxygen by the aid of the phosphoresence of other abyssal animals. Since the PHÆODARIA are, for the most part, dwellers in the deep-sea, and since the voluminous phæodium must be of great physiological importance, a positive solution of this hypothetical question would be of no small interest (compare § 89).

206. Respiration.--The respiration of the Radiolaria is animal in nature, since all Protista of this class, like all other true Rhizopoda, take in oxygen and give off carbon dioxide. Probably this process goes on continuously and is tolerably active, as may be inferred from the fact that Radiolaria cannot be kept for long in small vessels of sea-water unless either they contain numerous Xanthellæ or the water is well aërated. The oxygen is obtained from two sources, either from the surrounding water or from the enclosed Xanthellæ, which in sunlight evolve considerable quantities of this gas. Correspondingly, the carbon dioxide which is formed during the process of oxidation of the Radiolaria is either given up to the surrounding water or to the inquiline Xanthellæ, which utilise it for their own sustenance (§§ 204, 205).

The significance of the symbiotic Xanthellæ for the respiration of the enclosing Radiolaria may be shown experimentally in the following way. If two Polycyttarian colonies of equal size, both of which contain numerous Xanthellæ, be placed in equal quantities of filtered sea-water in sealed glass tubes, and if one tube be placed in the dark the other in the light, the colony in the former rapidly perishes, but not that in the latter; the Xanthellæ excrete only under the influence of sunlight the oxygen necessary for the life of the Radiolarian (compare Patrick Geddes, L. N. 42, p. 304).

207. Circulation.--In the protoplasm of all Radiolaria, both inside and outside the central capsule, slow currents may be recognised which fall under the general term circulation, and have already been compared to the cyclosis in the interior of animal and vegetable cells, as well as to the sarcode streams in the body of other Rhizopoda. These plasmatic currents or "plasmorrheumata" probably continue throughout the whole life of the Radiolaria, and are of fundamental importance for the performance of their vital functions. They depend upon slow displacements of the molecules of the plasma (plastidules or micellæ) and cause a uniform distribution of the absorbed nutriment and a certain equalisation of the metastasis. Furthermore they are of great importance also in the inception of nutriment, the formation of the skeleton, locomotion, &c. Sometimes the circulation is directly perceptible in the plasma itself; but usually it is only visible owing to the presence of granules (sarcogranula), which are suspended in the plasma in larger or smaller numbers. The movements of these granules are usually regarded as passive, due to the active displacement of the molecules of the plasma. Although the intracapsular protoplasm is in communication with the extracapsular through the openings in the capsule membrane, nevertheless the currents exhibit certain differences {cxxxiii}in the two portions of the malacoma. It is sometimes possible, however, to recognise the direct connection between them and to observe how the granules pass through the openings in the capsule-membrane.

208. Currents in the Endoplasm.--Intracapsular circulation or a certain slow flowing of the plasma within the central capsule is probably just as common in the Radiolaria as without it, but it is not so easy to observe in the former case as in the latter. A more satisfactory proof of these endoplasmatic currents is furnished by the arrangement of the protoplasm within the central capsule, since this is (at all events in part) an effect produced by them. In this respect the two main divisions of the class show characteristic differences. In the Porulosa (the SPUMELLARIA, § 77, and the ACANTHARIA, § 78) the endoplasm is in general distinguished by a more or less distinct radial structure, which is to be regarded as the effect of alternating centripetal and centrifugal radial streams. In the Osculosa, on the other hand, this radial structure is absent and the intracapsular plasmatic streams converge or diverge towards the osculum or main-opening in the central capsule which lies at the basal pole of its main axis, and through which the mass of the endoplasm issues into the calymma. The two legions of the Osculosa, however, present differences in this respect. In the NASSELLARIA (§ 79) the endoplasmatic currents appear to unite in an axial main stream at the apex of the monaxon central capsule, and this apical stream seems to split into a conical bundle, the individual threads of which pass diverging between the myophane fibrillæ of the podoconus towards the basis of the central capsule, and issue through the pores of the porochora. In the PHÆODARIA (§ 80), on the other hand, meridional currents of endoplasm are probably present on the inner surface of the capsule, which flow from the aboral pole of the vertical main axis to its basal pole, and return in the reverse direction.

