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).
B. The Astropodia, or free radiating pseudopodia, are in many Radiolaria more or less clearly distinguishable from the collopodia, which form the sarcoplegma within the calymma; how far these distinctions depend upon a permanent differentiation (especially in the Acantharia and Phæodaria) needs further investigation.
C. The sarcode-flagellum (perhaps better termed axoflagellum) was first described in my Monograph (1862, p. 115) in the case of various Discoidea (Taf. xxviii. figs. 5, 8; Taf. xxx. fig. 1). Hertwig has given a substantially similar account of the organ in some other Discoidea (L. N. 33, p. 67, Taf. vi. figs. 10, 11); probably this peculiar structure is confined to the order Discoidea among the Spumellaria, but is widely distributed within its limits. The axoflagellum is a thick cylindrical thread of sarcode, finely striated and pointed towards its free end. It always lies in the equatorial plane of the discoidal body, and always unpaired in one of its axes; in the triradiate Discoidea it is in the axis of the unpaired principal arm and opposite to it (Pl. 43, fig. 15). In the Ommatodiscida (p. 500, Pl. 48, figs. 8, 19, 20) the axoflagellum probably passes out through the peculiar marginal ostium of the shell. Perhaps it is always connected with the central nucleus by intracapsular axial fibres, and is to be regarded as a specially differentiated bundle of pseudopodia (or axopodia?).
95A. The Myxopodia and Axopodia.—The two forms of pseudopodia which we distinguish as myxopodia and axopodia differ markedly from each other both morphologically and physiologically. The myxopodia, or ordinary free pseudopodia, which are found in large numbers in all Radiolaria, and constitute their most important peripheral organs, are simple homogeneous exoplasmic threads, which arise from the sarcodictyum or extracalymmar sarcode network, and radiate freely into the water; here they may branch and combine by anastomosis to form a changeable network, but they never contain an axial thread. The axopodia, on the other hand, are differentiated pseudopodia, which consist of a firm radial thread, and a soft covering of exoplasm; they penetrate the whole calymma in a radial direction and project freely from its surface, and generally (if not always) they are produced inwards to the middle of the central capsule, perforating its membrane; their proximal end is lost in a dark central heap of granules. Such axopodia are at present known with certainty only in the Acantharia, where they are widely, and perhaps universally, distributed. Their development in this legion probably stands in direct causal relation to the peculiar structure of the central capsule and the centrogenous formation of the skeleton. Since the radial skeletal rods of the Acanthometra possess originally a thin coating of protoplasm, it may be said that the centrogenous axopodia of this group became differentiated in two ways, the firm axial threads of one section remaining very thin and covered by protoplasm, whilst those of the other section became metamorphosed into radial bars of acanthin. This hypothesis acquires more probability from the regular distribution and arrangement of the axopodia in the Acantharia; they usually stand at fixed intervals between the radial bars, singly or in groups; sometimes their number seems to be not greater than that of the bars, whilst in other cases a circlet or group of axopodia corresponds to each radial bar. Perhaps their fine axial thread consists of acanthin. At all events the axopodia are constant organs (probably sensory, like the "palpocils") and not retractile like the movable myxopodia.
The axial threads in the pseudopodia of the Acanthometra were first discovered by R. Hertwig, who accurately described their peculiar structure and arrangement (L. N. 33, pp. 16, 117).
