148 FORAMINIFERA, RADIOLARIA, AND DIATOMS 



The growth of Foraminifera is largely determined by the 

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

 when once formed is incapable of being enlarged. In the 

 young condition they consist of a simple mass of proto- 

 plasm covered by a more or less rounded shell, having at 

 least one aperture. But in most cases as the cell-body 

 grows it protrudes through the aperture of the shell as a 



FIG. 32. A, diagram of a Foraminifer in which new chambers are 

 added in a straight line : the smallest first-formed chamber is below, 

 the newest and largest is above and communicates with the exterior. 



B, diagram of a Foraminifer in which the chambers are added in a 

 flat spiral : the oldest and smallest chamber is in the centre, the newest 

 and largest as before communicates with the exterior. (From 

 Carpenter.) 



mass of protoplasm at first naked, but soon becoming 

 covered by the secretion around it of a second compartment 

 or chamber of the shell. The latter now consists of two 

 chambers communicating with one another by a small 

 aperture, and one of them the last formed communi- 

 cating with the exterior. This process may go on almost 

 indefinitely, the successive chambers always remaining in 

 communication by small apertures through which continuity 

 of the protoplasm is maintained, while the last formed 



