1. RIIIZOPODA: RADIOLARIA. 195 



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

 formed solely of organic substance (acanthin)', 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 capsule (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 

 by protoplasmic 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, which 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 Flo 127 _ Zoosporeg of 

 capsular membrane these swarm spores escape, SfcrospH l re- l6/ h" e 'zoo' 

 and at this point our knowledge of this type of gP r y e . ^^Jroapor!? 

 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 

 different swarm spores is necessary. 



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

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

 cellular algae (Zooxantliellce), which are also present in other animals. 

 (Thalamophora, actinians, 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 Zooxanthetta 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 float at 

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

 groups like the Phaeodaria occur only at great depths (1500-4000 fathoms) 

 where the temperature is about C. 



Sub Order I. PERIPYLEA or SPUMELLARIA. The capsule mem- 

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

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

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

 spines and connecting rods. SPH^ROZOID^E, colonial (fig. 126); THALASSI- 



