1. RHIZOPOBA: RADIOLARIA. 195 



in the central capsule (fig. 125), etc. In rare cases the siieleton is 

 formed solely of organic substance {acantldn); usually it is sili- 

 cious and of much firmness. Hence skeletons of Radiolaria occur 

 in rocks of various ages, as in Caltanisetta, Sicily, the Nicobars 

 (both tertiary), and the Barbadoes. 



In reproduction first comes fission, which begins a division of 

 the central cai)sule (in uni-nucleate forms with a division of the 

 nucleus) and usually extends through the extracapsulum. If this 

 latter does not divide a colony results, in which a gradually 

 increasing jelly contains numerous central capsules, bound together 

 Ijy jirotoplasmic threads, which form the pseudopodia on the sur- 

 face (fig. 126). A second type is reproduction 

 by swarm spores, which begins when the 

 nucleus has divided into hundreds or thousands 

 of daughter nuclei. The central capsule then 

 divides into as many portions as there are 

 nuclei, vvhicli become oval and develop two 

 flagella (fig. 127), which soon begin to vibrate 

 so that the central capsule is filled with a 

 tumultuous crowd. With the breaking of the p^^ m.-zoospores of 

 capsiilar membrane these swarm spores escape, Cuii^aoum merme. a, 



^ 1 Jr ' microspore; li, zoo- 



and at this point our knowledge of this type of ?P"J'^ "^'Ui fusiform 



^ , *^ . Dody; c, macrospore. 



development ceases. Since in many species 

 there are large and small spores — macrospores and microspores — 

 it is probable that for the further development a copulation of 

 difl'erent swarm spores is necessary. 



Common, if not constant, in the bodies of the Radiolaria are the yellow 

 cells vvhicli were formerly regarded as a part of the animal; they are uni- 

 cellular alg;e (Zooxantliella), which are also present in other animals. 

 (Thalamofihora, aotinians, sponges, etc.) They afford an instance of sym- 

 biosis, or the living together of different organisms for mutual good. This 

 new view rests upon the facts that the Zooxanthella: have a membrane, 

 secrete starch-like substances, divide independently of the radiolarian and 

 continue to live after its death. 



The Radiolaria are exclusively marine. In fair weather they flo.-it at 

 the surface, but sink in times of storm. Certain species and even large 

 gnjups like the Phseodaria occur only at great depths (1500-4000 fathoms) 

 where the temperature is about 0° C. 



Sub Order I. PERIPYLEA or SPUMELLARIA. The cai)sule mem- 

 brane everywhere perforated by pore canals; skeleton absent or formed of 

 loose needles, of silicions latticed spheres, often reduced to a spongy net- 

 work or flattened to discs. The latticed spheres can be provided with 

 spines and connecting rods. SphterozoiDjE, colonial (fig. 136); Thalassi- 



