HELIOZOA 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- 

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

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

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



-N,N,- 



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

 2, nucleus divides to form two, N2N2 ; 3, eaoli nucleus again divides to form two, 

 Nj and Mg, 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 N4 have 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 

 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.^ 



4. Eadiolaeia 



Sarcodina with the protoplasm divided hy a per/orated 

 chitinous central capsule into a central mass surrounding the 

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

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

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

 proteid matter (acanthin), sometimes also coalescing into a 

 latticed shell ; reproduction hy fission and hy zoospores formed 

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

 {plankton), at varying depths {zonarial), 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 Bimorpha Gruber 

 (Fig. 37 6, p. 112), Mastigophrys Frenzel, Ciliophrys Cienk., and Actinomonas 

 usually referred to Flagellates. 