209. Currents in the Exoplasm.--Extracapsular circulation, or a distinct flowing of the plasma outside the central capsule, may be readily observed in all Radiolaria which are examined alive; this is most readily seen in the astropodia, or those free pseudopodia which radiate from the sarcodictyum on the surface of the calymma into the surrounding water. The granular movement is often quite as clear in the sarcodictyum itself, and may be recognised in the collopodia, which compose the irregular plasmatic network within the calymma. More rarely it is possible to follow the granular stream thence through the sarcomatrix, and further into the interior of the central capsule. In general the direction of the extracapsular protoplasmic streams is radial, and it is frequently possible, even in a single free astropodium, to observe two streams opposite in direction, the granules on one side of the radial sarcode thread moving centripetally, those on the other side centrifugally. If the threads branch, and neighbouring ones {cxxxiv}become united by connecting threads, the circulation of the granules may proceed quite irregularly in the network thus formed. The rapidity and character of the extracapsular currents are subject to great variations.

The different forms of extracapsular sarcode currents have been already very fully described in my Monograph (L. N. 16, pp. 89-126), and in my critical essay on the sarcode body of the Rhizopoda (L. N. 19, p. 357, Taf. XXVI.).

210. Secretion.--Under the name secretions, in the strict sense, all the skeletal formations of the Radiolaria may be included. They may be divided according to their chemical composition into three different groups: pure silica in the SPUMELLARIA and NASSELLARIA, a silicate of carbon in the PHÆODARIA, and acanthin in the ACANTHARIA (compare § 102). It may indeed be assumed that these skeletons arise directly by a chemical metamorphosis (silicification, acanthinosis, &c.) of the pseudopodia and protoplasmic network; and this view seems especially justified in the case of the Astroid skeleton of the ACANTHARIA (§ 114), the Spongoid skeleton of the SPUMELLARIA (§ 126), the Plectoid skeleton of the NASSELLARIA (§ 125), the Cannoid skeleton of the PHÆODARIA (§ 127), and several other types. On closer investigation, however, it appears yet more probable that the skeleton does not arise by direct chemical metamorphosis of the protoplasm, but by secretion from it; for when the dissolved skeletal material (silica, acanthin) passes from the fluid into the solid state, it does not appear as imbedded in the plasma, but as deposited from it. However, it must be borne in mind that a hard line of demarcation can scarcely, if at all, be drawn between these two processes. In the ACANTHARIA the intracapsular sarcode is the original organ of secretion of the skeleton; in the other three legions, on the other hand, the extracapsulum performs this function (§§ 106, 107). In addition to the skeleton, we may regard as secretions (or excretions) the intracapsular crystals (§ 75) and concretions (§ 75A), and perhaps certain pigment-bodies (§§ 74, 88); and further, the calymma (§ 82) may be considered to be a gelatinous secretion of the central capsule, and perhaps also the capsule-membrane, in so far as it represents only a secondary excretory product of the unicellular organism.

211. Adaptation.--The innumerable and very various adaptive phenomena which we meet with in the morphology of the Radiolaria, and especially in that of their skeleton, are like other phenomena of the same kind, to be ultimately referred to altered nutritional relations. These may be caused directly either by the influence of external conditions of existence (nutrition, light, temperature, &c.), or by the proper activity of the unicellular organism (use or disuse of its organs, &c.), or, finally, by the combined action of both causes in the struggle for existence. In very many cases the cause to which the origin of a particular form of Radiolaria is due may be directly perceived or at least guessed at with considerable probability; thus, for example, the lattice-shells {cxxxv}may be explained as protective coverings, the radial spines as defensive weapons, and the anchor-hooks and spathillæ as organs of prehension, which are of advantage to their possessors in the struggle for existence; the regular arrangement of the radial spines in the Radiolaria may also be explained on hydrostatic grounds, it being advantageous that the body should be maintained in a definite position of equilibrium, &c. The well-known laws of direct or actual adaptation, which we designate cumulative, correlative, divergent adaptation, &c., here explain a multitude of morphological phenomena. The connection is less distinct in the case of the laws of indirect or potential adaptation, although this must play as important a part in the formation of the Radiolaria as in that of other organisms (compare on this head my Generelle Morphologie, Bd. ii. pp. 202-222).

212. Reproduction.--The most common form of reproduction in the Radiolaria is the formation of spores in the central capsule, which in this respect is to be regarded as a sporangium (§ 215). In many Radiolaria (Polycyttaria and PHÆODARIA), however, there occurs in addition an increase of the unicellular organism by simple division (§ 213); upon this the formation of colonies in the social Radiolaria is dependent (§ 14). Reproduction by gemmation is much less common, and has hitherto been observed only in the Polycyttaria (§ 214). In this group alone there also occur at certain times two different forms of swarm-spores which copulate, and thus indicate the commencement of sexual reproduction (Alternation of Generations, § 216). The general organ of reproduction is in all cases the central capsule, whilst the extracapsulum never takes an active part in the process.

