66. _The Form of the Nucleus._--The nucleus of the Radiolaria shows greater variations in form and structure than are to be found in the majority of cell-nuclei; { xxxv}exception must, however, be made in the case of many animal ovicells, which, in their peculiar form and composition, often recall large Radiolarian nuclei. With respect to the external shape two main forms may be distinguished, as primary and secondary. The _primary form_ of the Radiolarian nucleus is the sphere; it occurs not only in most swarm-spores, but also in most adult forms belonging to the legion SPUMELLARIA, and in individual instances in other groups; indeed the nuclei of most SPUMELLARIA, as also the concentric central capsules in which they lie, are true geometrical spheres. The _secondary forms_ of the nucleus are found in the majority of adult Radiolaria, and arise from the primary spherical forms in various ways, either by the elongation or contraction of one axis, or by the formation of apophyses or processes. The most important of these secondary forms are as follows:--
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. _Amoeboid 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).