/. RHIZOPODA: RADIOLARIA. 



193 



A second resemblance lies in the delicate pseudopodia, often 

 with an axial filament. The distinguishing characteristic is the 

 central capsule. This is the central portion of the body surrounded 

 by a membrane, outside of which is the extracapsulum. The 

 central capsule is the most important part of the animal. If it be 

 dissected out from the extracapsulum it not only lives but regen- 

 erates the lost parts, while the extracapsular portions die. Since 

 the protoplasm of both parts is identical, the difference in regen- 

 erative powers can only depend on the nuclei, which are confined 

 to the central capsule. 



The central capsule may be uni- or polynucleate. In the first case the 

 nucleus (fig. 123), a vesicle of appreciable size, lies in the centre of the 

 capsule, in the others the capsule is thronged by hundreds of small homo- 

 geneous nuclei. All Radiolaria are uninucleate in the young stage, and only 

 at the time of swarm-spore formation polynucleate. The fact that certain 

 species have almost always one nucleus, while others usually have many, is 

 explained in the first case by the long continuance of the uninucleate con- 

 dition, only giving place to the polynucleate condition just before the 

 formation of swarm-spores, while in the second the polynucleate condition 

 is reached early. In the central capsule are also included various 

 deposits which serve as food during reproduction, such as concretions, oil 

 globules, etc. 



The membrane surrounding the central capsule is either per- 

 forated on all sides by numerous pore- 

 canals or by small openings in certain 

 places. Through these pores and open- 

 ings the intracapsular protoplasm passes 

 out and spreads itself in the extracap- 

 sulum. This consists of a gelatinous 

 mantle through which the protoplasm 

 extends as a fine network before it forms 

 pseudopodia on the surface. In the 

 larger Radiolaria it contains vacuoles 

 (extracapsular alveoli) developed in the 

 protoplasmic net (fig. 123). 



With few exceptions the Radiolaria 

 possess skeletons of wonderful beauty; 

 latticed spheres, single or one within 

 another, and bound together with radial 

 rods (fig. 85), frequently ornamented on 

 the outer surface with spines, or latticed 

 discs, helmet-like or cage-like structures (fig. 124) or spongy 

 structures. In other cases occur rings, tubes, spines, which meet 



FIG. 124. Eucyrtidium crani- 

 oides. (After Haeckel). 



