60. 1871. Doenitz, W., Beobachtungen über Radiolarien. Archiv f. Anat. u. Physiol., 1871, p. 71, Taf. ii. (Compare L. N. 26, p. 7.)

252. Progress of our Knowledge of the Radiolaria from 1862 to 1885.—The history of our scientific knowledge of the Radiolaria extends over about half a century (from 1834 to 1885). A historical and critical discussion of the works which appeared within the first twenty-eight years of this period (from 1834 to 1862) is contained in the historical introduction to my Monograph (L. N. 16, pp. 1-24); I shall therefore give here only a brief survey of the investigations published during the last twenty-three years (from 1862 to 1885). The most important steps in our progress during this period we owe to the following naturalists:—Cienkowski (1871), Ehrenberg (1872 and 1875), Richard Hertwig (1876 and 1879), Karl Brandt (1881 and 1885), Bütschli (1882), and Rüst (1885). To the valuable works of these authors must be added a number of smaller contributions, which are recorded in the foregoing Bibliography. Some communications from dilettanti, written with insufficient knowledge of the subject, and hence of no value, are mentioned for the sake of completeness in the "Phaulographic Appendix" (compare L. N. 55-60, also L. N. 26, p. 9).

The first important advance in our knowledge of the organisation of the Radiolaria, made after the publication of my Monograph (1862), was the demonstration of the nature of the extracapsular "yellow cells." In the year 1870 I showed that these yellow cells contain starch (L. N. 21, p. 519). I regarded them, as did all authors up to that time, as integral parts of the Radiolarian organism, and hence considered this to be multicellular; for no doubt was possible regarding the true cellular nature of these remarkable, nucleated, yellow globules, which I had thoroughly studied in 1862. It was first shown by Cienkowski in 1871 that the yellow cells of the Collodaria remain unchanged even after the death of these organisms, "that they continue to grow uninterruptedly, and eventually multiply by division" (L. N. 22, pp. 378-380, Taf. xix. figs. 30-36). Cienkowski concluded from these important observations that the yellow cells are not integral parts of the Radiolarian body, but "parasitic structures," independent, unicellular organisms, which live only as parasites in the body of the Radiolaria (compare § 90).

This important recognition underwent ten years later a further development and complete establishment by the extensive investigations of Karl Brandt (L. N. 38, 39) and Patrick Geddes (L. N. 42, 43). This arrangement was compared by Brandt to the remarkable symbiosis of the Algoid gonidia and Fungoid hyphæ in the organisation of the Lichens, which had been recently discovered, and since he recognised the independent nature of the yellow cells, as unicellular Algæ, in all divisions of the Radiolaria, he founded for them the genus Zooxanthella. Geddes named them Philozoon, and showed experimentally that they give out oxygen under the influence of sunlight (compare § 90). The great physiological importance of the yellow cells in the metastasis of the Radiolaria, and, when they are developed in large quantities, in the economy of marine organisms in general, has recently been insisted upon by Brandt (see § 205 and L. N. 52, pp. 65-71, 86-94).

The proof that the yellow cells do not belong to the Radiolarian organism itself, but only live parasitically in it, was a necessary preliminary to the very important step which next took place in our knowledge of the organisation of the Radiolaria. This step consisted in the demonstration that the whole body of the Radiolaria, like that of all other Protista, is only a single cell. It was Richard Hertwig who in two remarkable works (L. N. 26, 33) firmly established this fundamental theorem of the unicellular nature of the Radiolaria. In his treatise on the histology of the Radiolaria (L. N. 26, 1876) he published complete investigations into the structure and development of the Sphærozoida and Thalassicollida. Since he made use of the modern methods of histological examination, and especially of staining fluids, which he was the first to apply to the study of the Radiolaria, he was able to show that no true cells (apart from the parasitic yellow cells) are to be found in their bodies, but rather that all their morphological components are to be regarded as differentiated parts of a single true cell, and in particular that the central capsule includes a genuine nucleus.

A wider foundation for this important discovery and its applicability to all divisions of this extensive class, was given by Hertwig in a second work on the organisation of the Radiolaria (L. N. 33, 1879). Among the numerous discoveries by which this work enriched the natural history of the Radiolaria must be specially mentioned the recognition of the fundamental differences exhibited by the main divisions of the class in the structure of their central capsule. Hertwig first observed that the capsular membrane is double in the Phæodaria but single in the other Radiolaria (§ 56); the former he named "Tripylea" because he discovered in their capsular membrane a large, peculiarly constructed main opening and two small accessory openings. The Nassellaria, in which he found a single porous area at the basal pole of the main axis, with a cone of pseudopodia rising from it, he called on this account "Monopylea"; whilst the other Radiolaria, whose capsular membrane is perforated on all sides with fine pores, were termed "Peripylea." Besides the central capsule, Hertwig laid stress upon the significance of the gelatinous envelope as a constant and important constituent of the body. He also devoted attentive consideration to the morphology of the skeleton, and on the basis of certain phylogenetic conclusions which he drew from it, he arrived at an improved systematic arrangement in which he distinguished six orders:—(1) Thalassicollea, (2) Sphærozoea, (3) Peripylea, (4) Acanthometrea, (5) Monopylea, (6) Tripylea. The numerous isolated discoveries with which Hertwig enriched the morphology of the Radiolaria, have been already alluded to in the appropriate paragraphs in the anatomical portion of this Introduction (see L. N. 42, pp. 340, 341).

