The _nucleus_ is contained in the endoplasm, and is always at first single, though it may divide again and again. The nuclear wall is a firm membrane, sometimes finely porous. If there are concentric shells it at first occupies the innermost, which it may actually come to enclose, protruding lobes which grow through the several perforations of the lattice-work, finally coalescing outside completely, so as to show no signs of the joins. In the Nassellaria a similar process usually results in the formation of a lobed nucleus, contained in an equally lobed central capsule. The chromatin of the nucleus may be concentrated into a central mass, or distributed into several "nucleoli," or it may assume the form of a twisted, gut-like filament, or, again, the nuclear plasm may be reticulated, with the chromatin deposited at the nodes of the network. {82}[Illustration: FIG. 26.—A, _Lithocercus annularis_, with sagittal ring (from Parker and Haswell). B, _Aulactinium actinastrum_. _C_, calymma; _cent.caps._, _km_, central capsule; _Ext.caps.pr._, Extracapsular, and _Int.caps.pr._, intracapsular protoplasm; _n_, _nu_, nucleus; _op_, operculum; _ph_, phaeodium; _psd_, pseudopodium; _Skel._, skeleton; _z_, Zooxanthella. (From Lang's _Comparative Anatomy_, after Haeckel.)] The skeleton of this group varies, as shown in our conspectus, in the several divisions.[90] The Acantharia (Figs. 24, 25, A) have a skeleton of radiating spines meeting in the centre of figure of the endoplasm, and forcing the nucleus to one side. The spines are typically 20 in number, and emerge from the surface of the regular spherical forms (from which the others may be readily derived) radially, in five sets of four in the regions corresponding to the equator and the tropics and polar circles of our world. {83}The four rays of adjacent circles alternate, so that the "polar" and "equatorial" rays are on one set of meridians 90° apart, and the "tropical" spines are on the intermediate meridians, as shown in the figures. By tangential branching, and the meeting or coalescence of the branches, reticulate (Figs. 23, 24, 25) and latticed shells are formed in some families, with circles of openings or pylomes round the bases of the spines. In the Sphaerocapsidae the spines are absent, but their original sites are inferred from the 20 circles of pylomes. In the Spumellaria the simplest form of the (siliceous) skeleton is that of detached spicules, simple or complex, or passing into a latticed shell, often with one or more larger openings (pylomes). Radiating spines often traverse the whole of the cavity, becoming continuous with its latticed wall, and bind firmly the successive zones when present (Fig. 23).