213. Cell-Division.--Increase by cell-division among the Radiolaria in the early stage, before the formation of the skeleton, is widely distributed (perhaps even general?); in the adults of this class it is rather rare and limited to certain groups. It is most readily observed in the Polycyttaria; the growth of the colonies in this social group depends mainly (and in many species exclusively) upon repeated spontaneous division of the central capsule; all the individuals of each colony (in so far as this has not arisen by the accidental fusion of two or more colonies) are descendants of a single central capsule, which has arisen from an asexual swarm-spore (§ 215) or from the copulation of two sexual swarm-spores (§ 216). Whilst the central capsules of the colonies continually increase by division, their calymma remains a common gelatinous sheath. Among the SPUMELLARIA reproduction by simple cell-division probably occurs also in many monozootic #Collodaria#. Among the ACANTHARIA the peculiar group Litholophida has perhaps arisen by the spontaneous division of #Acanthonida# (see p. 734). Among the PHÆODARIA increase by cell-division seems to occur commonly in many groups, as in the #Phæocystina#, which have no skeleton (Phæodinida, Pl. 101, {cxxxvi}fig. 2), or only an incomplete Beloid skeleton (Cannorrhaphida, Pl. 101, figs. 3, 6, and Aulacanthida, Pl. 104, figs. 1-3). The #Phæosphæria# also (Aulosphærida, Coelacanthida) and the #Phæogromia# (Tuscarorida, Challengerida) appear sometimes to divide; at all events, their central capsule often contains two nuclei. Of special interest is the spontaneous division of the #Phæoconchia#, especially the Concharida (Pl. 124, fig. 6). In all monozootic Radiolaria, the nucleus first divides by a median constriction into two equal halves (usually by the mode of direct division); then the central capsule becomes constricted in the middle (in the PHÆODARIA in the vertical main axis), and each portion of the capsule retains its own nucleus. In the #Phæoconchia# each half or daughter-cell corresponds to one valve of the shell, dorsal or ventral, so that probably on subsequent separation each daughter-cell retains one valve of the mother-cell, and forms a new one for itself by regeneration (as in the Diatoms). In the polyzootic Radiolaria, which already contain many small nuclei, but usually only a single central oil-globule in each central capsule, the division of the latter is preceded by that of the oil-globule. In many Polycyttaria the colony as a whole multiplies by division.

The increase of the central capsule by division was first described in 1862 in my Monograph (L. N. 16, p. 146); since then R. Hertwig (L. N. 26, p. 24) and K. Brandt (L. N. 52, p. 144) have confirmed my statement. In the PHÆODARIA the division of the central capsule appears always to take place in the main axis; in the bilateral sometimes in the sagittal, sometimes in the frontal plane. In the Tripylea each daughter-cell seems to retain one parapyle and half the astropyle (compare the general description of the PHÆODARIA, Pl. 101, figs. 1-6, Pl. 104, figs. 1-3, and also Hertwig, L. N. 33, p. 100, Taf. x. figs. 2, 11). Regarding the spontaneous division of colonies of the Polycyttaria, see K. Brandt, L. N. 52, p. 142.

214. Cell-Gemmation.--Reproduction by gemmation has hitherto been observed only in the social Radiolaria, but in them it appears to be widely distributed, and in very young colonies is perhaps almost universally present. The gemmules or capsular buds (hitherto described as "extracapsular bodies") are developed on the surface of young central capsules before these had secreted a membrane. They grow usually in considerable numbers, from the surface of the central capsule, which is sometimes quite covered with them. Each bud usually contains a raspberry-like bunch of shining fatty globules, and by means of reagents a few larger or a considerable number of smaller nuclei may be recognised in them; the naked protoplasmic body of the bud is not enclosed by any membrane. As soon as the buds have reached a certain size they are constricted off from the central capsule and separated from it, being distributed in the meshes of the sarcoplegma by the currents in the exoplasm. Afterwards each bud becomes developed into a complete central capsule by surrounding itself with a membrane when it has attained a definite size. From the special relations of the process of nuclear formation, which take place in the multiplication of the {cxxxvii}social central capsules by gemmation and by cell-division, it would appear that the capsules produced by the former method afterwards produce anisospores, whilst those in the latter way yield isospores (§ 216).

The gemmules or capsular buds of the Polycyttaria were first accurately described by Richard Hertwig (L. N. 26, pp. 37-39), under the name "extracapsular bodies," and their significance rightly indicated; earlier observers had incidentally mentioned and figured them, but had not seen their origin from the central capsule. Quite recently Karl Brandt has given a very painstaking account of them in the different Polycyttarian genera (L. N. 52, pp. 179-198). In the Monocyttaria such a formation of buds has not yet been observed. The basal lobes of the central capsule, which occur in many NASSELLARIA, are not buds, but simple processes of the capsule, due to its protrusion through the collar pores of the cortinar septum (§ 55).