96. The Myophriscs of the Acanthometra.—The Acanthometra are characterised by a very peculiar differentiation of the exoplasm, namely, by the formation of myophriscs or contractile threads from the sarcodictyum. In most (and perhaps in all) Acantharia of this order each radial bar is surrounded by a circlet of such contractile threads, which was first described as a "ciliary corona" (see note A, below). The number of contractile threads in each circlet usually amounts to from ten to twenty, rarely being more than thirty and less than eight; it often appears to be constant in the individual species (see note B). In the living state the myophriscs are long, thin filaments, the pointed distal end of which is inserted into the radial bar, whilst the thicker proximal end is attached to the surface of the calymma, which is elevated round the base of each rod into the form of a gelatinous cone or skeletal sheath (see note C). Probably the myophriscs lie on the outer surface of the apical portion of this gelatinous cone, and are hence to be regarded as exoplasmic threads differentiated from the sarcodictyum. Sometimes, however (as in Acanthochiasma), they fuse into a contractile membrane and form the envelope of a cone, whose interior is occupied by a gelatinous papilla of the calymma. On mechanical irritation the myophriscs contract rapidly and suddenly, like muscle-fibrillæ, becoming at the same time thicker, and hence are very different from pseudopodia. Their distal point of insertion being fixed to the firm acanthin rod, they raise by their contraction the skeletal sheath, to which their bases are attached or in the surface of which they lie. The result of their contraction is therefore a distention and increase in volume of the calymma, with which is no doubt connected an inception of water into the gelatinous mass, and hence a diminution in its specific gravity. Probably the Acanthometra contract their myophriscs voluntarily when they wish to rise in the water; when these relax the calymma collapses owing to its elasticity, water is then expelled and the specific gravity increases. From a physiological point of view, then, the myophriscs are to be regarded as a hydrostatic apparatus, morphologically as myophanes or muscular fibrillæ, such as also occur in the intracapsular protoplasm (see §§ 77-80). On more violent irritation and after the death of the Acanthometra the myophriscs separate from the radial bars and remain attached to the distal ends of the conical gelatinous sheaths as free "ciliary coronas." At the same time, they melt into short, thick, hyaline rods, the so-called "gelatinous cilia." The myophriscs are found only in the order Acanthometra, and are wanting in the Acanthophracta, as well as in the other three legions of Radiolaria.
A. The "ciliary coronas" on the skeletal rods of dead Acanthometra were first described by the discoverer of this order, Johannes Müller, and referred to as "the stumps of the contracted, thickened threads" (L. N. 12, p. 11, Taf. xi.).
B. The "number of the gelatinous cilia" I found constant in certain species of Acanthometra, and stated in my Monograph (L. N. 16, p. 115) "that here is to be found the first differentiation of the diffuse sarcode into definite organs of regular definite number, size, and position, which deserve the name tentacles rather than pseudopodia."
C. The nature of the myophriscs as fibrillæ allied to muscles was first discovered by R. Hertwig, who described them as "structures of peculiar nature," under the name of "contractile threads," and pointed out in detail their histological and physiological peculiarities (L. N. 33, pp. 16-19, Taf. i.).
97. The Exoplasm of the Peripylea.—The extracapsular protoplasm of the Spumellaria or Peripylea is in communication with the intracapsular sarcode by the innumerable fine pores of the capsule-membrane, and like these pores is evenly distributed over the whole surface. The sarcomatrix which immediately surrounds the central capsule is moderately strong, and sends out innumerable long, thin pseudopodia, which probably correspond to the pores of the membrane. Their number is markedly greater in the Spumellaria than in the other three legions. The ramifications and communications which the radiating fibres of the sarcomatrix undergo within the calymma, apparently present the most manifold variations, so that the sarcoplegma or intracalymmar network thus formed has very diverse forms. On the surface of the calymma the exoplasmic threads constitute a variously disposed sarcodictyum, a regular or irregular exoplasmic network, by the silicification of which a primary lattice-shell arises in the majority of the Spumellaria. The free ends of the pseudopodia, which arise from this extracalymmar network and radiate out into the water, appear in most Spumellaria to be relatively short, but exceedingly numerous. Specially modified pseudopodia and axial threads in particular do not seem to occur in this legion. Perhaps, however, among the latter may be reckoned the remarkable pseudopodia which combine to form the sarcode flagellum in many Discoidea (and perhaps in other Spumellaria). This axoflagellum is a particularly strong thread of sarcode, arising from a definite point in the central capsule; it is cylindrical or slenderly conical in form, much longer, stronger, and more contractile than the ordinary pseudopodia; it contracts in a serpentine fashion on mechanical irritation and seems to originate by the fusion of a bundle of pseudopodia (compare § 95, C).
