RADIOLARIA, so called by E. Haeckel in 1862 (Polycystina, by C. G. Ehrenberg, 1838), the name given to Marine Sarcodina, in which the cytoplasmic body gives off numerous fine radiating pseudopods (rarely anastomosing) from its surface, and is provided with a chitinous “central capsule,” surrounding the inner part which encloses the nucleus, the inner and outer cytoplasm communicating through either one or three apertures or numerous pores in the capsule. The extracapsular cytoplasm is largely transformed into a gelatinous substance (“calymma”), through which a granular network of plasm passes to form a continuous layer bearing the pseudo pods at the surface; this gelatinous layer is full of large vacuoles, “alveoli,” as in other pelagic Sarcodina (Heliozoa, q.v.), Globigerinidae, &c., among Foraminifera (q.v.). The protoplasm may contain oil-globules, pigment-grains, reserve-grains and crystals. There is frequently a skeleton present, either of silica (pure or containing a certain amount of organic admixture), or of “acanthin” (possibly a proteid, allied to vitellin, but regarded by W. Schewiakoff as a hydrated silicate of calcium and aluminium); never calcareous or arenaceous. The skeleton may consist of spicules, isolated or more or less compacted, or form a latticed shell, which, in correlation with the greater resistance of its substance, is of lighter and more elegant structure than in the Foraminifera. The alveoli contain a liquid, which, as shown by Brandt, is rich in carbon dioxide, and in proportion to its abundance may become much lighter than sea-water; and possibly the gelatinous substance of the calymma is also lighter than the medium. In Acantharia the protoplasm at the base of the projecting spines is often differentiated into a bundle of fibres converging on to the spines some way up (distally); these, comparable to the myonemes of Infusoria (q.v.), &c., and termed “myophrisks”, possibly serve to drag outwards the surface and so extend it, with concurrent dilatation of the alveoli, and lower the specific gravity of the animal. In this group also a thick temporary flagellum “sarcoflagellum” may be formed, apparently by the coalescence of a number of pseudopodia. The pigmented mass or “phaeodium” in the ectoplasm of Phaeodaria appears to be an excretory product, formed within the central capsule and passing immediately outwards; a similar uniform deposit of pigmented granules occurs in the Colloid species, Thalassicolla nucleata. The wall of the central capsule is simple in the Spumellaria, but formed of two layers in the Nassellaria and Phaeodaria. In the Nassellaria the oscule is simply a perforated area, and a cone of differentiated fibres in the intracapsular cytoplasm has its base on it: it is termed the “porocone,” and the fibres may possibly be muscular (myonemes). In Phaeodaria, the inner membrane at each oscule is prolonged through the outer into a tube (“proboscis”): the outer membrane of the principal oscule forms a large radially striated circular plate, the “astropyle,” or “operculum.” The innermost shell of some with concentric shells may lie within the central capsule, or even within the nucleus; this is due to the growth of these organs after the initial shell is formed, so that they pass out by lobes through the latticed openings of the embryonic shell, which lobes ultimately coalesce outside the embryonic chamber, and so come finally to invest it (fig. III. 17). In some, a symbiosis occurs with Zooxanthella, Brandt, a Flagellate of the group Chrysomadineae, which in the resting state inhabits the extracapsular cytoplasm growing and dividing freely therein, and only (under study) becoming free and flagellate on the death of the host (fig. III. 4, 6-13). The Silicoflagellata or Dictyochidae, also possessing a vegetable-colouring matter, but with a skeleton of impure silica (like that of Phaeodaria), may pass some of their lives in symbiosis with Radiolaria.
Fig. I.—Thalassicolla pelagica, Haeckel; ×25. CK, central capsule; EP, extracapsular protoplasm; al, alveoli, liquid-holding vacuoles in the protoplasm similar to those of Heliozoa, Hastigerina, &c.; ps, pseudopodia. The minute unlettered dots are the “yellow cells.”
