28 



POLTCTSTINa 



CHAPTER III. 



POLYCTSTIXA. 

 I. Nature. — 2. Distribution. 



I. Ifatwre. — The name Polycystina was first 

 given by Ehrenberg to a gi"oup of minute shell- 

 bearing organisms, apparently allied to the Fora- 

 minifera. They are usually of smaller size than 

 the latter, from which also they differ in the nature 

 of their shelly investment, the composition of 

 which is siliceous. These shells are remarkable 

 for the great beauty and variety of their forms. 



Fig. 5. 



Polycystina 



a, Podocyrtis Schomhurgkii ; b, Haliomma 

 Humboldtii. 



and the peculiar appearance of the spine-like 

 projections with which they are frequently fur- 