98. The Exoplasm of the Actipylea.—The extracapsular protoplasm of the Acantharia or Actipylea differs in several important respects from that of other Radiolaria, and appears to undergo more significant differentiations than that of the three other legions. Since the pores in the wall of the central capsule are not distributed evenly and at equal intervals over its whole surface (as in the Peripylea), but rather exhibit a regular disposition in groups at unequal intervals, the number of projecting pseudopodia is much less and the law of their arrangement different from that which obtains in the Peripylea (§ 58). In many and probably in all Acantharia they are divided into two groups, those which arise from the centre of the capsule and possess firm axial threads, and those which have not these characters (compare § 95, A). The axopodia, or stiff pseudopodia with axial threads, arise from the centre of the capsule, are present in much smaller numbers than the soft and flexible myxopodia, and are regularly disposed between the radial bars of acanthin, usually so that they are as far removed from them as possible, i.e., in the centre between each three or four bars; these latter may indeed be regarded as strongly developed axial threads, which have become changed into acanthin (§ 95, A). The soft myxopodia, or pseudopodia without axial threads, are much more numerous than the others, and arise from the sarcodictyum or exoplasmic network which ramifies over the surface of the calymma. Their number and arrangement seem, however, in many (if not in all) Acantharia to be regular and not to possess the extraordinary variability seen in the other three legions. In many Acanthometra the sarcodictyum exhibits a symmetrical conformation, with regular or subregular, polygonal (mostly hexagonal) meshes, and generally the stronger threads of the sarcodictyum secrete a firm, homogeneous or fibrillar, striated substance, which forms a network of ridges on the surface of the calymma. In the Acanthophracta the place of this is taken by the acanthin network of the primary lattice-shell. The axopodia of the Acanthometra are usually about as long as the radial spines between which they stand; their stiff axial thread is surrounded by a soft sheath of protoplasm, communicating with the thin sarcomatrix which surrounds the central capsule. Numerous branches pass into the calymma from the exoplasmic sheath of the axial threads, and form by their interweaving a loose sarcoplegma. The most peculiar differentiated products of the exoplasm of the Acantharia, however, are the myophane fibrillæ of the Acanthometra, which have already been described under the name of myophriscs (§ 96).
99. The Exoplasm of the Monopylea.—The extracapsular protoplasm of the Nassellaria or Monopylea arises only from the porochora, or the intracapsular podoconus, the oral base of which is formed by this porous area. The pseudopodia or protoplasmic threads which pass through the pores of the latter, united into a bundle, are not very numerous (in most Nassellaria probably between thirty and ninety), and unite just outside it to form a thick discoid sarcomatrix; this covers the porochora completely below, and spreads out in the form of a thin envelope of exoplasm over the whole surface of the central capsule; at the apical portion of the latter the sarcomatrix is often so thin that it can only be recognised by the aid of reagents; it separates the membrane of the central capsule from the surrounding calymma. The pseudopodia, which penetrate the latter and by loose anastomoses from a wide-meshed sarcoplegma within it, are usually not very numerous. The greater part of them radiate in a bunch downwards from the basal disc of the sarcomatrix, and a smaller number arise from the thinner envelope which covers the remainder of the central capsule (Pl. 51, fig. 13; Pl. 65, fig. 1; Pl. 81, fig. 16). On the outer surface of the calymma the collopodia, which have passed through it, unite to form the sarcodictyum, and through the silicification of this the primary lattice-shell arises in the great majority of the Nassellaria. From the surface of the sarcodictyum arise the astropodia, or free pseudopodia which radiate outwards into the water. Their number in most Monopylea is relatively small, but their length appears to be very great.
100. The Exoplasm of the Cannopylea.—The extracapsular protoplasm of the Phæodaria or Cannopylea is much better developed as regards volume than in the other three legions, and is connected with the intracapsular sarcode by only a few apertures in the capsule-membrane. In most Phæodaria three of these are present, the astropyle or main-opening at the oral pole of the main axis, and the two lateral parapylæ or accessory openings on either side of the aboral pole (§ 60). In several families the latter appear to be wanting, whilst in others their number is increased; these families have not yet, however, been observed during life. The protoplasm projects both from the oral main-opening and from the two aboral accessory openings in the form of a thick cylindrical rod; the tube into which each opening is produced in many Phæodaria (longer in the case of the astropyle, shorter in the parapylæ) being regarded as an excretion from this protoplasmic cylinder. The sarcode threads within the tube appear like a bundle of fibrils, either quite hyaline or finely striated. After issuing from the mouth of the aperture they pass over into a thick sarcomatrix, which surrounds the central capsule entirely and separates it from the enclosing calymma. In the neighbourhood of the basal astropyle the sarcomatrix is usually swollen into a thick lenticular disc, which is in direct contact with the peculiar phæodium of this legion (§ 89). The pseudopodia, which radiate from the sarcomatrix, and form by anastomosis a wide-meshed sarcoplegma within the calymma, are usually not very numerous in the Phæodaria, but are very strong. Sometimes two stronger bundles of collopodia may be distinguished at the two poles of the main axis, an oral bundle (in the direction of the proboscis of the astropyle) and an aboral bundle (at the opposite pole between the parapylæ). The collopodia of the sarcoplegma unite at the surface of the calymma into a regular or irregular sarcodictyum, which, in most Phæodaria produces by the secretion of a peculiar silicate the primary lattice-shell. The free astropodia, which pass outwards from the sarcodictyum into the water, are in most Phæodaria very numerous (Pl. 101, fig. 10). Since, however, only a few species of this great legion have been observed in a living state, their pseudopodia require further accurate examination.
