[lELIOZOA — RADIOLARIA 



75 



the surface) and a pairing nucleus/ The two cells then fuse 

 completely, and surround themselves with a second gelatinous 

 cyst wall, separated from the outer one by a layer of siliceous 

 spicules. The nucleus appears to divide at least twice before the 

 young creep out, to divide immediately into as many Actino- 

 jiJtri/s -like cells as there were nuclei; then each of these 

 multiplies its nuclei, to become apocytial like the adult form. 

 Schaudinn admits 2-4 genera (and 7 doubtful) and 41 species 

 12 :J 4 5 



^^ 



NoNo 



ji^N^N^-^w^ 



N, 



Fig. 21. — Diagram illustrating the conjugation of Actinosphaerium. 1, Original cell ; 

 2, nucleus divides to form two, N^Ni ; 3, each nucleus again divides to form two, 

 Ng and n^, the latter jiassing out with a little cytoplasm as an abortive cell ; 4, 

 repetition of the same process as in 3 ; 5, the two nuclei N4 hiive fused in syngamy 

 to form the zygote nucleus N.. 



(and 18 doubtful). None are known fossil. Their geographical 

 distribution is cosmopolitan, as is the case with most of the 

 minute fresh-water Protista ; 8 genera are exclusively marine, and 

 Orhulinella has only been found in a salt-pond ; Acfdnoj^hrys sol 

 is both fresh-water and marine, and Actinoloiihus has 1 species 

 fresh-water, the other marine. One of the 14 species of Aeantho- 

 ci/sfis is marine ; the remaining genera and species are all 

 inhabitants of fresh water." 



4. liADIOLAKIA 



Sarcodina tcith the 'jirotoplasm divided Inj a j^)erforat€d 

 chiiinous central cctpside into a central mass surrounding the 

 nucleus, and an outer layer ; the pseudopodia radiate, never anasto- 

 mosing enough to form a Tnarlced netiuork ; skeleton either siliceous, 

 of spicides, or perforated; or of definitely arranged spicules of 

 proteid matter {acanthiii), sometimes cdso coalescing into a 

 latticed shell ; reproduction hy fission and hi/ zoospores formed 

 in the central capside. Habitat marine, susp)ended at the surface 

 (planJdon), at varying dep>ths (zonaricd), or near the bottom (abyssal). 



^ Such divisions into functional and abortive sister nuclei arc termed "reduc- 

 ing divisions," and are not infrequent in the formation of pairing-cells, especially 

 oospheres of Metazoa, where the process is termed the maturation of the ovum. 



- Besides these genera enumerated by Schaudinn, we include Z)('/?iorjj/irt Gruber 

 (Fig. 37 5, p. 112), Mastigophrys Frenzel, Cilicphrys Cienk., and Adinomonas 

 Usually referred to Flagellates. 



