C. Perhaps the phæodellæ are to some extent symbiontes with the Phæodaria; the xanthellæ present in most other Radiolaria are absent in this legion.
90. The Extracapsular Xanthellæ.—Xanthellæ or Zooxanthellæ, symbiotic "yellow cells," are very commonly found in the extracapsulum of the Radiolaria, especially in many Spumellaria and Nassellaria; whilst in the Acantharia similar yellow cells usually only occur within the central capsule, and in the Phæodaria their presence has not been certainly demonstrated. The extracapsular Xanthellæ are found most abundantly in the Collodaria, both in the monozootic Thalassicollida and in the polyzootic Sphærozoida. They occur in smaller numbers in the Sphærellaria, and in many divisions of the latter they seem to be entirely absent. Also it sometimes happens that, though present in large numbers in some Spumellaria, they are entirely absent in others nearly related to them; indeed, this has also been observed in the case of different individuals of the same species. This fact alone is sufficient to show that the Xanthellæ are not an integral part of the Radiolarian organism (as was formerly believed) but parasites or more correctly symbiontes, which live as inhabitants of the calymma. More recent investigations have shown, that besides the yellow pigment-grains they contain starch or an amyloid substance, that is to say, vegetable reserve materials, that their thin envelope contains cellulose, and that their yellow colouring-matter resembles chlorophyll and is related to that of the Diatomaceæ ("Diatomin"). Hence they are now generally regarded as unicellular Algæ, nearly related to those which occur as symbiontes in other marine animals (Exuviella, &c.). The starch, which they develop with the formation of oxygen, may serve as nutriment to the Radiolaria, while the carbonic acid yielded by the latter is also beneficial to the Xanthellæ. The form of the Xanthellæ is usually spherical and elliptical, often also sphæroidal or discoidal. Their diameter is usually between 0.008 and 0.012 mm., rarely more or less. The differences exhibited by Xanthellæ which live in different groups of Radiolaria demand further investigation, which will perhaps lead to the establishment of several species of the genus Zooxanthella. At present Zooxanthella extracapsularis, in the calymma of Spumellaria and Nassellaria, may be clearly distinguished from Zooxanthella intracapsularis, in the central capsule of the Acantharia.
The "yellow cells" were first described in 1851 by Huxley, in the Collodaria, and afterwards by J. Müller (1858) in many Spumellaria and Nassellaria. In my Monograph (1862, pp. 84-87) I gave a detailed account of their structure and increase by division, and laid special emphasis on the fact that they are the only elements in the Radiolarian organism which "are undoubtedly cells in the strict histological sense of the word." Afterwards, in my Beiträge zur Plastiden-Theorie, I showed the constant presence of "starch in the yellow cells of the Radiolaria" (1870, L. N. [21]). Shortly afterwards Cienkowski observed that the yellow cells live independently and reproduce themselves after the death of the Radiolaria, and in consequence first put forth the hypothesis that they do not belong to the Radiolarian organism, but that they are unicellular Algæ parasitic upon it (1871, L. N. [22]). This view was ten years later more fully established by Karl Brandt, and elucidated by comparison with the symbiosis of the gonidia of Algæ, and the hyphæ of Fungi in the formation of Lichens, which had in the meantime become known (1881, L. N. [38]). Brandt gave this unicellular yellow Alga the name Zooxanthella nutricola, and afterwards gave fuller details regarding its remarkable vital relations (L. N. [39]). Patrick Geddes, who named it Philozoon, supplemented this account and showed experimentally that it gives off oxygen under the influence of sun-light (1882, L. N. [42], [43]). In consequence of this there is no doubt that all Xanthellæ (the Zooxanthella extracapsularis of Spumellaria and Nassellaria, and the Zooxanthella intracapsularis of the Acantharia, and possibly also the Zooxanthella phæodaris of the Phæodaria) do not originally belong to the Radiolarian organism, as was believed up to the time of Cienkowski, but penetrate actively into it from without, or are taken in passively by means of the pseudopodia. In any case their symbiosis, when they are associated with the Radiolarian cell in large numbers, may be of great advantage to both parties, since the metastasis of the Xanthella is vegetable, that of the Radiolarian animal in character. In any case their symbiosis is to a large extent accidental, by no means as necessary as in the case of the Lichens. See on these points in addition to Brandt and Geddes (loc. cit.) also Geza Enz, Das Consortial-Verhältniss von Algen und Thieren, Biol. Centralbl., Bd. ii. No. 15, 1883, Oskar Hertwig, Die Symbiose oder das Genossenschaftsleben im Thierreich, Jena, 1883, and Bütschli, Die Radiolarien, in Bronn's Klass. u. Ord. d. Thierreichs, 1882 (L. N. [41], pp. 456-462).