Living Radiolaria were first observed and partially described by W. I. Tilesius in 1803-6 and 1814, by W. Baird in 1830, and by C. G. Ehrenberg in 1831, as luminous organisms in the sea; F. J. F. Meyen in 1834 recognized their animal character and the siliceous nature of their spicules. Ehrenberg a little later described a large number of Nassellarian skeletons under the name of Polycystina (1838), but without more than a very slight knowledge of a few living forms. T. H. Huxley in 1851 made the first adequate study of the living animal, and was followed by Joh. Müller in the same decade. E. Haeckel began his publications in 1862, and in two enormous, abundantly illustrated, systematic works, besides minor publications, has dealt exhaustively with the cytology, classification, and distribution of the class. Next in value come the contributions of Richard Hertwig (largely developmental), besides those of L. Cienkowsky, Karl Brandt and A. Borgert, while to F. Dreyer and V. Häcker we owe valuable studies on the physical relations of the skeleton.
Fig. II.—Eucyrtidium cranioides, Haeck.; ×150; one of the Nassellaria. Entire animal as seen in the living condition. The central capsule is hidden by the beehive-shaped siliceous shell within which it is lodged.
Our classification is taken from Haeckel.
A. Spumellaria, Haeck. (Peripylaea, Hertwig). Central capsule perforated with numerous evenly distributed pores. Skeleton siliceous, latticed or of detached spicules, or absent. Form homaxonic or with at least three planes of symmetry intersecting at right angles, rarely irregular or spiral, sometimes forming colonies, i.e. with several central capsules in a common external cytoplasm.
I. Skeleton of detached spicules, or absent.
Fam. 1. Colloidea. Skeleton absent. Thalassicolla, Huxl. (figs. I. and III. 1); Thalassophysa, Haeck.; Collozoum, Haeck. (fig. III. 2-5, 15, 16); Actissa, Haeck.
Fam. 2. Beloidea. Skeleton spicular. Sphaerozoum, Haeck.; Raphidozoum, Haeck.
II. Skeleton latticed or spongy-reticulated.
Fam. 3. Sphaeroidea. Skeleton homaxial, sometimes colonial. Collosphaera, Mull.; Haliomma, Ehrb.; Actinomma, Haeck. (fig. III. 17), showing concentric latticed shells, the smallest intranuclear, all connected by radial spines; Spongosphaera, Haeck. (fig. IV. 8); Heliosphaera, Haeck. (fig. III. 14).
Fam. 4. Prunoidea. Skeleton a prolate spheroid or cylinder of circular section, sometimes constricted like a dice-box.
Fam. 5. Discoidea. Shell flattened, of circular plan, rarely becoming spiral.
Fam. 6. Larcoidea. Shell with three unequal axes, elliptical in the plane of any two, more rarely becoming irregular or spiral.
B. Acantharia, Haeck. (Actipylaea, Hertw.). Skeleton of spicules of acanthin radiating from a centre, and usually twenty disposed on five successive zones of four on alternating meridians, the zones corresponding to equator, tropics and circumpolar circles on the globe; pores of central capsule in scattered groups.
Fam. 1. Actinelida. Spines numerous, more than twenty, irregularly grouped. Litholophus, Haeck.; Xiphacantha, Haeck.
Fam. 2. Acanthonida. Spines twenty, simple, usually equal. Acanthometra, J. Müll. (fig. iv. 6, 7); Astrolonche, Haeck.; Amphilonche, Haeck. (fig. III. 18).
Fam. 3. Sphaerophractida: Spines equal, branching and often coalescing into a latticed shell, homaxonic.
Fam. 4. Prunophractida: Branching spines coalescing into a latticed shell which is elongated and elliptical in at least one plane.
C. Nassellaria, Haeck. (Monopylaea, Hertw.). Silico-skeletal Radiolaria in which the central capsule is typically monaxonic (cone shaped), with a single perforate area (pore-plate) placed on the basal face of the cone; the membrane of the capsule, the nucleus single; the skeleton is extracapsular, and forms a scaffold-like or beehive-like structure of monaxonic form, a tripod or calthrop, a sagittal ring, or a combination of these.
Fam. 1. Nassoidea, Haeck. Skeleton absent. Cystidium, Haeck.
Fam. 2. Plectida, Haeck. Skeleton formed of a single branching spicule, a tripod or usually a 4-radiate calthrop, its branches sometimes reticulated. Genera: Plagiacantha, Haeck.; Plegmatium, Haeck.
