THE RADIOLARIA 121 



perfectly investigated, and the following statement can only be 

 regarded as a provisional account of its coarser features. The 

 two chief phases of life are signalised by distinctive characters in 

 the nucleoplasm. In the vegetative phase it consists of a single 

 large vesicular structure, or of a few derived from this by mitotic 

 division, or of many equivalent, amitotically produced, small nuclei. 

 In only a few cases are chromidia or other nuclear derivatives as 

 yet known to occur in this phase (Collospkaera, Siphonosphaera, and 

 Aulacantlia), and there is no separation of somatic and germinal 

 nucleoplasm. The Radiolaria are, in fact, homokaryota. Neverthe- 

 less, at the advent of the sporulating phase, the nucleus displays 

 new characters. Either it becomes differentiated and divides into 

 spore nuclei ; or it fragments partly into chromidia and plasma, which 

 recombine to form the spore nuclei, and partly into a residue which 

 perishes with the parental exuviae. In this process we can detect 

 a certain analogy with the extrusion of nucleoplasm during the 

 formation of the spores in the Heliozoa. But since the fate of 

 the Radiolarian spores is unknown, a just comparison of the two 

 -cases is at present impossible. 



The nucleus lies wholly in the endoplasm, and no chromidia or 

 other nuclear products have yet been recognised in the extra- 

 capsulum ; but the axopodia which radiate from the neighbourhood 

 of the nucleus in certain Nassellaria, the similar fibrillae that run 

 from the nucleus outwards to form the flagellum of the Discoidea, 

 are indications of the paths along which the nucleus probably exerts 

 its influence upon the ectoplasm, and vice versa. Further evidence 

 of this peri nuclear sphere of influence is found in the apparently 

 porous character of the nuclear membrane (Physematium, Thalasso- 

 lampe, and certain Sphaeroidea) and the radial arrangement of its 

 peripheral plasma. 



The characters of the nucleus vary according as to whether it 

 is a single or multiple structure. The Collodaria, Sphaeroidea, 

 Nassellaria, and Phaeodaria are generally mononuclear : the Sphaero- 

 zoa and Acantharia, polynuclear forms. In the first group the 

 nucleus is vesicular and differentiated into membrane, sap, chromatin, 

 and achromatin. In the second the nuclei are without a distinct 

 membrane, and, in the vegetative stage, homogeneous ; their origin 

 from the spore or zygote nucleus has been traced in no single 

 instance. 



One or two special forms of nucleus may be referred to. 

 Among the Phaeodaria the majority possess a nucleus such as that 

 shown in Fig. 15, A, together, in some cases (Aulacantha scolymantha), 

 with chromatin particles, scattered through the endoplasm. The 

 Tuscaroridae, however, are peculiar in having (Figs. 13 and 30) 

 an elongate nucleus, with a loop of chromatin enclosed by the 

 nuclear sap. 



