HELIOZOA RADIOLARIA 



75 



" 



he surface) and a pairing nucleus. 1 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- 

 phrys- like cells as there were nuclei ; then each of these 

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

 Schaudinn admits 24 genera (and 7 doubtful) and 41 species 



*Wr 



N 4 N 4 - 



N. 



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

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

 N 3 and n 3 , the latter passing out with a little cytoplasm as an abortive cell ; 4, 

 repetition of the same process as in 3 ; 5, the two nuclei N 4 have fused in syiigamy 

 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 

 Orbulinella has only been found in a salt-pond ; Actinophrys sol 

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

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

 cystis is marine ; the remaining genera and species are all 

 inhabitants of fresh water. 2 



4. Eadiolaria 



Sarcodina loith the protoplasm divided by a perforated 

 chitinous central capsule into a centred mass surrounding the 

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

 mosing enough to form a marked network ; skeleton either siliceous, 

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

 proteid matter (cicanthiii), sometimes also coalescing into a 

 latticed shell ; reproduction by fission and by zoospores formed 

 in the centred capsule. Habitat marine, suspended at the surface 

 {plankton), at varying depths (zonaricd), or near the bottom (abyssal). 



1 Such divisions into functional and abortive sister nuclei are 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. 



2 Besides these genera enumerated by Schaudinn, we include Dimorpha Gruber 

 (Fig. 37 5, p. 112), Mastigophrys Frenzel, Ciliophrys Cienk., and Actinomonas 

 usually referred to Flagellates. 