The new and interesting group, which was thus erected into an order under the name Tripylea, I had already a year previously separated from the other Radiolaria as "Pansolenia" in my Protistenreich (L. N. 32, p. 102). Since, however, neither the three capsular openings of the Tripylea nor the skeletal tubes of the Pansolenia are present in all the families of this extensive order, I substituted in 1879 the more suitable name Phæodaria, which is applicable to all members of the group (L. N. 34). In the preliminary memoir then published regarding the Phæodaria, a New Group of Siliceous Marine Rhizopods, I distinguished four orders, ten families, and thirty-eight genera. The great majority of these new forms (among which were no less than 465 different species) were first discovered by the deep-sea investigations of the Challenger. John Murray was the first who called attention to the great abundance in the deep sea of these remarkable Rhizopods, and to the constant presence of their peculiar, dark, extracapsular pigment body (phæodium); even in 1876 he described a portion of them as Challengerida (L. N. 27, p. 536; L. N. 53, p. 226). The earliest observations on the Phæodaria were made at Messina in 1859, where I examined five genera of this remarkable group alive (compare p. 1522 and L. N. 16).

By the discovery that the Phæodaria, although differing in important respects from the other Radiolaria, still conform to the definition of the class, a new and extensive series of forms was added to this latter, and by their closer investigation a fresh source of interesting morphological problems was disclosed. In other groups, however, morphology was advanced by comparative anatomical studies. In addition to the smaller contributions of various authors, mentioned in the foregoing bibliography, I may specially refer to the valuable Beiträge zur Kenntniss der Radiolarien-Skelete, insbesondere der der Cyrtida by O. Bütschli (L. N. 40, 1882). On the basis of careful comparative anatomical studies, investigations into the skeletal structure of a number of fossil Cyrtoidea and critical application of the recently published researches of Ehrenberg into the Polycystina of Barbados (L. N. 25), Bütschli attempted to derive the complicated relations of the Monopylean skeletons phylogenetically from a simple primitive form,—the primary sagittal ring. Even if this attempt did not actually solve the very difficult morphological problem in question, still the critical and synthetic mode in which it was carried out deserves full recognition, and furnishes the proof that the comparative anatomy of the skeleton in the Radiolaria not less than in the Vertebrata, is a most interesting and fruitful field of phylogenetic investigation. A further demonstration of this was furnished by Bütschli in the general account of the organisation of the Radiolaria which he published in 1882 in Bronn's Klassen und Ordnungen des Thierreichs (L. N. 41).

In our knowledge of the developmental history of these Protista the last two decades have witnessed less progress than in their comparative anatomy. The most important advance in this direction has been the proof that in all the main groups of the class the contents of the central capsule are used in the formation of swarm-spores. The movements of these zoospores in the central capsule had indeed been observed by several previous authors in the case of the Spumellaria and Acantharia (L. N. 10, 13, 16; compare also § 142, Note A). The origin of the flagellate spores from the contents of the central capsule and their peculiar constitution were, however, first described fully by Cienkowski in 1871 (L. N. 22, p. 372). Soon after this, R. Hertwig discovered that in the social Radiolaria (Polycyttaria or Sphærozoea) two different forms of zoospores are formed, one with, the other without crystals, and that the latter are also divided into macrospores and microspores (compare L. N. 26, and § 142). Recently this sexual differentiation has been shown by Karl Brandt to exist in all the groups of Sphærozoea, and its regular interchange with the formation of crystal-spores has been interpreted as a true "alternation of generations" (compare L. N. 52 and also § 216). The other forms of development also, especially reproduction by cell-division (§ 213) and gemmation (§ 214), have been elucidated by the recent investigations of the same author.

