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). 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; {xxxvi}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. 1_l_, 4_l_), but sometimes a variable number of small excentric nucleoli (Pl. 1, figs. 1_a_, 2_a_). 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. 2_a_). In the