PROTOZOA 



77 



Fig. 44 



B ^ \ 



-LIFE HISTORY OF A FORAMINIFERON, POLYSTOMELLA CRISPA 



Two gametes fuse to form a fertilized cell or zygote, A. This develops into an individual with 

 a minute initial chambered shell, B. When mature this individual breaks up into a myriad of 

 small ameboid spores, C and D, each of which develops into a new many-chambered individual, 

 E, in which the initial chamber is large. When E is mature it breaks up into a cloud of 

 gametes, F, which fuse two by two, forming zygotes 



After Schandinn 



Magnification, 150 



and calcium carbonate, or lime, is precipitated between these two 

 layers in much the same way as concrete walls are made between 

 frameworks of boards. At the points where pseudopodia extend 

 outward no lime is precipitated; hence pores or windows (fora- 

 mina) in the shell result, a characteristic which gave rise to the 

 name Foraminifera. 



Radiolaria* 



In Radiolaria there is no such membrane formation, nor is 

 there a constant deposition of mineral matter. These organisms 

 precipitate silica instead of lime and this glass-like product is 

 laid down in definite skeleton patterns which are often of ex- 

 quisite design (Fig. 45). The deposition of silica is not a con- 

 tinuous process, but occurs apparently only at periods of satura- 

 tion, and at such times an entire skeleton is laid down. These 

 periods, known as dictyotic^ moments, may recur a number of 



6 



* See "Fossils," page 23, in this Scries. 

 t Dictyotic, like a net. 



§3Q 



>D« A, 



sV 



lu LIBRA 