The palæontology of the Radiolaria has of late made important and interesting advances. Until ten years ago fossil remains of this class were known exclusively from the Tertiary period; almost the only source of our information was to be found in the researches of Ehrenberg, commenced in 1838, continued in his Mikrogeologie in 1854, and concluded in his last work (L. N. 25) published in 1875 (compare L. N. 16, pp. 3-9, 191-193). In the year 1876 a number of Mesozoic Radiolaria from the chalk were described by Zittel (L. N. 28), and afterwards others from the Jura by Dunikowski (L. N. 44). That fossil Radiolaria occur in Mesozoic formations, especially in the Jura, as well preserved and as abundantly as in the Tertiary rocks of Barbados, was shown in 1883 by Rüst (L. N. 48). By the examination of numerous thin sections he discovered that in all the main divisions of the Jurassic formation (Lias, Dogger, Malm) there are distributed jaspers, flints, cherts, and other quartzites, which consist largely of the siliceous shells of Polycystina; the same is true also of many Coprolites found in the Jura. The full account of these and the descriptions and figures of 234 Jurassic species, distributed in 76 genera, are contained in the Beiträge zur Kenntniss der fossilen Radiolarien aus Gesteinen des Jura (L. N. 51, 1885). But even in the older rocks, the Trias, the Permian, and Carboniferous systems, and even as far downwards as the Silurian and Cambrian formations, Rüst has recently shown the existence of fossil Radiolaria, and thus increased the known period of the developmental history of the class by many millions of years (§ 244).

The great significance of the Radiolaria in geology and palæontology has been brought into new light not only by these extensive discoveries, but also by the important relations which have been shown to exist between the Radiolarian rocks and the deep-sea deposits of the present day. In this direction the wonderful discoveries of the Challenger, and especially the investigation of the deep-sea deposits by Wyville Thomson (L. N. 31) and John Murray (L. N. 27), have furnished us with new and valuable information (compare §§ 236-239, and §§ 245-250). The Tertiary Polycystine formations of Barbados and the Nicobar Islands, with which we have been acquainted for the last forty years, as also the Mesozoic Radiolarian quartzes, which have only recently been made known to us from the Jura, are ascertained to be fossil representatives of the same deep-sea deposits which now occur in the form of Radiolarian ooze (§ 237), and to some extent also of Globigerina ooze and red clay (§§ 238, 239), on the bottom of the ocean, at depths of from 2000 to 4500 fathoms.

These investigations into fossil Radiolaria and their comparison with recent deep-sea forms have a further general significance, inasmuch as the identity of many living and fossil species from the Tertiary formation has been shown beyond all doubt. In this direction the numerous measurements and accurate comparisons which I have made during the last ten years of the abyssal forms in the Challenger collection, and of fossil species from Barbados and Caltanisetta, have brought to light many important facts. In this I had the able assistance of my friend, Dr. Reinhold Teuscher (compare § 250, and p. 1760). Further valuable contributions in this direction are found in the careful observations and comparative measurements recently published by Emil Stöhr (L. N. 35, 1880), regarding the Radiolarian fauna of the Tripoli of Grotte in the province of Girgenti, Sicily. From these it appears that the number of Miocene species which are still extant, is much greater than would appear from the results of Ehrenberg.

Ehrenberg himself, towards the end of his long and laborious life, collected the results of the systematic and palæontological researches, which he had begun thirty-seven years previously (L. N. 16, pp. 3-12) into the Polycystina, in two large works (L. N. 24, 25). The first treatise (L. N. 24, 1872) contains the Mikrogeologische Studien über das Kleinste Leben der Meeres-Tiefgründe aller Zonen und dessen geologischen Einfluss, with a list of 279 Polycystina observed by him from the deep-sea, as well as figures of 127 species. The second work (L. N. 25, 1875) contains the Fortsetzung der Mikrogeologischen Studien, mit specieller Rücksicht auf den Polycystinen-Mergel von Barbados; the list of fossil Polycystina observed by him includes 325 species, of which 26 are still extant; 282 of them are figured on the thirty plates accompanying the memoir. By means of these numerous figures, as well as by the appended systematic and chorological tables, Ehrenberg furnished a welcome supplement to the numerous communications regarding the Polycystina, which he had made to the Berlin Academy since 1838, and which he had published in his Mikrogeologie in 1854. It will always be the merit of this zealous and indefatigable microscopist that he first called attention to the great wealth of forms existing in this class; he separated systematically about 500 species, and published drawings of about 400; in addition to which he was the first to lay stress upon the great chorological and geological importance of the Radiolaria.

