1. Ellipsoidal nuclei, arising by elongation of one principal axis; very common among the Nassellaria, as well as in many Prunoidea and Larcoidea among the Spumellaria; also in several Acantharia.
2. Discoidal nuclei, arising by contraction of one principal axis, sometimes lenticular or spheroidal, biconvex, sometimes shaped like a disc or coin; especially common in the Discoidea among the Spumellaria, also in some Acantharia; the large nucleus of the Phæodaria is always spheroidal or almost spherical, with a slightly shortened main axis.
3. Stellate nuclei, spherical, and armed with evenly distributed radial club-shaped or conical processes; rare but very characteristic, especially in the two large Thalassicollida Thalassopila (Pl. [1], fig. 3), and Thalassophysa (Monogr. d. Radiol., Taf. i.); also in some Sphærellaria (Pl. [11], fig. 5).
4. Amœboid nuclei, with unequal processes irregularly arranged, in certain irregular forms of Spumellaria and Acantharia.
5. Lobate nuclei, with several (usually two or three) large ovoid or pyriform lobes, which protrude into corresponding larger lobes of the central capsule, in many Nassellaria, especially the multiarticulate Cyrtoidea (Pl. [59], figs. 12, 13). The budding nucleus of the Acantharia is also lobate (Pl. [129], figs. 6-11).
67. The Nucleus of the Peripylea.—The nucleus of the Spumellaria or Peripylea shows in certain groups a very primitive arrangement, indeed the archaic structure from which the various forms of nuclei of other Radiolaria may be derived; but on the other hand, in other groups it exhibits very peculiar and remarkable differentiations. In the first place it may be noted that the monozootic or solitary Spumellaria usually possess a single serotinous nucleus, which only divides into numerous swarm-spores at a late period; whilst, on the contrary, the polyzootic colonial Spumellaria (or Polycyttaria) are uninuclear only in the young state (Pl. [3], fig. 12), and speedily present numerous small homogeneous nuclei, which have arisen by precocious division of a single nucleus; these are usually spherical and 0.008 to 0.012 mm. in diameter. The serotinous nucleus of the monozootic Spumellaria, in many divisions of this large legion, and especially in the simply constituted Sphæroidea, is a homogeneous sphere of nuclein, lying in the middle of the central capsule. In many other cases it assumes the form of a spherical vesicle ("Binnen-Bläschen"), whose fluid or semi-fluid contents are enclosed by a more or less firm membrane. This vesicle often contains a single central spherical nucleolus (Pl. [1], figs. 1l, 4l), but sometimes a variable number of small excentric nucleoli (Pl. [1], figs. 1a, 2a). The nuclear membrane is often somewhat thick, presenting a double contour, and in such cases may even exhibit a fine radial striation, the expression of minute pores (Pl. [1], fig. 2a). In the colossal nuclei (as much as 1 to 2 mm. in diameter) of certain large Thalassicollida the nucleolus presents a very remarkable form, becoming stellate by the protrusion of processes, which may again branch in a dendritic fashion (as in the common Thalassicolla nucleata), or it may develop into a very long cylindrical thread, which is disposed in serpentine coils, and in Thalassophysa pelagica passes into the different cæcal processes of the stellate nucleus. In many Sphæroidea, whose skeleton is composed of numerous concentric lattice spheres, the small central spherical nucleus lies at first within the innermost of these (the medullary shell); but afterwards it grows through the meshes of the lattice-work, and the radiating club-shaped processes thus formed (Pl. [11], fig. 5) unite with each other outside the medullary shell, and form an external nuclear sphere which completely encloses the latter. In the Polysphærida (with several concentric lattice-shells) and in the Spongosphærida (with spongy lattice-spheres), this process may be several times repeated, so that eventually the central spherical nucleus attains considerable dimensions, and encloses two or more concentric lattice-shells with their radial connecting rods. The nuclear membrane is in these cases usually penetrated by radial bars, which connect the outermost of the enclosed shells with the remaining cortical shells which surround the central capsule. The same remarkable arrangement is also very common among the Discoidea. The small spherical primary nucleus is in such instances immediately surrounded by the innermost earliest developed lattice-shell, around which the concentric rings are subsequently deposited; it then grows out through the meshes, and the processes fuse outside the ring to form a homogeneous lentiform nucleus (Pl. [43], fig. 15). The same process recurs in certain Prunoidea and Larcoidea, whilst in other Spumellaria of these groups (e.g., Pylonida) the lobate processes of the nucleus remain free.