101. The Significance of the Skeleton.—The skeleton of the Radiolaria is developed in such exceedingly manifold and various shapes, and exhibits at the same time such wonderful regularity and delicacy in its adjustments, that in both these respects the present group of Protista excels all other classes of the organic world. For, in spite of the fact that the Radiolarian organism always remains merely a single cell, it shows the potentiality of the highest complexity to which the process of skeleton formation can be brought by a single cell. All that has been brought to pass in this direction by single tissue-cells of animals and plants does not attain the extremely high stage of development of the Radiolaria. Only very few Rhizopoda of this very rich and varied class fail to exhibit the power of forming this firm supporting and protecting organ—indeed, only ten of the seven hundred and thirty-nine genera which are enrolled in the list of the Challenger collection, namely, six genera of Spumellaria (five Thalassicollida, Actissa, Thalassolampe, Thalassopila, Thalassicolla, Thalassophysa, Pl. 1, and one genus of Collozoida, Collozoum, Pl. 3), and in addition two genera of Nassellaria (the Nassellida, Cystidium and Nassella, Pl. 91, fig. 1), and two genera of Phæodaria (the Phæodinida, Phæocolla and Phæodina, Pl. 101, figs. 1, 2). These skeletonless forms of Radiolaria are, however, of extreme interest, since they include the original stem-forms of the whole class as well as of its four legions. All Radiolaria which form skeletons have originated from soft and skeletonless stem-forms by adaptation, and that polyphyletically, for the skeletal types of the four legions have been developed independently of each other (§ 108).
102. The Chemical Peculiarities of the Skeleton.—The chemical composition of the skeleton shows very marked variations in the different legions of the Radiolaria. The two legions Spumellaria and Nassellaria (united formerly as "Polycystina") form their skeleton of pure silica (see note A, below); the legion Phæodaria of a silicate of carbon (see note B), and the Acantharia of a peculiar organic substance—acanthin (see note C). This explains the well-known fact that the deposits of fossil Radiolaria (or Polycystine marls) are composed exclusively of the skeletons of Spumellaria and Nassellaria, those of the Acantharia and Phæodaria being entirely absent (in the case of the last group, however, exception must be made in favour of the Dictyochida, or those Phæodaria whose skeleton is made up of isolated scattered tangential siliceous fragments). The enormous deposits of Radiolarian skeletons in the deep sea of today, which constitute the Radiolarian ooze, consist, like the fossil Polycystine marls, almost exclusively of the shells of Spumellaria and Nassellaria, though here the acanthin skeletons of the Acantharia may be present in very small numbers, and the silicate skeletons of the Phæodaria, which offer more resistance to the solvent action of sea-water, somewhat more abundantly. Calcareous skeletons do not occur in the Radiolaria (see note D).
A. The pure siliceous skeletons of the Polycystina were first recognised in 1833 by Ehrenberg in chalky marls (L. N. 2, p. 117). Since the two legions Acantharia and Phæodaria were entirely unknown to Ehrenberg, his name Polycystina has reference only to the Spumellaria and Nassellaria.