215. Sporification.--Asexual reproduction by the formation of movable flagellate spores has been hitherto observed only in a very small number of genera; but since these belong to very different groups, and since the comparative morphology of the capsule appears to be similar throughout as regards the structure and development of its contents, it may be safely assumed that this kind of reproduction occurs quite generally in the Radiolaria. In all cases it is the contents of the central capsule, from which the swarm-spores are formed, both nucleus and endoplasm taking an equal share in the process; in all cases the spores produced are very numerous, small, ovoid or reniform, and have one or two very long slender flagella at one extremity (see §§ 141, 142). Since the whole contents of the mature central capsule are used up in the formation of these flagellate zoospores, it discharges the function of a sporangium. The division of the simple primary nucleus into numerous small nuclei, which usually (serotinous Radiolaria) takes place only shortly before sporification, but sometimes (precocious Radiolaria, § 63) happens very early, is the commencement of the often repeated process of nuclear division, which terminates with the production of a very large number of small spore-nuclei. The nucleolus often divides very peculiarly (§ 69, C). Each spore nucleus becomes surrounded by a portion of endoplasm and usually receives in addition one or more fatty granules, and sometimes also a small crystal (hence the "crystal-spores"). The size of the flagellate zoospores which emerge from the ruptured central capsule and swim freely in the water by means of their flagellum, varies generally between 0.004 and 0.008 mm. The extracapsulum is not directly concerned in the sporification, but undergoes degeneration during the process and perishes at its conclusion.

The first complete and detailed observations on the formation of spores in the Radiolaria were published by Cienkowski in 1871 and related to two genera of Polycyttaria, the skeletonless Collozoum and the spherical-shelled Collosphæra (L. N. 22, p. 372, Taf. xxix.). These were subsequently continued and supplemented by R. Hertwig (1876, L. N. 26, pp. 26-42, and L. N. 33, p. 129), and a general summary of these results has been given by Bütschli (L. N. 41, pp. 449-455). {cxxxviii}Recently Karl Brandt has given a very detailed and fully illustrated account of the sporification of the Polycyttaria (L. N. 52, pp. 145-178). I have also had the opportunity during my sojourn in the Canary Islands (1866), in the Mediterranean at Corfu (1877), and Portofino (1880), as well as in Ceylon (1881), of observing the development of flagellate zoospores from the central capsule of individuals of all four legions: among the SPUMELLARIA in certain #Colloidea#, #Beloidea#, #Sphæroidea#, and #Discoidea#, among the ACANTHARIA in several #Acanthometra# and #Acanthophracta#, among the NASSELLARIA in individuals belonging to the #Stephoidea#, #Plectoidea#, and #Cyrtoidea#, and among the PHÆODARIA in one Castanellid. In most zoospores I could distinctly observe only a single long flagellum; sometimes, however, two or even three appeared to be present, but the determination of their number is very difficult.

216. Alternation of Generations.--A peculiar form of reproduction, which may be designated "alternation of generations," appears to occur generally in the Polycyttaria, but has not yet been observed in the Monocyttaria. All #Collozoida#, #Sphærozoida#, and Collosphærida which have hitherto been carefully and completely examined with respect to their development, are distinguished by the production of two different kinds of swarm-spores, isospores and anisospores. The Isospores (or monogonous spores) correspond to the ordinary asexual zoospores of the Monocyttaria (§ 215); they possess a homogeneous, doubly refracting nucleus of uniform constitution and develop asexually, without copulation. The Anisospores (or amphigonous spores), on the other hand, are sexually differentiated and possess a heterogeneous, singly refracting nucleus of twofold constitution; they may therefore be distinguished as female macrospores and male microspores. The Macrospores (or gynospores, comparable with the female macrogonidia of many Cryptogams) are larger, less numerous, and possess larger nuclei, which are less easily stained, and have a fine filiform trabecular network. On the other hand the Microspores (or androspores, comparable with the male microgonidia) are much smaller and more numerous, and are distinguished by their smaller nuclei, which have thicker tuberculæ and become stained more intensely. The gynospores and androspores are developed in the #Collozoida# and #Sphærozoida# in the same individual, but not in the Collosphærida. It is very probable that these two forms of anisospores copulate with each other after their exit from the central capsule and thus produce a new cell by the simplest mode of sexual reproduction. But, since the same Polycyttaria, which produce these anisospores, at other times give rise to ordinary or asexual isospores, it is further possible that these two forms of reproduction alternate with each other, and that the Polycyttaria thus pass through a true alternation of generations. This has not yet been observed in the Monocyttaria, and hence these latter seem to bear to the Polycyttaria a relation similar to that in which the sexless solitary Flagellata (Astasiea) stand to the sexual social Flagellata (Volvocinea). In the two analogous cases the sexual differentiation may be regarded as a consequence of the social life in the gelatinous colonies.

