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.
Chapter IV.—THE SKELETON.
(§§ 101-140).
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]).