55. The Exometamorphic Forms of the Central Capsule.—The secondary forms of the central capsule, which are brought about by external causes, chiefly dependent on the formation of the skeleton, are very various and in many cases devoid of special interest; in other instances, on the contrary, they are of great importance, because of the clear relation of cause and effect which can be traced between the development of the skeleton and of the capsule. The most important phenomena to be recorded in this connection are as follows:—
I. Spumellaria.—(A) In many of the Sphæroidea, the central capsule of which is originally enclosed by a simple lattice-sphere, it puts out protrusions through the meshes of the shell, thus forming club-shaped processes, corresponding in number with the meshes of the lattice (Pl. [11], figs. 1, 5; Pl. [20], fig. 1a; Pl. [27], fig. 3, &c.). The whole surface of the spherical capsule may thus be covered with numerous independent radial clubs of equal size, but usually they unite again outside the shell to form a simple sphere with smooth surface. (B) In many Prunoidea whose originally ellipsoidal body has become cylindrical by the marked prolongation of the main axis, the central capsule is divided by a series of constrictions into segments, which correspond with the annular constrictions of the skeleton (Pls. [39], [40]). (C) In most Discoidea whose lentiform or discoidal shell develops radial arms at its margin, the central capsule sends out processes into these arms, and adapts itself to the stellate form of the skeleton (p. [409], Pl. [43], fig. 15; Pl. [47], &c.) (D) In many Larcoidea whose growth is originally lentelliptical, but later spiral or irregular, the central capsule follows the mode of growth and develops irregular protuberances.
II. Acantharia.—Whilst the central capsule of most Acantharia retains its primitive spherical form, in a minority of the group it passes over into various secondary forms, which are directly determined by the growth of the skeleton; especially common are lappet or club-shaped prominences which follow the larger radial spines. Hence the central capsule may assume the form of a violin, with two lobes corresponding to the two poles of the elongated main axis, as in many Amphilonchida (p. [782], Pl. [132], fig. 10), and the Diploconida (p. [884], Pl. [140]). On the other hand the central capsule becomes cruciform, with four lobes disposed at right angles, as in Lithoptera and other Quadrilonchida (p. [768], Pl. [131], fig. 10, &c.).
III. Nassellaria.—The primitive ellipsoid or ovoid form of the central capsule persists only in a few Nassellaria, such as the simplest and most archaic forms, the Nassellida, many Plectoidea, Stephoidea, Monocyrtida, &c. In the great majority of the Nassellaria, on the contrary, the ellipsoid or ovoid form passes over into a secondary form which is usually characterised by the presence of lobes, and is obviously dependent upon the previous development of the skeleton. In many Stephoidea and Spyroidea (probably the majority), a bilobed central capsule is formed (with symmetrically equal right and left lobes), since the primary vertical sagittal ring interferes with the growth in the median plane (Pl. [90], figs. 7-10). In other Spyroidea, on the contrary, and the majority of the Cyrtoidea, the central capsule forms at its basis rounded lobes, which protrude and hang down from the meshes of the cortinar plate; and since this latter has usually three or four large pores, the capsule similarly develops three or four processes (Pl. [53], fig. 19; Pl. [55], figs. 4-11; Pl. [59], figs. 4-13; Pl. [60], figs. 3-7; Pl. [65], fig. 1).
56. The Membrane of the Central Capsule.—The capsule-membrane or envelope of the central capsule is both morphologically and physiologically one of the most important parts of the Radiolarian body, for it separates its two main constituents, the capsule with its nucleus and endoplasm and the extracapsulum with the calymma and exoplasm. The capsule-membrane is invariably present at some time or other during the life of the organism, even though in a few species it may persist only for a short time. It is characterised in general by its power of resistance to chemical and physical reagents, and appears to be related to the elastic tissues or perhaps even more to the chitinous substances. Its thickness is usually less than 0.0001, though in certain groups it ranges between 0.001 and 0.002, and in many of the larger Radiolaria (such as Collida and Phæodaria) it may attain a thickness of 0.003 to 0.006 or more. In the three legions Spumellaria, Acantharia, and Nassellaria the capsule-membrane is single, while in the Phæodaria it is always double, being composed of a firm outer and a delicate inner membrane, which are in contact at only few points. Usually it is quite structureless, except for its apertures; the thicker membrane showing occasionally a fine concentric lamination. In certain large Colloidea (e.g., Thalassicolla, Pl. [1], fig. 5b) the membrane is covered on the inner surface by a network of polygonal ridges, and in some large Phæodaria with remarkable small curved rods (Pl. [114], fig. 13). In all Radiolaria the membrane is perforated by definite openings or pores, through which the intracapsular and extracapsular protoplasm are in direct communication. These openings (or "pylae") show very characteristic and constant differences in the four legions, which have given rise to the names—Peripylea, Actipylea, Monopylea, Cannopylea.