{cxxxix}The sexual differentiation of the Polycyttaria was first discovered in 1875 by R. Hertwig, and accurately described in the case of Collozoum inerme as occurring in addition to the formation of the ordinary crystal-spores (L. N. 26, p. 36); compare also the general discussion of Bütschli (L. N. 41, p. 52). Recently Karl Brandt has demonstrated the formation of both homogeneous isospores (crystal-spores) and heterogeneous anisospores (macro- and microspores) in seven different species of Polycyttaria, and has come to the conclusion that in all social Radiolaria there is a regular alternation between the former and latter generations. Compare his elaborate account of the colonial Radiolaria of the Gulf of Naples (L. N. 52, pp. 145-178).

217. Inheritance.--Inheritance is to be regarded as the most important accompaniment to the function of reproduction, and especially in the present case, because the comparative morphology of the Radiolaria furnishes abundant instances of the action of its different laws. The laws of conservative inheritance are illustrated by the comparative anatomy of the larger groups; thus, in the four legions the characteristic peculiarities of the central capsule are maintained unaltered in consequence of continuous inheritance, although great varieties appear in the skeleton in each legion. The individual parts of the skeleton furnish by their development on the one hand and their degeneration on the other, especially in the smaller groups, examples of progressive inheritance. Thus in the SPUMELLARIA the constant formation of the primary lattice-shell (a central medullary shell) and its ontogenetic relation to the secondary one, which is developed concentrically round it, can only be explained phylogenetically by conservative inheritance, whilst on the other hand the characteristic differentiation of the axes in the various families of SPUMELLARIA is to be explained by progressive inheritance. In the ACANTHARIA the arrangement of the twenty radial spines (in accordance with Müller's law, §§ 110, 172) was first acquired by a group of the most archaic #Actinelida# (Adelacantha) through hydrostatic adaptation, and has since been transmitted by inheritance to all the other families of the legion (Icosacantha). The morphology of the NASSELLARIA is not less interesting, because here several different heritable elements (the primary sagittal ring and the basal tripod) combine in the most manifold ways in the formation of the skeleton (compare §§ 123, 124, 182). The affinities of the genera in the different families yield an astonishing variety of interesting morphological phenomena, which can only be explained by progressive inheritance. The same is true also of the PHÆODARIA. In this legion the primary inheritance is especially manifested in the constant and firm structure of the central capsule with its characteristic double wall and astropyle, whilst the formation of the skeleton in this legion proceeds in different directions by means of divergent adaptation. The morphology of the Radiolaria thus proves itself a rich source of materials for the physiological study of adaptation and inheritance.

{cxl}CHAPTER VIII.--ANIMAL FUNCTIONS.

(§§ 218-225.)

218. Motion.--In addition to the internal movements which appear generally in the unicellular Radiolaria and have already been mentioned as plasmatic currents in treating of the circulation (§§ 207-209), two different groups of external motor phenomena are to be observed in this class: first, the contraction of individual parts, which brings about modifications of form (§ 220), and secondly, voluntary or reflex locomotion of the whole body (§ 220). These movements are partly due to changes in form of undifferentiated plasmatic threads or sarcode filaments, partly to the actual contraction of differentiated filaments which are comparable to muscle fibrillæ, and must therefore be distinguished as myophanes. In addition to this, endosmose and exosmose probably play an important part in some of the locomotive phenomena, but nothing is yet certainly known regarding these osmotic processes. We are at present equally ignorant whether all the movements of the Radiolaria are simply reflex (direct consequences of irritation) or whether they are in part truly spontaneous.

219. Suspension.--From direct observation of living Radiolaria, as well as from deductive reasoning, based upon their morphology (and especially their promorphology, §§ 17-50), the conclusion appears justified that all Protista of this class in their normal condition float suspended in the sea-water, either at the surface or at a definite depth. A necessary condition of this hydrostatic suspension is that the specific gravity of the Radiolarian organism must be equal to, or but slightly greater than that of sea-water. The increase in specific gravity brought about by the production of the siliceous skeleton, is compensated by the lighter fatty globules, and partly perhaps by the calymma, especially when the latter contains vacuoles or alveoles. The fluid or jelly contained in the latter appears to be for the most part lighter than sea-water (containing no salt, or only a very small quantity?). But if the specific gravity of the whole body should be generally (or perhaps always) slightly greater than that of sea-water, then the organism would be prevented from sinking, partly by the increased friction, due to the radiating pseudopodia and the radial spines usually present, and partly perhaps by active (if only feeble) movements of the pseudopodia.

