LESS. XIV 



THE SHELL 



149 



But what gives the Foraminifera their special character is 

 the fact that around the protoplasm is developed a cell-wall, 

 sometimes membranous, but usually impregnated with cal- 

 cium carbonate, and so forming a shell. In some cases, as 

 in the genus Rotalia (Fig. 30), this is perforated by nume- 

 rous small holes, through which the pseudopods are pro- 

 truded, in others it has only one large aperture (Fig. 31), 





FIG. 30. A living Foraminifer (Rotalia), showing the tine radiating 

 pseudopods passing through apertures in the chambered shell : at x 

 several of them have united. (From Gegenbaur. ) 



through which the protoplasm protrudes, sending off its 

 pseudopods and sometimes flowing over and covering the 

 outer surface of the shell. Thus while in some cases the 

 shell has just the relations of a cell-wall with one or more 

 holes in it, in others it becomes an internal structure, being 

 covered externally as well as filled internally by protoplasm. 

 The mode of growth of Foraminifera is largely determined 

 by the hard and non-distensible character of the cell-wall, 



