ISO FORAMINIFERA, RADIOLARIA, DIATOMS less. 



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 globular 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 

 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 



Fig. 31. — 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. (After 

 Carpenter. ) 



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 

 chamber has a terminal aperture placing its protoplasm in 

 free communication with the outer world. 