91. The Exoplasm or Extracapsular Protoplasm.—The extracapsular protoplasm, which may be shortly termed the "exoplasm" (or ectosarc), is primitively in all Radiolaria (and especially in their earliest development stages) the only important constituent of the extracapsulum, besides the calymma. Although the extracapsular and intracapsular protoplasm of the Radiolaria are everywhere in direct communication, and although the openings in the membrane of the central capsule bring about an interchange between them, still the two portions of sarcode show certain constant and characteristic differences, which are due to the physiological division of labour between the central and peripheral parts of the body and their corresponding morphological differentiation. The extracapsular, like the intracapsular, protoplasm is originally homogeneous, but may afterwards become differentiated in various ways, producing the special constituents of the extracapsulum. Such "external protoplasmic products" are vacuoles, pigment-bodies, &c. More important, however, are the topographically different sections into which the exoplasm may be divided according to its relations to the central capsule and the calymma. In this respect the following parts may be generally distinguished—(1) the Sarcomatrix, or fundamental layer of the exoplasm, which surrounds the central capsule as a continuous sheath of sarcode and separates it from the calymma; (2) the Sarcoplegma, an irregular network of the exoplasm, which spreads throughout the gelatinous material of the calymma; (3) the Sarcodictyum or network of sarcode on the outer surface of the calymma; and (4) the Pseudopodia, which project outwards from the latter and radiate into the water.
92. The Sarcomatrix.—The sarcomatrix, being "the fundamental layer of the pseudopodia" (or "matrix of the exoplasm"), constitutes the proximal innermost section of the extracapsular sarcode, and in all Radiolaria forms a thin continuous mucous layer, which covers the whole outer surface of the central capsule and separates it from the surrounding calymma (see note A, below). The sarcomatrix communicates internally through the openings of the central capsule with the endoplasm, whilst externally the pseudopodia or mucous threads arise from it, which by their union form the sarcoplegma. The sarcomatrix is only interrupted in the Spumellaria and Acantharia by those parts of the skeleton which perforate the membrane of the central capsule. In all Nassellaria and Phæodaria, as in the Collodaria, it appears as a perfectly continuous sarcode-envelope of the central capsule. Its thickness is variable; in general it is most strongly developed in the Spumellaria and Phæodaria, less so in the Nassellaria, and is thinnest in the Acantharia. The thickness seems, however, to vary even in one and the same individual, the difference depending partly upon the different stages of development and partly upon nutritional conditions. After abundant inception of nutriment the thin protoplasmic layer of the matrix is thickened and turbid, rich in granules and irregular masses, which are probably due to enclosed but only half-digested food; xanthellæ also, as well as foreign bodies taken up with the nutriment, such as frustules of Diatoms and shells of smaller Radiolaria, and of pelagic infusoria, larvæ, &c., are often, especially in large individuals, aggregated in considerable quantities in the matrix. After long fasting, on the contrary, this is poor in these enclosed bodies and in granules; it then forms a thin colourless more or less hyaline mucous coating to the central capsule. From a physiological standpoint the sarcomatrix is to be regarded as the central organ of the extracapsulum, and as of pre-eminent significance. Probably it is not only the most important organ for the nutrition of the Radiolaria (especially for digestion and assimilation in particular), but perhaps is also the central organ of perception. On the other hand the sarcomatrix belongs to those components of the Radiolarian organism which take no part in the formation of the skeleton.
A. The sarcomatrix was first described in my Monograph in 1862 (p. 110) as the "Mutterboden der Pseudopodien," possessing a pre-eminent physiological importance. Compare also my paper on the sarcode elements of the Rhizopoda (Zeitschr. f. wiss. Zool., Bd. xv. p. 342, 1865).
93. The Sarcoplegma.—By the name sarcoplegma, as distinguished from the remaining extracapsular sarcode, is understood the intracalymmar web of exoplasm or "ectosarcode network," which ramifies within the gelatinous mass of the calymma. Internally it is in direct connection with the continuous sheath (sarcomatrix), which encloses the central capsule, whilst externally it is in contact with the superficial sarcode network (sarcodictyum) which surrounds the calymma. The configuration of this exoplasmic web, which penetrates the jelly-veil in all directions, is exceedingly variable; in most Radiolaria it is extremely irregular in form, like the protoplasmic network in the ground-substance of many kinds of connective tissue. In some groups, however, it assumes a rather regular shape which it appears to retain (e.g., in many Acantharia). It must be assumed also that in those instances where the consistency of the calymma approaches that of cartilage, the tracks of the exoplasmic threads remain constant, but accurate observations are wanting as to how far the configuration of the sarcoplegma is constant or variable in the different groups, as well as regarding its peculiar behaviour in those Radiolaria whose calymma is characterised by the formation of vacuoles or alveoles (see § [86]). Usually it envelops the larger alveoles in the form of a reticulate veil. In many Collodaria the exoplasm is aggregated at certain points of the intracalymmar web, so that large balls or amœboid bodies appear to be distributed between the alveoles, e.g., in Thalassophysa pelagica and Thalassicolla melacapsa (Pl. [1], figs. 4, 5). The sarcoplegma is metamorphosed directly into silex in the Radiolaria spongiosa, or those genera which possess a spongy cortical skeleton, and were formerly known as Spongurida; to this category belong the Spongosphærida (Pl. [18]) and Spongodiscida (Pl. [47]) as well as certain Nassellaria and Phæodaria. The single siliceous spicules, which are irregularly interwoven to form the spongy web, are to be regarded as the silicified threads of the intracalymmar sarcode network. From a physiological point of view the sarcoplegma is of importance both for the nutrition and motion of the Radiolaria, since it brings the sarcomatrix and the sarcodictyum, with the pseudopodia which radiate from it, into direct communication.
