. A manual of zoology. Zoology. 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 result
. A manual of zoology. Zoology. 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^^ 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 tha
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