Radiolaria

“ 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. ”

“ 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) . ”

“ The extracapsulum is partly the general organ for intercourse with the outer world (by means of the pseudopodia) , partly the special organ of protection (calymma) and nutrition (sarcomatrix) . The majority of Radiolaria develop also a skeleton for support and protection, which presents the utmost variety of form, and is generally composed of silica, sometimes of an organic substance (acanthin) . The Radiolarian cell usually leads an isolated existence (Monozoa vel Monocyttaria) ”

“ To this category belong all Phæodaria and Nassellaria, as well as all the solitary or monozoic Spumellaria and some Acantharia. On the other hand, the name "precocious," or early dividing, is applied to those Radiolaria in which the division of the nucleus takes place very early, and in which, therefore, the cell is multinuclear during the greater part of its existence. This is the case in all the social or polyzootic Radiolaria (Polycyttaria, Pls. 3-8) , and also in the great majority of the Acantharia, both Acanthometra and Acanthophracta. ”

“ From these true abyssal Radiolaria must be carefully distinguished those species whose empty skeletons, devoid of all soft parts, occur also in the Radiolarian ooze of the deep-sea, but are clearly only the sunken remains of dead forms, which have lived at the surface or in some of the upper zones. ”

“ 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) . ”

“ 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. ”

“ 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. ”

“ Among these remains, however, the siliceous skeletons of the Radiolaria play a significant and often the most important part. Furthermore, John Murray has called attention to the fact that in many deep-sea deposits yellow and red insoluble particles remain, which unmistakably present the form of Radiolarian shells (L. N. 27, p. 513) . At Station 303 he found "amorphous clayey matter, rounded yellow minerals, many Radiolaria-shaped;" ”

“ In most Radiolaria the exoplasm is clearly different from the endoplasm, and the former is more voluminous than the latter. 2. In all Radiolaria the division of labour is so carried out between the central capsule and the extracapsulum, that the physiological significance and independence of both principal parts of the cell is almost equally great. 3. It is only in the Acantharia that the formation of the skeleton takes place within the central capsule; in all the other three legions it is quite independent of it. ”

“ From the tropics the abundance of species seems to diminish regularly towards the poles, and more rapidly in the northern than in the southern hemisphere; the latter also appears, considered as a whole, to possess more species than the former. A limit to the life of the Radiolaria towards the poles has not yet been found; the expeditions towards the North Pole (see note C) , like those towards the South (see note D) , have obtained bottom-deposits and ice enclosures which contained Radiolaria ”

“ The fact is, nevertheless, certain that the hyaline spheres, which may be isolated on rupturing the central capsule of many Radiolaria, in certain cases, particularly in large species, possess a clear, anatomically demonstrable membrane, whilst in others no such appearance is presented. It may be assumed that the vesicular alveoles are developed from the drop-like vacuoles by increase in size, and by the precipitation of a delicate envelope from the endoplasm. ”

“ In the calymma of many Radiolaria thin, straight, radial lines are to be seen, which are probably pseudopodia, and not to be attributed to any structural modification, or they may be slender canals which serve for the exit of the pseudopodia. On the outer surface of the calymma of different Radiolaria, and especially in the Acantharia, a peculiar network of fibres is to be found, composed of polygonal meshes, like elastic fibres, probably due to a local thickening of the jelly. ”

“ On the other hand, there are very many species which are hitherto known only from one locality, and probably many small local faunas exist, characterised by the special development of particular groups. The observations which we at present possess are too incomplete, and the rich material of the Challenger is too incompletely worked out, to enable any definite conclusions to be drawn regarding the local distribution of Radiolaria. ”

“ Extracapsular vacuoles or alveoli.
6. Fat-granules or oil-globules.
7. Pigment-granules or a peculiar large body of dark extracapsular pigment, the "phæodium."
8. "Xanthellæ" or "zooxanthellæ," peculiar yellow cells, which contain starch and are unicellular yellow Algæ, living as "Symbiontes" in true Symbiosis with a great many Radiolaria.
The Nucleus of the Radiolaria is a large true simple cell-nucleus, originally a solid spherical, roundish or longish body of nuclein. It is placed either in the centre of the capsule (in most Peripylea) or excentrically (in most other Radiolaria) . ”

“ Besides this the endoplasm of the Radiolaria seems to have a great significance for the nutrition, mainly for the interchange of materials. It becomes very often vacuolate or alveolate, filled with smaller or larger spherical drops of fluid; it produces very commonly smaller fat-granules or larger oil-globules, and further pigment-granules of different colours, more rarely crystals and other peculiar enclosed parts.
The Membrane or "capsule-membrane" is the most typical and characteristic part of the body of a Radiolarian, sufficient of itself to separate this class from all other Rhizopoda. ”

“ Formations poor in Radiolaria, which contain only a few species of Spumellaria and Nassellaria mingled with other fossil remains and mineral particles, occur in all formations and are probably very widely distributed. ”

“ The order Collodaria, the first order of Radiolaria, comprises all those Spumellaria in which the skeleton is either entirely wanting, or represented by numerous single, solid, siliceous needles or spicules, loosely scattered in the calymma around the central capsule. ”

“ The genus Actissa is the most simple and typical form of all Radiolaria, and may be regarded as the common ancestral form of the whole class. The spherical body is composed of a simple spherical capsule and a concentric spherical calymma or jelly-envelope. Neither the former nor the latter contains alveoli. The central capsule possesses a strong membrane perforated by small pores, and contains in the intracapsular sarcode numerous small pellucid vacuoles, and in its middle a simple, concentric, spherical nucleus (often with some nucleoli) ; sometimes also one or more oil-globules. ”

“ 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) . ”

“ 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) . ”

“ 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. ”

“ They differ, however, the amphipleural body not being made up of two antimeres, but of at least three pairs of antimeres (or three parameres) , being therefore primitively radial. Hence each of the symmetrical halves of the body contains more than one antimere. Among the Radiolaria this form does not occur in the Spumellaria, Acantharia, or Phæodaria; it is very common, however, among the Nassellaria ”

“ 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) . ”

“ 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. ”

“ 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) . ”

“ Among the Miocene Radiolaria numerous species, both of Spumellaria (especially Sphæroidea and Discoidea) and of Nassellaria (especially Spyroidea and Cyrtoidea) are not to be distinguished from the corresponding still living forms (see notes C, D) . On the other hand, those genera, which are rich both in species and individuals (recent as well as fossil) , present continuous series of forms which lead gradually and uninterruptedly from old Tertiary species to others still living, which are specifically indistinguishable from them. ”

“ Only one other genus of Radiolaria exhibits the same characteristic disposition of thirty-two radial spines as Actinastrum, and this is Chiastolus; but here the two opposite spines of each pair are grown together and form one diametral spine; and the sixteen diametral spines are crossed in the centre of the capsule. In Actinastrum, as in Astrolophus and Actinelius, the central ends or bases of all the spines are pyramidal, and the triangular faces of the neighbouring spines rest one upon another (as in the greater number of Acanthonida) . ”