Both the simple serotinous nucleus of the monozootic Spumellaria, and the numerous precocious nuclei of the Polycyttaria, were first described in my Monograph in 1862, the former as the "endocyst" ("Binnen-Bläschen"), the latter as "spherical transparent vesicles" ("Kugelige wasserhelle Bläschen"). I was in error, however, in regarding the latter as identical with the so-called "hyaline spherules" in the central capsule of many Monozoa, which rather belong to the category of intracapsular vacuoles (see § [72]). The credit of recognising, by the aid of the modern methods of staining, the distinctness of these two structures, which may readily be mistaken for each other, and of demonstrating the true nature both of the serotinous and precocious nuclei, belongs to Richard Hertwig (1879, L. N. [33]).
68. The Nucleus of the Actipylea.—The nucleus of the Acantharia or Actipylea shows very peculiar relations in respect of structure and division, particularly special forms of lobular budding, which belong to the characteristic peculiarities of this singular legion, and are not found among other Radiolaria. The position of the nucleus is always excentric, even in the youngest Acantharia, for the centrogeneous formation of the skeleton, the constant development of the earliest radial portions of it in the middle of the central capsule, forces the nucleus from its normal central position. The majority of the Acantharia, like most Polycyttaria, are precocious, the primary nucleus early dividing into numerous small nuclei (see note A below). Nevertheless there are many exceptions to this rule in different families, e.g., Stauracantha, Xiphacantha, Phatnacantha, and Pristacantha among the Acanthometra, and Stauraspis, Echinaspis, Dodecaspis, and Phatnaspis among the Acanthophracta. In these instances the primary nucleus remains for a long time as a simple excentric ellipsoidal or irregularly round body, even in the fully developed stage, and only at a very late period (sometimes just before the formation of the spores) divides into many small nuclei. Since this serotinous division of the nucleus takes place in different genera of very various groups, it can only be decided by further investigations how widely it is spread among the Acantharia, and upon what circumstances it is dependent (see note B). The division of the nucleus appears to be precocious in the majority of this legion, and a number of small nuclei appear to be early formed by a peculiar process of budding; in most fully developed Acantharia these are disposed in one or two layers under the surface of the central capsule, but if their numbers increase to any considerable extent, the whole space between the skeletal rods becomes filled with small nuclei; sometimes these are homogeneous, sometimes vesicular, 0.002 to 0.012 mm. in diameter; usually they are spherical and have a small nucleolus (compare Pl. [129], figs. 6-11, and note C).
A. The numerous nuclei, which are to be found in the central capsule of most mature Acantharia, were first described in my Monograph (1862) as "spherical, transparent vesicles, provided with a small dark granule" (p. 374, Taf. xv. figs. 2, 5; Taf. xvi. figs. 2, 4; Taf. xxi. fig. 7, &c.). Their more minute constitution and peculiar origin were first accurately delineated by R. Hertwig (1879, loc. cit., pp. 11-24, Taf. i-iii.).
B. The fact that in a number of Acantharia the nucleus does not divide early as in the majority of the legion, but only at a later period, was first observed by R. Hertwig in a species of Acanthometra (Xiphacantha serrata), and a species of Acanthophracta (Phatnaspis mülleri = Haliommatidium mülleri) (loc. cit., pp. 11 and 27). This serotinous division of the nucleus seems, however, to be rather widely spread in both sublegions of the Acantharia; I have found, not only in the forms above mentioned, but also in several others belonging to different genera, a single large excentric nucleus, even in those individuals in which the skeleton was fully developed.