With these systematic and descriptive, chorological and palæontological works, however, which relate exclusively to the Polycystina, the merits of the famous naturalist of Berlin are exhausted as regards this class of animals. Of the organisation of the Radiolaria, Gottfried Ehrenberg remained entirely ignorant up till his death in 1876. All that a number of famous naturalists had observed during a quarter of a century as to the structure and life-history of the Radiolaria, all the important discoveries of Huxley (1851), Johannes Müller (1858), Claparède (1858), Cienkowski (1871), and many others (L. N. 1-22), and all that I had published in my Monograph (1862) on the basis of three years' study of their anatomy and physiology—all this Ehrenberg ignored, or rather, he regarded it all as worthless rubbish of science, as a chaos of devious errors, resting upon incomplete observations and false conclusions. His strange "special considerations regarding the Polycystina" (L. N. 24, pp. 339-346) and the general "concluding remarks" (L. N. 25, pp. 146-147) leave no room for doubt on this point. Ehrenberg indeed doubted to the last whether any observer had seen living Radiolaria at all (L. N. 25, p. 108).

The invincible obstinacy with which Ehrenberg maintained his preconceived opinion of the high organisation of the Radiolaria, and entirely ignored the contrary observations of other naturalists, is explained by the consistency with which he held to the end the "principle peculiar to himself of the universally equal development of the animal kingdom" (L. N. 16, p. 7). From the complicated arrangement of their siliceous shells he concluded that the animals inhabiting them must possess a structure correspondingly complex, and nearly related to that of the Echinodermata (Holothuria). Like all other animals the Radiolaria must possess systems of organs for locomotion, sensation, nutrition, circulation, and reproduction. Whilst Ehrenberg originally interpreted the Polycystina as siliceous Infusoria polygastrica, and regarded them as compound Arcellina, he afterwards classed them sometimes with the Echinodermata (Holothuria), sometimes with the Bryozoa, sometimes with the Oscillaria (see L. N. 41, p. 336). Although a decided opponent of the cell-theory he called them "multicellular animalcules" (Polycystina), interpreting the pores of the siliceous shell as cells. To-day the opposite term (Monocystina) might be adopted to express their unicellular organisation. It was a remarkable irony of fate that in the self-same year (1838) in which Schwann of Berlin made by his foundation of the cell theory the greatest advance in the whole of Biological Science, that Ehrenberg, all his life the most zealous opponent of that theory, published his great work on the Infusoria, and at the same time established the "family of multicellular animalcules or Polycystina" (L. N. 16, p. 4).

The "short systematic survey of the genera of cellular animalcules" given by Ehrenberg in 1875 (L. N. 25, p. 157), is only a new edition, increased by sixteen genera, of his first systematic arrangement of the Polycystina of 1847 (L. N. 4, p. 53). Since I have already given a full discussion of this in my Monograph (L. N. 16, pp. 214-219), I need only here remark that a correct understanding of his very inadequate generic diagnoses is only possible by the aid of his figures. Relying upon these I have retained almost all Ehrenberg's genera, although entirely new definitions of most of them have been necessary.

The same is true also of the two orders which Ehrenberg distinguished in his class of "Zellenthierchen." The first order is constituted by his "Netzkörbchen" (Monodictya or Nassellaria) formerly known as "Polycystina solitaria"; they include our Cyrtoidea, the greater part of Hertwig's Monopylea. Ehrenberg's second order is the "Schaumsternchen" (Polydictya or Spumellaria), previously called "Polycystina composita"; they include the Peripylea of Hertwig, as well as the Spyridina (our Spyroidea), which belong properly to the Nassellaria. Although Ehrenberg's statements regarding the organisation of both these orders were quite erroneous, and his knowledge even of the structure of their shells very defective, I still thought it advisable to retain his names for the groups, since they constituted his one successful effort in the systematic treatment of the Radiolaria (compare L. N. 41, p. 336).

The sketch of a systematic arrangement of the Radiolaria (L. N. 37), which I published in 1881 on the basis of the study of the Challenger Radiolaria, resembles, in respect of seven orders being distinguished, the new system which R. Hertwig founded in 1879, in consequence of the variations which he discovered in the structural relations of the central capsule (L. N. 33, p. 133). It differs, however, inasmuch as his Sphærozoea (my Polycyttaria) are here divided into two orders, Symbelaria (Collosphærida) and Syncollaria (Sphærozoida). In that sketch too I separated for the first time the two subclasses Holotrypasta (Porulosa) and Merotrypasta (Osculosa). The fifteen families established by Hertwig were then raised to twenty-four. The six hundred and thirty genera, which I then distinguished, are still for the most part retained, some, however, in a restricted sense, or with amended definitions.

The differential characters of the orders and families of the Radiolaria, given in the Prodromus in 1881, were amended in a further communication which I gave in 1883 regarding the orders of the Radiolaria (L. N. 46, p. 17). There I reduced the seven orders to four, the structural relations of the central capsule being precisely the same in the Polycyttaria and Collodaria as in the Peripylea. The survey of the affinities of the class was thus rendered much simpler and clearer, and the hypothetical genealogical tree, which I then published, has been still further carried out in Chapter VI. of the present Introduction (see §§ 153-200).