B. The silicate skeleton of the Phæodaria was formerly taken by me for a purely siliceous one. When I described the first Phæodaria in my Monograph in 1862, I was only acquainted with five genera and seven species, whilst the number of Phæodaria here described from the Challenger amounts to eighty-four genera and four hundred and sixty-five species. In the vast majority of these (though not in all) the skeleton becomes more or less intensely stained by carmine, and is also more or less charred at a red heat, in some even becoming of a blackish-brown. In many Phæodaria, furthermore, the hollow skeletal tubes are destroyed by the continued action of heat. They are also, for the most part, strongly acted upon, or even destroyed by boiling caustic alkalis, whilst boiling mineral acids have no effect upon them. The best method of cleaning the skeletons of Phæodaria from their soft parts is to heat them in concentrated sulphuric acid, and then add a drop of fuming nitric acid; in this they are not dissolved even on prolonged heating. From these facts it would appear that the skeletons of the Phæodaria consist of a compound of organic substance and silica, or a "carbonic silicate." The more intimate composition yet remains to be discovered, as also the manifold differences which the various families of Phæodaria seem to show in respect of its composition. The small skeletal fragments of the Dictyochida (the only remains of Phæodaria which occur as fossils) appear to consist of pure silica.
C. The acanthin skeleton of the Acantharia was first described as such in my Monograph (1862, pp. 30-32). Johannes Müller, the discoverer of this legion, took them for siliceous skeletons and defined the Acanthometra as "Radiolaria without lattice-shell, but with siliceous radial spines" (L. N. 12, p. 46). I formerly supposed that the acanthin skeletons in some of the Acantharia were partially or wholly metamorphosed into siliceous skeletons, but, according to the investigations of R. Hertwig, this does not appear to be the case; he showed that the skeletons of the most varied Acanthometra and Acanthophracta are completely dissolved under the longer or shorter action of acids, and supposes that in all Acantharia, without exception, the skeleton is composed of acanthin (1879, L. N. 33, p. 120). Quite recently Brandt has found that the acanthin spines dissolve not only in acids, alkalis, and "liquor conservativus" (as I had shown), but also in solutions of carbonate of soda (1 per cent.), and even of common salt (10 to 20 per cent.); he concludes from this that they consist of an albuminoid substance (vitellin) (L. N. 38, p. 400). I am unable to share this view, for I have never been able to see some of the most important reactions of albumen in any of the skeletons which I have examined, such for example as the xanthoproteic reaction, the red coloration with Millon's test, &c. They do not become yellow either with nitric acid or with iodine. In dilute mineral acids they dissolve more rapidly than in concentrated. My usual method of cleansing the skeleton of Acantharia (which has been practised with the same result on thousands of specimens) consists in heating the preparation in a small volume of concentrated sulphuric acid and then adding a drop of fuming nitric acid; all other constituents (the whole central capsule and the calymma) are thus very rapidly destroyed; the skeleton remains quite uninjured and withstands the combined action of the mineral acids for a longer or shorter time, though on prolonged heating it also is dissolved. I do not therefore regard acanthin as an albuminous substance, but as one related to chitin.
D. Calcareous skeletons have not been certainly demonstrated in the Radiolaria, and probably do not occur. Sir Wyville Thomson in his Atlantic (1877, L. N. 31, vol. i. p. 233, fig. 51) described under the name Calcaromma calcarea, a Radiolarian which contained scattered in its calymma numerous calcareous corpuscles "resembling the rowels of spurs." These are identical with the "toothed bodies, recalling crystal balls," which Johannes Müller figured in the Mediterranean Thalassicolla morum so early as 1858, and compared with the "siliceous asterisks of Tethya" (L. N. 12, p. 28, Taf. vii. figs. 1, 2). I formerly regarded these peculiar calcareous corpuscles, whose solubility in mineral acids I had observed, as spicules of a Thalassicollid, and hence described the species in my Monograph as Thalassosphæra morum (L. N. 16, p. 260). I have, however, seen reason to change my view, and am now led to suppose that those peculiar calcareous corpuscles, which may be named "Calcastrella," are not formed by the Radiolarian itself, but are foreign bodies which have been accidentally incorporated into the calymma of a Thalassicollid (Actissa). These corpuscles occur, often in large numbers, in many preparations in the Challenger collection, and in the calymma of other Radiolaria, chiefly Discoidea, hence it would appear that they are foreign bodies taken up by the pseudopodia and carried into the calymma by the circulation of the sarcode. The Radiolaria which Sir Wyville Thomson figured as Calcaromma calcarea, and Müller as Thalassicolla morum, I regard as species of Actissa (see p. 13), perhaps Actissa radiata of the Pacific, and Actissa primordialis of the Mediterranean (compare the description of the Thalassosphærida of the Challenger collection, pp. 30, 31).