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 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 Cœlodendrum 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 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 cœnobium, and, having become isolated, establish the formation of a new colony, is very doubtful. The various forms which the cœnobium 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 cœnobia 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).

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, 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.
I. Legion (or Subclass)
Spumellaria
(Peripylea)

[Porulosa peripylea.]		 brace I. Collodaria
(Spumellaria palliata)		 brace 1. Colloidea, brace 1a. Thalassicollida.
1b. Collozoida.
2. Beloidea, brace 2a. Thalassosphærida.
2b. Sphærozoida.
II. Sphærellaria
(Spumellaria loricata)		 brace 3. Sphæroidea, brace 3a. Ethmosphærida.
3b. Collosphærida.
4. Prunoidea, brace 4a. Ellipsida.
4b. Zygartida.
5. Discoidea, brace 5a. Phacodiscida.
5b. Porodiscida.
6. Larcoidea, brace 6a. Larnacida.
6b. Pylonida.
II. Legion (or Subclass)
Acantharia
(Cannopylea).

[Osculosa cannopylea.]		 brace III. Acanthometra
(Acantharia palliata)		 brace 7. Actinelida, brace 7a. Astrolophida.
7b. Litholophida.
7c. Chiastolida.
8. Acanthonida, brace 8a. Astrolonchida.
8b. Quadrilonchida.
8c. Amphilonchida.
IV. Acanthophracta
(Acantharia loricata)		 brace 9. Sphærophracta, brace 9a. Sphærocapsida.
9b. Dorataspida.
9c. Phractopeltida.
10. Prunophracta, brace 10a. Belonaspida.
10b. Hexalaspida.
10c. Diploconida.

III. Legion (or Subclass)
Nassellaria
(Monopylea)

[Osculosa monopylea.]

 brace V. Plectellaria
(Nassellaria palliata)		 brace 11. Nassoidea, 11. Nassellida.
12. Plectoidea, brace 12a. Plagonida.
12b. Plectanida.
13. Stephoidea, brace 13a. Stephanida.
13b. Tympanida.
VI. Cyrtellaria
(Nassellaria loricata)		 brace 14. Spyroidea, brace 14a. Zygospyrida.
14b. Androspyrida.
15. Botryodea, brace 15a. Cannobotryida.
15b. Lithobotryida.
15c. Pylobotryida.
16. Cyrtoidea, brace 16a. Monocyrtida.
16b. Dicyrtida.
16c. Tricyrtida.
16d. Stichocyrtida.
IV. Legion (or Subclass)
Phæodaria
(Actipylea)
[Porulosa actipylea.]		 brace VII. Phæocystina
(Phæodaria palliata)		 brace 17. Phæocystina, brace 17a. Phæodinida.
17b. Cannorrhaphida.
17c. Aulacanthida.
18. Phæosphæria, brace 18a. Orosphærida.
18b. Aulosphærida.
18c. Cannosphærida.
VIII. Phæocoscina
(Phæodaria loricata)		 brace 19. Phæogromia, brace 19a. Challengerida.
19b. Castanellida.
19c. Circoporida.
20. Phæoconchia, brace 20a. Concharida.
20b. Cœlodendrida.
20c. Cœlographida.
Four Legions.
Eight Sublegions.
Twenty Orders.
Typical Families.

I. Legion (or Subclass) (Peripylea)

[Porulosa peripylea.]
I. Collodaria (Spumellaria palliata)
1. Colloidea,
1a. Thalassicollida.
1b. Collozoida.
2. Beloidea,
2a. Thalassosphærida.
2b. Sphærozoida.
II. Sphærellaria (Spumellaria loricata)
3. Sphæroidea,
3a. Ethmosphærida.
3b. Collosphærida.
4. Prunoidea,
4a. Ellipsida.
4b. Zygartida.
5. Discoidea,
5a. Phacodiscida.
5b. Porodiscida.
6. Larcoidea,
6a. Larnacida.
6b. Pylonida.

II. Legion (or Subclass) Acantharia (Cannopylea).

[Osculosa cannopylea.]
III. Acanthometra (Acantharia palliata)
7. Actinelida,
7a. Astrolophida.
7b. Litholophida.
7c. Chiastolida.
8. Acanthonida,
8a. Astrolonchida.
8b. Quadrilonchida.
8c. Amphilonchida.
IV. Acanthophracta (Acantharia loricata)
9. Sphærophracta,
9a. Sphærocapsida.
9b. Dorataspida.
9c. Phractopeltida.
10. Prunophracta,
10a. Belonaspida.
10b. Hexalaspida.
10c. Diploconida.

III. Legion (or Subclass) Nassellaria (Monopylea)

[Osculosa monopylea.]
V. Plectellaria (Nassellaria palliata)
11. Nassoidea,
11. Nassellida.
12. Plectoidea,
12a. Plagonida.
12b. Plectanida.
13. Stephoidea,
13a. Stephanida.
13b. Tympanida.
VI. Cyrtellaria (Nassellaria loricata)
14. Spyroidea,
14a. Zygospyrida.
14b. Androspyrida.
15. Botryodea,
15a. Cannobotryida.
15b. Lithobotryida.
15c. Pylobotryida.
16. Cyrtoidea,
16a. Monocyrtida.
16b. Dicyrtida.
16c. Tricyrtida.
16d. Stichocyrtida.

IV. Legion (or Subclass) Phæodaria (Actipylea)

[Porulosa actipylea.]
VII. Phæocystina (Phæodaria palliata)
17. Phæocystina,
17a. Phæodinida.
17b. Cannorrhaphida.
17c. Aulacanthida.
18. Phæosphæria,
18a. Orosphærida.
18b. Aulosphærida.
18c. Cannosphærida.
VIII. Phæocoscina (Phæodaria loricata)
19. Phæogromia,
19a. Challengerida.
19b. Castanellida.
19c. Circoporida.
20. Phæoconchia,
20a. Concharida.
20b. Cœlodendrida.
20c. Cœlographida.

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 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 Cœlographida. 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."