POLYCY8TIXA 



8 S l 



outwards, as in Oladococcas (fig. 647, 5). Sometimes the central 

 sphere is inclosed in two, three, or even more concentric spheres 

 connected by radii, as in the beautiful Actinomma (fig. 647, 2), re- 

 minding us of the wonderful concentric spheres carved in ivory by 



FIG. 648. Varietal modifications of Astro) 



the Chinese. One of the most common examples of this group is 

 the Haliomma Htimboldtii (fig. 651), in which the shell is double. 



Polycystina. This name, which originally included the preceding 

 group, is now restricted to those which have the shell formed outside 



FIG. GW.Perichlaniyflium prcctcxtn m, FIG. Q50.Stylodyctya gracilis. 



the capsule. This shell may, as in the preceding, be a simple sphere 

 composed of an open silicious network, as in Ethmosphcera (fig. 647, l) ; 

 or it may consist of two or three concentric spheres connected by 

 radii ; or, again, it may put forth radial outgrowths, which sometimes 



3 i2 



