POLYSTOMELLA 



115 



Like many other Foraminifera, Polystomella shows 

 a remarkable dimorphism. It occurs in two forms, 

 outwardly indistinguishable, but differing in internal struc- 

 ture. In the megalospheric form the central chamber is 

 large (a megalosphere), and there is a single large nucleus, 

 placed about the middle of the series of chambers ; in 

 the microspheric form the central chamber is small (a 

 microsphere), being about one-tenth of the diameter of 

 the megalosphere, and there are numerous small nuclei. 

 The megalospheric individuals are about thirty times 

 as numerous as the microspheric individuals. 



Fig. 49. — Polystomella, microspheric form, with small 

 central chamber {c.c), numerous nuclei {N.), bridges of 

 protoplasm between chambers {B.). — After Lister. 



Life-history. — The microspheric form has its nuclei 

 replaced by chromidia (chromatin bodies detached from 

 the nuclei into the protoplasm). These chromidia form 

 the centres of amoeboid nucleated spores which leave the 

 shell or are liberated by the protoplasm creeping out and 

 forming a halo of anastomosing threads round the deserted 

 test. The spores secrete a shell and grow into the typical 

 megalospheric forms. 



When the megalospheric form is about to reproduce, 

 its nucleus disintegrates and is replaced by numerous 

 scattered nuclei formed around chromidia. The proto- 