94. The Sarcodictyum.—The sarcodictyum may be defined as the extracalymmar network of exoplasm, and is a reticular covering which lies upon the outer surface of the gelatinous calymma. Internally, the sarcodictyum is in direct communication with the sarcoplegma, or the web of exoplasmic threads which ramifies in the gelatinous substance of the calymma; externally, on the other hand, the pseudopodia radiate freely from it; thus its relation to these is similar to that which the sarcomatrix bears to the roots of the sarcoplegma. Relations similar to those which have led to the separation of the primary from the secondary calymma, induce us to distinguish also a primary and secondary sarcodictyum. The original or primary sarcodictyum ramifies over the surface of the original or primary calymma, and like this is of pre-eminent importance in the formation of the primary lattice-shell; if we regard the surface of the primary calymma as the indispensable foundation for the deposition of this latter, then the primary sarcodictyum furnishes the material from which it is developed: silex in the Spumellaria and Nassellaria, a silicate of carbon in the Phæodaria, and acanthin in the Acantharia. It may indeed be said that the primary lattice-shell of the Radiolaria arises by a direct chemical metamorphosis of the primary sarcodictyum, by a chemical precipitation of the dissolved skeletal material (silex, silicate, or acanthin), which was stored up in the exoplasm of the sarcodictyum. Hence a deduction from the special conformation of the former to that of the latter is permissible. The particular form of the primary lattice-sphere with its regular or irregular meshes is due to the corresponding form of the primary sarcodictyum; both regular and irregular forms of this commonly occurring. The form of the regular sarcodictyum with circular or regular polygonal, usually hexagonal, meshes is constantly maintained during the formation of the regular lattice-shells (e.g., Pl. [12], figs. 5-10; Pl. [52], figs. 8-20; Pl. [96], figs. 2-6; Pl. [113], figs. 1-6). The form of the irregular sarcodictyum, on the other hand, with irregular polygonal or roundish meshes, persists during the development of the irregular lattice-shells (e.g., Pls. [29], [70], [97], [106]). All this is true also of the secondary sarcodictyum, or the exoplasmic network which ramifies over the surface of the secondary calymma. The secondary lattice-shells, which are deposited on the surface of the latter, retain the configuration of the secondary sarcodictyum, by the chemical metamorphosis of which they have originated; this is the case in many Spumellaria which develop several concentric lattice-shells (Pl. [29]), in some Nassellaria (Pl. [54], fig. 5), in the Phractopeltida among the Acantharia (Pl. [133]), and in the double-shelled Phæodaria, Cannosphærida, and part of the Cœlodendrida and Cœlographida (Pls. [112], [121], [128]). In those Radiolaria which form no lattice-shell whatever, the conformation of the sarcodictyum is usually irregular, with meshes of irregular form and unequal size; sometimes, however, they seem to be very regular, as in many Acanthometra (Pl. [129], fig. 4).
95. The Pseudopodia.—On the whole the pseudopodia or thread-like processes of the exoplasm exhibit in the Radiolaria the same characteristic peculiarities as in all true Rhizopoda; they are usually very numerous, long and thin, flexible and sensitive filaments of sarcode, which show the peculiar phenomena of granular movement. Their physiological significance is in several respects very great, for they serve as active organs for the inception of nutriment, for locomotion, sensation, and the formation of the skeleton (see note A, below). The presence of a calymma, however, which distinguishes the Radiolaria from the other Rhizopoda, brings about certain modifications in the behaviour of the pseudopodia. If in general all the threads, which arise from the sarcomatrix or fundamental layer and radiate outwards, be called "pseudopodia," then that part of them which is included in the gelatinous substance of the calymma and forms the sarcoplegma may be termed the "collopodia" (or intracalymmar pseudopodia), and the remaining portion, which passes outwards from the sarcodictyum freely into the water, may be described as "astropodia" (or extracalymmar pseudopodia). In many Radiolaria these two portions present some differences in morphological and physiological respects, and certain distinctions are probably generally present (see note B). Apart from this universal differentiation in the different groups of the Radiolaria, specially modified forms of pseudopodia may be recognised as the axopodia and myxopodia of the Acantharia (see § [95, A]), and the sarcode-flagellum of certain Spumellaria (see note C).
A. The pseudopodia of the Radiolaria have been so fully described in my Monograph, in 1862, both morphologically and physiologically, that I need only refer to the account there given (pp. 89-127); for supplementary observations see R. Hertwig (1879, L. N. [33], p. 117) and Bütschli (1882, L. N. [41], pp. 437-445).