Fam. 3. Spyroidea. Shell latticed around the sagittal ring (“cephalis”), sometimes with a lower chamber added.
Fam. 4. Botridea, Haeck. Shell latticed, composed of several chambers agglomerated without definite order; a single central capsule. Genera: Botryocyrtis, Haeck.; Lithobotrys, Haeck.
Fam. 5. Cyrtoidea, Haeck. Skeleton a monaxonic or triradiate shell, or continuous piece (beehive-shaped). Genera: Halicalyptra, Haeck.; Eucyrtidium, Haeck. (fig. II.); Carpocanium, Haeck. (fig. IV. 3).
Fam. 6. Stephoidea, Haeck. Skeleton a sagittal ring continuous with the branched spicule, and sometimes growing out into other rings or branches. Genera: Acanthodesmia, Haeck.; Zygostephanus, Haeck.; Lithocircus, Haeck. (fig. IV. 1).
D. Phaeodaria, Haeck. (Tripylaea, Hertw.). Radiolaria of cruciate symmetry, prolonged into tubular processes with three oscula to the central capsue, one inferior, the principal, and two symmetrically placed on either side of the opposite pole; skeleton of spicules, a network of hollow filaments, or a minutely alveolate shell, of a combination of silica with organic substance; extracapsular protoplasm containing in front of the large oscule an agglomeration of dusky purplish or greenish pigment (“phaeodium”).
Fam. 1. Phaeocystida, Haeck. Siliceous skeleton absent or of separate needles. Genera: Aulacantha, Haeck.; Thalassoplancta, Haeck.
Fam. 2. Phaeosphaerida. Spicules united into a latticed shell. Genera: Aulosphaera, Haeck. (fig. IV. 9); Auloplegma, Haeck.; Cannacantha, Haeck.
Fam. 3. Phaeogromida, Haeck. Shell continuous, traversed by fine canals or finely alveolate, provided with at least one pylome. Genera: Challengeria, Wyv., Thomson; Lithogromia, Haeck.
Fam. 4. Phaeoconchida. Shell as in Phaeosphaerida, but of two symmetrical halves (valves), which meet in the plane of the three oscules (“frontal” of Haeckel, who terms the plane of symmetry through the shells “sagittal”). Genera: Conchidium, Haeck.; Coelodendrum, Haeck. (fig. IV. 4).
|Fig. III.-Radiolaria. 1. Central capsule of Thalassicolla
nucleata, Huxley, in radial section. a, the large nucleus (Binnenbläschen); b, corpuscular structures of the intracapsular protoplasm containing concretions; c, wall of the capsule (membranous shell), showing the fine radial pore-canals; d, nucleolar fibres (chromatin substance) of the nucleus. 2, 3. Collozoum inerme, J. Müller, two different forms of colonies, of the natural size. 4. Central capsule from a colony of Collozoum inerme, showing the intracapsular protoplasm and nucleus, broken up into a number of spores, the germs of swarm-spores or flagellulae; each encloses a crystalline rod. c, yellow cells lying in the extracapsular protoplasm. 5. A small colony of Collozoum inerme, magnified 25 diameters. a, alveoli (vacuoles) of the extracapsular protoplasm; b, central capsules, each containing besides protoplasm a large oil-globule. 6-13. Yellow cells of various Radiolaria: 6, normal yellow cell; 7. 8, division with formation of transverse septum; 9, a modified condition according to Brandt; 10, division of a yellow cell into four; 11, amoeboid condition of a yellow cell from the body of a dead Sphaerozoon; 12, a similar cell in process of division; 13, a yellow cell the protoplasm of which is creeping out of its cellulose envelope. 14. Heliosphaera inermis, Haeck., living example; a, nucleus; b, central capsule; c, siliceous basket-work skeleton. 15. Two swarm-spores (flagellulae) of Collozoum inerme, set free from such a central capsule as that drawn in 4; each contains a crystal b and a nucleus a. 16. Two swarm-spores of Collozoum inerme, of the second kind, viz. devoid of crystals, and of two sizes, a macrospore and a microspore. They have been set free from central capsules with contents of a different appearance from that drawn in 4. a, nucleus. 17. Actinomma asterocanthion, Haeck.; one of the Peripylaea. Entire animal in optical section. a, nucleus; b, wall of the central capsule; c, innermost siliceous shell enclosed in the nucleus; c1, middle shell lying within the central capsule; c2, outer shell lying in the extracapsular protoplasm. Four radial siliceous spines holding the three spherical shells together are seen. The radial fibrillation of the protoplasm and the fine extracapsular pseudopodia are to be noted. 18. Amphilonche messanensis, Haeck.; one of the Acanthometridea. Entire animal as seen living.
