filGH SCHOOL ^OOLOGt. 



255 



fera have a very great geological importance, rocks of the chalk 

 and other formations being often formed largely out of the 

 remains of their shells. A deposit of a similar nature is being 

 formed at the present day at the bottom of the Atlantic, dead 

 shells of Glohigerina and similar pelagic forms being rained 

 down upon the bottom. 



16. The pseudopodia in the Rhizopoda may flow together 

 round a particle of food as represented in Fig. 190, but in the 

 remaining orders of Sarcodina, they are less subject to altera- 

 tion in form, rarely coalesce, stand out radially from the body, 

 and are sometimes strengthened by an axial filament. These 

 orders are the Heliozoa and the Radiolaria, the former a small 

 group, chiefly of fresh-water forms, sometimes naked, sometimes 

 with a spicular or perforated shell (Fig. 189, e) ; the latter, a ma- 

 rme group containing numerous forms with siliceous skeletons, 

 which offer the most surprising beauty and wealth of form (Fig. 

 191). The Radiolaria are afeo important from a geological, 

 l)oint of view, for deposits of " infusorial earth " are found 

 consisting almost entirely of their shells, and similar deposits 

 are being formed in some parts of the ocean at the present day. 

 The body is more differentiated than in the Heliozoa, the endo- 





v'yw-:*^ 



L 



¥'v^. 191. — Living Radiolarian : — Heliosphcera actinota. 



