RADIOLARIA 



75 



group (Acantharia) they are said to be of strontium sulphate. They are 

 rarely absent, occasionally loose, but usually united into a lattice-work 

 (Figs. 67, 68), which is often very complicated, with projecting spines. 

 The latter maybe radial but do not meet at a central point except in the 

 Acantharia. The outer layer of the body differs from that of the pelagic 

 Foraminifera in that the vacuoles are contained in a layer of jelly 

 (calymma) traversed by strands of protoplasm, which secrete it and the 

 vacuoles, and in that it cannot be withdrawn. 



There is no contractile vacuole. 



The Radiolaria reproduce by binary fission and by spore formation. 

 The spores found in them are sometimes alike (isospores) and some- 

 times of two kinds {anisospores). The latter are held to be gametes, and 

 it is said that union between them has been observed. On account of 

 their resemblance to the Dinoflagellata it has been suggested that they 



Fig. 67. Fossil Radiolaria. From Woods. A, Lithocampe tschernyschevi, 

 Devonian. B, Trochodiscus longispinus, Carboniferous. C, Podocyrtis schom- 

 burgki, Barbados Earth (Tertiary). A and C, Nassellaria; B, Spumellaria. 



belong to parasitic members of that group. It is possible, on the other 

 hand, that the Radiolaria have an alternation of generations like that 

 of the Foraminifera. 



Peculiarities of the mitoses in this group have been mentioned 

 above (pp. 19, 20). 



Symbiotic flagellates, known as "yellow cells" {Zooxanthellae^ see 

 pp. 43, 46), are present in large numbers in the cytoplasm of many 

 of the Radiolaria. 



Thalassicolla(Fig.22A). (Suborder Spumellaria.) Skeleton absent 

 or represented by some loose siliceous spicules ; one nucleus ; yellow 

 cells in extracapsular protoplasm. 