Fig. IV.—Radiolaria. 1. Lithocircus annularis, Hertwig; one of the Monopylaea. Whole animal in the living state (optical section); a, nucleus; b, wall of the central capsule; c, yellow cells; d, perforated area of the central capsule (Monopylaea). 2. Cystidium inerme, Hertwig; one of the Monopylaea. Living animal. An example of a Monopylaeon destitute of skeleton. a, nucleus; b, capsule-wall; c, yellow cells in the extracapsular protoplasm. 3. Carpocanium diadema, Haeck.; optical section of the beehive-shaped shell to show the form and position of the protoplasmic body. a, the tri-lobed nucleus; b, the siliceous shell; c, oil-globules; d, the perforate area (pore-plate) of the central capsule. 4. Coelodendrum gracillimum, Haeck.; living animal, complete; one of the Tripylaea. a, the characteristic dark pigment (phaeodium) surrounding the central capsule b. The peculiar branched siliceous skeleton, consisting of hollow fibres, and the expanded pseudopodia are seen. 5. Central capsule of one of the Tripylaea, isolated, showing a, the nucleus; b, c, the inner and the outer laminae of the capsule wall; d, the chief or polar aperture; e, e, the two secondary apertures. 6, 7. Acanthometra claparedei, Haeck. 7 shows the animal in optical section, so as to exhibit the characteristic meeting of the spines at the central point as in all Acanthometridea; 6 shows the transition from the uninuclear to the multinuclear condition by the breaking up of the large nucleus. a, small nuclei; b, large fragments of the single nucleus; c, wall of the central capsule; d, extracapsular jelly (not protoplasm); e, peculiar intracapsular yellow cells. 8. Spongosphaera streptacantha, Haeck.; one of the Peripylaea. Siliceous skeleton not quite completely drawn on the right side. a, the spherical extracapsular shell (compare fig. III. 17), supporting very large radial spines which are connected by a spongy network of siliceous fibres. 9. Aulosphaera elegantissima, Haeck.; one of the Phaeodaria. Half of the spherical siliceous skeleton.
The following passages may be repeated here from Sir E. Ray Lankester's article “Protozoa” in the 9th edition of this Encyclopaedia:—
“The important differences in the structure of the central capsule of different Radiolaria were first shown by Hertwig, who also discovered that the spines of the Acanthometridea consist not of silica but of an organic compound (but see above). In view of this latter fact and of the peculiar numerical and architectural features of the Acanthometrid skeleton, it seems proper to separate them altogether from the other Radiolaria. The Peripylaea maybe regarded as the starting-point of the Radiolarian pedigree, and have given rise on the one hand to the Acanthometridea, which retain the archaic structure of the central capsule whilst developing a peculiar skeleton, and on the other hand to the Monopylaea and Phaeodaria, which have modified the capsule but retained the siliceous skeleton.
“The occasional total absence of any siliceous or acanthinous skeleton does not appear to be a matter of classificatory importance, since skeletal elements occur in close allies of those very few forms which are totally devoid of skeleton. Similarly it does not appear to be a matter of great significance that some forms (Polycyttaria) form colonies, instead of the central capsules separating from one another after fission has occurred.
