REPORT ON THE RADIOLARIA. 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 Discoid ea among the Spumellaria, also in 

 some Acantharia ; the large nucleus of the Ph.eodaria 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. Radioh, Taf. i.) ; also in some Sphasr ell aria (PL 11, fig. 5). 



4. Arnceboicl 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 Nasellaria, 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 tbe 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 ; 