220. Locomotion.--Active locomotion of the whole body, which is very probably to be regarded as voluntary, occurs in the Radiolaria in three different modes; (1) the vibratile movement of the flagellate swarm-spores; (2) the swimming of the floating organisms; (3) the slow creeping of those which rest accidentally upon the bottom. {cxli}The vibratile movement of the swarm-spores is the result of active sinuous oscillation of the single or multiple flagellum, and is not essentially different from that of ordinary flagellate Infusoria (see note A). Of the active swimming of mature Radiolaria, only that form is known which is vertical in direction and causes the sinking and rising in the sea-water. This is probably, for the most part (perhaps exclusively), due to increase or diminution in the specific gravity, which is perhaps brought about by the retraction or protrusion of the pseudopodia; slow, oscillating, sinuous motions of these organs have been directly observed to take place (though very slowly) in suspended living Radiolaria. The most important hydrostatic organ is probably the calymma, by the contraction of which the specific gravity is increased, while it is diminished by its expansion; the contraction is probably brought about by active contraction of the sarcodictyum, and is connected with exosmosis, while the expansion is probably due to the elasticity of the calymma and the inception of water by endosmosis. In the #Acanthometra# (§ 96) the peculiar myophriscs appear to be charged with the duty of distending the gelatinous envelope, and thus diminishing the specific gravity; the latter increases again when the myophriscs are relaxed, and the calymma contracts by virtue of its own elasticity (see note B). The slow creeping locomotion exhibited by Radiolaria on a glass slide under the microscope, does not differ from that of the Thalamophora (Monothalamia and Polythalamia), but can only occur normally when the animal accidentally comes into contact with a solid surface or sinks to the bottom of the sea. Whether this actually occurs periodically is not known (see note C). The slow or gliding locomotion exhibited by creeping Monozoa on a glass slide is due to muscle-like contractions of bundles of pseudopodia, just as in the case of the social central capsules of Polyzoa, which live together in the same coenobium and are able to move within their common calymma sometimes centrifugally to its surface, sometimes towards the centre where they aggregate into a roundish mass (see note D).

A. Regarding the movement of the flagella in mature swarm-spores compare L. N. 22, p. 375; L. N. 26, pp. 31, 35; L. N. 41, p. 452, and L. N. 52, p. 170.

B. On the active vertical swimming movements of mature Radiolaria, especially the cause of sinking and rising, see L. N. 16, p. 134; L. N. 41, p. 443, and L. N. 52, pp. 97-102.

C. On the active horizontal creeping movements of mature Radiolaria on a firm ground, compare L. N. 12, p. 10, and L. N. 16, pp. 132-134.

D. Regarding the motion of social central capsules within the same coenobium and the changes thus brought about in the structure of the calymma, see L. N. 16, pp. 119-127, and L. N. 52, pp. 75-82.

221. Contraction.--Motions, which are due to the contraction of individual portions and cause changes in volume or form, have been partly already spoken of under the head of locomotion (§ 220) and are partly connected with other functions. Examples may be seen in the contraction of the central capsule and of the calymma. A certain {cxlii}contraction of the central capsule is probably brought about by the myophanes, which arise by differentiation of the endoplasm and hence may assume different forms in the four legions. In the SPUMELLARIA, where numerous radial fibrillæ run from the central nucleus to the capsule membrane (§ 77), the endoplasm is probably driven out evenly through all the pores of the capsule membrane by their simultaneous contraction, and hence the volume of the capsule is diminished in all directions. The ACANTHARIA probably behave similarly, but are different, inasmuch as the number of their contractile radial fibrillæ is less, and special axial threads (§ 78) are already differentiated. In the NASSELLARIA it is probable that owing to the contraction of the divergent myophane fibrillæ in the podoconus the vertical axis of the latter is shortened, the opercular rods of the porochora are lifted, and the endoplasm driven out of its pores, so that the volume of the monaxon central capsule is diminished (§ 79). In the PHÆODARIA the same result is probably brought about by the contraction of the cortical myophane fibrillæ, which run meridionally along the inside of the capsule membrane from the apical to the basal pole of the vertical main axis, where they are inserted into the periphery of the astropyle; since the volume of the capsule is diminished by their contraction (their spheroidal figure becoming more nearly spherical) the endoplasm will be driven out through the proboscis of the astropyle. Whilst these contractions of the central capsule are largely due to differentiated muscle-like threads of endoplasm (myophanes), this appears to be but rarely the case with the contractions of the extracapsulum (e.g., the myophriscs of the #Acanthometra#, § 96). Most of the phenomena of contraction which can be observed in the calymma and pseudopodia depend upon exoplasmatic currents (§ 209).