“It is important to note that the skeleton of silex or acanthin does not correspond to the shell of other Sarcodina, which appears rather to be represented by the membranous central capsule. The skeleton does, however, appear to correspond to the spicules of Heliozoa, and there is an undeniable affinity between such a form as Clathrulina and the Sphaerid Peripylaea (such as Heliosphaera, fig. III. 14). The Radiolaria are, however, a very strongly marked group, definitely separated from all other Sarcodina by the membranous central capsule sunk in their protoplasm. Their differences inter se do not affect their essential structure. The variations in the chemical composition of the skeleton and in the perforation of the capsule do not appear superficially. The most obvious features in which they differ from one another relate to the form and complexity of the skeleton, a part of the organism so little characteristic of the group that it may be wanting altogether. It is not known how far the form-species and form-genera which have been distinguished in such profusion by Haeckel as the result of a study of the skeletons are permanent (i.e. relatively permanent) physiological species. There is no doubt that very many are local and conditional varieties, or even merely stages of growth, of a single Protean species. The same remark applies to the species discriminated among the shell-bearing Reticularia. It must not be supposed, however, that less importance is to be attached to the distinguishing and recording of such forms because we are not able to assert that they are permanent species.
“The streaming of the granules of the protoplasm has been observed in the pseudopodia of Radiolaria as in those of Heliozoa and Reticularia; it has also been seen in the deeper protoplasm; and granules have been definitely seen to pass through the pores of the central capsule from the intracapsular to the extracapsular protoplasm. A feeble vibrating movement of the pseudopodia has been occasionally noticed.
“The production of swarm-spores has been observed only in Acanthometra and in the Polycyttaria and Thalassicollidae, and only in the two latter groups have any detailed observations been made. Two distinct processes of swarm-spore production have been observed by Cienkowski, confirmed by Hertwig,—distinguished by the character of the resulting spores, which are called ‘crystalligerous’ and ‘isospores’ (fig. III. 15) in the one case, and ‘dimorphous’ or ‘anisospores’ in the other (fig. III. 16). In both processes the nucleated protoplasm within the central capsule breaks up by a more or less regular cell-division into small pieces, the details of the process differing a little in the two cases. In those individuals which produce crystalligerous swarm-spores, each spore encloses a small crystal (fig. III. 15). On the other hand, in those individuals which produce dimorphous swarm-spores, the contents of the capsule (which in both instances are set free by its natural rupture) are seen to consist of individuals of two sizes, ‘megaspores’ and ‘microspores,’ neither of which contain crystals (fig. III. 16). The further development of the spores has not been observed in either case. Both processes have been observed in the same species, and it is suggested that there is an alternation of sexual and asexual generations, the crystalligerous spores developing directly into adults, which in their turn produce in their central capsules dimorphous swarm-spores (megaspores and microspores), which in a manner analogous to that observed in the Volvocinean Flagellata copulate (permanently fuse) with one another (the larger with the smaller) before proceeding to develop. The adults resulting from this process would, it is suggested, produce in their turn crystalligerous swarm-spores. Unfortunately we have no observations to support this hypothetical scheme of a life-history.
“Fusion or conjugation of adult Radiolaria, whether preliminary to swarm-spore-production or independently of it, has not been observed—this affording a distinction between them and Heliozoa.
“Simple fission of the central capsule of adult individuals, preceded of course by nuclear fission, and subsequently of the whole protoplasmic mass, has been observed in several genera of Acantharia and Phaeodaria, and is probably a general method of reproduction in the group. In Spumellaria it gives rise to colonial ‘Polycyttarian’ forms when the extra capsular protoplasm does not divide.
“The siliceous shells of the Radiolaria are found abundantly in certain rocks from Palaeozoic times onwards. They furnish, together with Diatoms and Sponge spicules, the silica which has been segregated as flint in the Chalk formation. They are present in quantity (as much as 10%) in the Atlantic ooze, and in the celebrated ‘Barbados earth’ (a Tertiary deposit) are the chief components.”
Bibliography.—The most important systematic works are those of E. Haeckel, Die Radiolarien (1862-87), and the “Report” on the Radiolaria of the “Challenger” Expedition (vol. xviii., 1887), which contains full lists of the older literature. Among the most important recent studies we cite K. Brandt, “Die Koloniebildenden Radiolarien” in Fauna and Flora des Golfes von Neapel, xii. (1885); A. Borgert in Zeitschrift f. Wissenschaftliche Zoologie, li. (1891), and Zoologische Jahrbücher (Anatomie), xiii. (1900); F. Dreyer in Jenäischer Zeitschr., xix. (1892); V. Häcker in Zeitsch. f. Wiss. Zool., lxxxiii. (1905).
- (M. Ha.)