170 



PROTOZOA 



With few exceptions the Radiolaria possess skeletons of wonderful beauty; 

 latticed spheres, single or one within another, and bound together with radial 

 rods (fig. 89), frequently ornamented on the outer surface with spines, or latticed 

 discs, helmet-like or cage-like structures (fig. 12-7). In other cases occur rings, 

 tubes, spines, which meet in the central capsule (fig. 125), etc. In rare cases 

 the skeleton is formed solely of organic substance (acanthiri) ; usually it is silicious 

 and resistant. Skeletons of Radiolaria occur in rocks of various ages, as in 

 Sicily, the Nicobars (both tertiary), and the Barbadoes. 



In reproduction there are numerous forms with a skeleton in which division 

 begins with a cleavage of the central capsule and usually extends through the 

 extracapsulum. If this latter does not divide a colony results, in which a jelly 



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FIG._ 124. Thalassicolla pelagica. In centre the nucleus with coiled nucleolus, 

 around it central capsule with oil globules; still outside the extracapsulum with vacuoles 

 (extracapsular alveoli), yellow cells (black) and pseudopodia. 



contains numerous central capsules, bound together by protoplasmic threads, 

 which form the pseudopodia on the surface (fig. 128). A second type is repro- 

 duction by swarm or zoospores, which begins when the nucleus has divided 

 into hundreds or thousands of daughter nuclei. The contents of the central 

 capsule then divides into as many portions as there are nuclei, these become oval 

 and develop two flagella (fig. 126), which soon begin to vibrate so that the central 

 capsule is filled with a tumultuous crowd. With the breaking of the capsular 

 membrane these swarm spores escape; here our knowledge of this type ceases. 

 Since in many species there are macrospores and microspores it is probable that 

 a copulation is necessary. 