The capsule-membrane was first indicated as the most important and absolutely constant component of all Radiolaria, and as the differential character of the class, in my Monograph (1862, pp. 69-71). The careful investigations of R. Hertwig have confirmed this view and at the same time have yielded the most important conclusions regarding the nature and systematic significance of the openings in the capsule (op. cit., 1879, pp. 105-107). On the contrary, Karl Brandt has recently propounded the theory that the capsule-membrane is by no means a constant part of the Radiolarian organism, but is lacking in certain species of Collozoum and Sphærozoum (1881, p. 392). This contradiction is explained by the fact that in some Collodaria and Acanthometra the formation of the central capsule takes place much later than in the other Radiolaria, in some species indeed only just prior to the development of the swarm spores. I have recognised the presence of it in all species which I have investigated (more than a thousand), and even in those in which Brandt denies its existence. It is often very delicate and may easily be overlooked, especially when the contents of the capsule are colourless, but in all cases by the prudent application of staining fluids and other reagents its presence may be demonstrated. Even in those cases in which the contour of the capsule was not visible, and its contents appeared to pass without definite boundary into the matrix of the extracapsulum, it was possible by the use of appropriate stains or reagents, which would not penetrate the capsule, or of those solvents which were capable of dissolving its contents and of causing it to swell up like a distended bladder, to recognise the existence of the membrane. Those Radiolaria in which it is truly absent are young animals of species in which the membrane is only formed immediately before sporification, and persists but for a short time (e.g., species of Collozoum, Sphærozoum, Acanthometra, Acanthochiasma, &c.).
57. The Capsule-Openings of the Peripylea (or Spumellaria).—The capsule-membrane of the Peripylea is generally perforated by extremely fine and numerous pores, which are distributed at equal distances over the whole surface, and are precisely alike in all parts of the capsule. Hence the Spumellaria may be called "Holotrypasta" or "Porulosa"; they agree with the Actipylea in being devoid of an osculum or operculum; they are distinguished from the latter group mainly in that their pores are equally distributed over the whole surface of the capsule, whilst in the Actipylea the pores are disposed in definite groups or lines, separated by large imporous areas.
The central capsule of the Spumellaria, with its innumerable fine and evenly distributed pores, must be regarded as the primitive arrangement, from which the different central capsules of the three other legions have been developed. The central capsule of the Actipylea has been derived from that of the Peripylea by reduction in the number of the pores and their distribution in definite, regularly disposed areas in the membrane. The central capsule of the Osculosa is characterised by the formation of a special main-aperture (osculum) at the basal pole, which is closed in the Monopylea by the porochora, and in the Cannopylea by the astropyle; the remaining pores, with the exception of the accessory openings of many Cannopylea, remain undeveloped in both these legions. In the same way Hertwig regards the central capsule of the Peripylea as the primitive form (1879, L. N. [33], p. 107).
58. The Capsule-Openings of the Actipylea (or Acantharia).—The capsule-membrane of the Actipylea is perforated by very numerous fine pores, which are regularly distributed over the surface of the central capsule, and separated by imporous intervals. Hence the Acantharia belong to the "Holotrypasta" or "Porulosa"; they have neither osculum nor operculum, and agree in this particular with the Peripylea; but they are separated from these latter chiefly by the fact that their pores are much less numerous, and marked off into regularly arranged groups or lines by imporous intervals. In the Peripylea, on the contrary, the pores are much more numerous and are evenly distributed over the whole surface of the capsule.
The central capsule of the Acantharia has hitherto been for the most part confounded with that of the Spumellaria, and no clear distinction has been drawn in this respect between the two legions of the Porulosa. Hertwig, who in 1879 first discovered the remarkably different structure of the Osculosa (Nassellaria and Phæodaria), recognised no distinction between the structure of the capsules in the Peripylea and Actipylea (his Acanthometrea), and supposed that in both these legions "very fine pores were evenly distributed in large numbers over the capsule-membrane" (loc. cit., p. 106). I have, however, during the last few years convinced myself, by the careful comparative investigation of numerous Acantharia, that in this respect they are quite distinct from the Spumellaria (with perhaps the exception of the Astrolophida, which are nearly related to the primitive Actissa). The number of pores in the Actipylea is usually very much smaller than in the Peripylea, and they are regularly arranged in groups.