222. Protection.--Of the utmost importance, both for the physiology and for the morphology of the Radiolaria are their manifold protective functions, which we now consider under the heading "protection." From the physiological point of view the consideration of the exposed situation in which the delicate, free-swimming Radiolarian organism lives, and the numerous dangers which beset it in the struggle for existence, would lead a priori to the expectation, that many special protective adaptations would be developed by natural selection. On the other hand, morphological experience shows us that this latter has been in action for immeasurable periods, and has gradually produced an abundance of the most remarkable protective modifications. Examples of these may be found in the formation of the voluminous calymma, as a gelatinous protective covering for the central capsule, and further, the formation of the capsule membrane itself, which separates the generative contents of the central capsule from the nutritive exoplasm. The phosphorescence of the central capsule, too (§ 223), may be regarded as a useful protective arrangement; as also the radiating of the numerous pseudopodia in all directions from the surface of the calymma; for they are of great significance to the {cxliii}well-being of the organism, both as sensory organs and as prehensile organs. By far the most important and most varied means for the actual defence of the soft body is to be seen in the endless modifications of the skeleton; first, in the production of the enclosing lattice-shells and projecting radial spines, but especially also in the very varied structure of the individual parts of the skeleton, and in the special differentiation of the small appendicular organs which grow out from it (hairs, thorns, spines, scales, spathillæ, anchors, &c.). Finally "mimicry" possesses a considerable significance among the different forms of adaptation which are to be observed in this class.

223. Phosphorescence.--Many Radiolarians shine in the dark, and their phosphorescence presents the same phenomena as that of other luminous marine organisms; it is increased by mechanical and chemical irritation, or renewed if already extinguished. The light is sometimes greenish, sometimes yellowish, and appears generally (if not always) to radiate from the intracapsular fatty spheres (§ 73). Thus these latter unite several functions, inasmuch as they serve, firstly, as reserve stores of nutriment, secondly, as hydrostatic apparatus, and thirdly, as luminous organs for the protection of the Radiolaria; probably the light acts by frightening other animals, for the phosphorescent animals are provided with spines, nettle-cells, poison glands or other defensive weapons. The production of the light depends probably, as in other phosphorescent organisms, upon the slow oxidation of the fat-globules, which combine with active oxygen in the presence of alkalis. Phosphorescence is very likely widely distributed among the Radiolaria.

The shining of the Radiolaria in the dark has been noticed by the earliest observers of the class (see L. N. 1, p. 163, L. N. 16, p. 2, and L. N. 52, pp. 136-139). In the winter of 1859 I observed the production of light in the case of many monozootic and polyzootic Radiolaria, but inadvertently omitted to record the fact in my Monograph. I made more accurate observations in the winter of 1866 at Lanzerote in the Canary Islands, and convinced myself the the light emanates from the central capsule, and in particular from the fat-globules contained in it. In most Polycyttaria (both #Collosphærida# and #Sphærozoida#), when each central capsule contains a large central oil-globule the light radiates from it. In Collozoum serpentinum (Pl. 3, figs. 2, 3) each cylindrical central capsule contains a row of luminous spherules like a string of beads. In Alacorys friderici (Pl. 65, fig. 1) the four-lobed central capsule contains four shining points. Karl Brandt has recently made more detailed communication on this point (L. N. 52, p. 137).

224. Sensation.--The general irritability which we ascribe to all organisms, and as the basis of which we regard the protoplasm, remains at an inferior stage of development in the Radiolaria. For although they are subject to various stimuli, and certainly possess a power of discrimination, special sensory organs are not differentiated; the peripheral portions of the protoplasm, and especially the pseudopodia, rather act both as organs of the different kinds of sensation and various modes of motion. That different Radiolaria have attained different degrees of development in this respect may be seen {cxliv}partly by direct observation of the reaction of the living organism towards various stimuli, and partly by the comparison of the different conditions of existence under which Radiolarians exist, both in the most various depths of the ocean and in all climatic zones (see note A). In general the Radiolaria seem to be sensitive to the following stimuli; (1) pressure (see note B); (2) temperature (see note C); (3) light (see note D); (4) chemical composition of the sea-water (see note E). The reaction towards these stimuli, corresponding to the sensation of pleasure or dislike which they call forth, is shown in various forms of motion of the protoplasm, changes in the currents in it, contraction of the central capsule, changes in the size, position, and form of the pseudopodia, changes in the volume of the calymma (by the evocation of water), &c. Among the sensory functions of the Radiolaria must be especially mentioned their remarkably developed perception of hydrostatic equilibrium (see note F), as well as their perception of distances, so clearly shown in the production of equal lattice-meshes and other regularly formed skeletal structures (see note G).

