SKELETON 151 



caps, pr.) in communication with the surrounding or extra- 

 capsular protoplasm (ext. caps. pr.). But besides this simple 

 membranous shell there is often developed, mainly in the 

 extra-capsular protoplasm, a skeleton (skel ) formed in the 

 majority of cases of pure silica, and often of surpassing 

 beauty and complexity. One very exquisite form is shown 



FIG. 35. Skeleton of a Radiolarian (Actiuoinma), consisting of 

 three concentric perforated spheres the two outer partly broken away 

 to show the inner connected by radiating spicules. (From Gegenbaur 

 after Haeckel.) 



in Fig. 35 : it consists of three perforated concentric spheres 

 connected by radiating spicules : the material of which it is 

 composed resembles the clearest glass. 



The student should examine mounted slides of the silicious 

 shells of these organisms sold under the name of Poly- 

 cystinece. and should consult the plates of Haeckel's Die 



