256 



THE FOKMS OF CELLS 



[CH. 



seems to shew the natural, physical tendency of the long semifluid 

 cylinder of protoplasm to contract, at its limit of stability, into 

 unduloid constrictions, as a step towards the breaking up into 

 separate spheres : the completion of which process is restrained or 

 prevented by the rigidity and friction of the arenaceous covering. 

 Passing to the typical, calcareous-shelled Foraminifera, we have 

 the most symmetrical of all possible types in the perfect sphere of 

 Orbulina ; this is a pelagic organism, whose floating habitat places 



Fig. 84. Various species of Lagena. (After Brady.) 



it in a position of perfect symmetry towards all external forces. 

 Save for one or two other forms which are also spherical, or 

 approximately so, like Thurammina, the rest of the monothalamic 

 calcareous Foraminifera are all comprised by naturalists within 

 the genus Lagena. This large and varied genus consists of "flask- 

 shaped" shells, whose surface is simply that of an unduloid, or 

 more frequently, like that of a flask itself, an unduloid combined 

 with a portion of a sphere. We do not know the circumstances 