A. I can add but little to the communication which I made twenty-four years ago regarding sensation in the Radiolaria (L. N. 16, pp. 128-131). The most important point would be the great difference in irritability which must obtain between the pelagic, zonarial and abyssal Radiolaria, which may be assumed from a consideration of their very different conditions of existence as regards pressure, light, warmth, nutrition, &c. It is natural to suppose that the numerous abyssal Radiolaria, discovered by the Challenger, which live at great depths (2000 to 4500 fathoms) in complete darkness, in icy cold and under an enormous pressure, must have quite different sensations of pleasure from their pelagic relatives which live at the surface of the sea under an equatorial sun. Karl Brandt has recently added much to our knowledge regarding the special action of different vital conditions upon the various Polycyttaria and the degrees of their irritability (L. N. 52, pp. 113-132).

B. Regarding the sensation of pressure or sensation of touch of the Radiolaria and the various degrees of their mechanical irritability, see L. N. 16, p. 129; L. N. 41, p. 464.

C. Regarding the sensation of warmth or temperature-sense and its dependence upon different climatic relations, see L. N. 16, p. 129; L. N. 52, pp. 114-129.

D. Regarding the sensation of light, compare L. N. 16, p. 128; L. N. 42, p. 304; L. N. 52, pp. 102-104, 114.

E. Regarding the sense of taste of the Radiolaria or their peculiar sensitiveness towards the different chemical composition of the water, change in its salinity, presence of organic impurities, &c., see L. N. 16, p. 130; L. N. 52, pp. 103, 113. This chemical irritability seems to be the most highly developed sense in the Radiolaria, even more so than their mechanical irritability.

F. The perception of hydrostatic equilibrium among the Radiolaria is immediately visible from the position which their bodies, floating freely in the water, assume spontaneously, and from the symmetrical development of the skeleton, which by its gravitation necessitates a definite position. It may be assumed that the development of the various geometrical ground forms which correspond to a definite position of equilibrium, is the result of this particular kind of perception (compare §§ 40-45).

{cxlv}G. The plastic perception of distance of the pseudopodia is shown by the symmetry with which the forms composing the regular skeletal structures (e.g., the ordinary lattice-spheres with regular hexagonal meshes, the radial spines with equidistant branches) are excreted from the exoplasm. Both this form of sensation and the one first mentioned (note F) have hitherto received scarcely any attention, but are deserving of a thorough physiological investigation.

225. The Cell-Soul (Zellseele).--The common central vital principle, commonly called the "soul," which is considered to be the regulator of all vital functions, appears in the Radiolaria as in other Protista in its simplest form, as the cell-soul. By the continual activity of this central "psyche" all vital functions are maintained in unbroken action, and in uniform correlation. It is also probable that by it the stimulations which the peripheral portions of the cell receive from the outer world are first transmitted into true sensation, and that, on the other hand, the volition, which alone calls forth spontaneous movements, proceeds from it. The central capsule is most likely the sole organ of this cell-soul or central psychic organ, and the active portion may be either the endoplasm or the nucleus, or both. The central capsule may thus (apart from its function as a sporangium, § 215) be regarded as a simple ganglion cell, physiologically comparable to the nervous centre of the higher animals, whilst the exoplasm (sarcomatrix and pseudopodia) are to be compared to the peripheral nervous system and sense organs of the latter. The great simplicity of the functions of the cell-soul which appear in the Radiolaria, and the intimate connection of their different psychic activities, give to these unicellular Protista a special significance for the comprehension of the monistic elements of a natural psychology.

Regarding the theory of the cell-soul as the only psychological theory which is able to explain naturally the true nature of the life of the soul in all organisms as well as in man, see my address on cell-souls and soul-cells ("Zellseelen und Seelenzellen") in Gesammelte populäre Vorträge aus dem Gebiete der Entwickelungslehre, Heft 1, p. 143; Bonn, 1878.

{cxlvi}CHOROLOGICAL SECTION.

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V. ONTOGENY OR INDIVIDUAL DEVELOPMENT

VII. VEGETATIVE FUNCTIONS