70 THE PROTOZOA . 



these, thus forming a bristling coat. Pinaciophora is very similar, 

 but the spicules are not so prismatic. 



Similarly the Radiolaria may have either simple isolated spicules 

 or compact and strong skeletons. In many cases the outer plasm 

 (calymma) is free from spicules, but in other cases isolated spicules of 

 sharp and needle-like, or tri- or tetra-radiate form, are present. The 



Fig. 40. Types of spicules in Heliozoa. [PENARD.] 



A. Raphidiophrys pallida F. E. Sch., with curved silicious rods. B. Pinaciophora rubiconda 

 Hert. and Less. C. Acanthocystls turfacea Carter. D. Pinaciophora fluviatilis Greeff. 



substance of the skeleton of Radiolaria is either silica or acanthin, a 

 horn-like modification of protoplasm. According to Haeckel, the 

 deposition of silica in many cases occurs only at certain periods, and 

 an entire skeleton may be laid down at one time (Dictyotic moment, 

 Haeckel, or Lorication moment, Dreyer). Again, it may be formed 

 during the entire period of life. The material of the shell is 

 secreted from the sarcodictyum, and as the deposition of the silica 



