92 PHYLUM PROTOZOA THE SIMPLEST ANIMALS. 



In the allied genus Actinospharium^ with very numerous 

 nuclei, there is a strange and complicated formation and 

 fusion of cysts within a single individual. 



Third Type of Protozoa POLYSTOMELLA 



Polystomella (see Fig. 50) is a type of Foraminifera with 

 a calcareous perforate shell or test. 



Description. Polystomella crispa is common on the 

 shore, especially among Zostera. It looks like a miniature 

 of an Ammonite shell, and Foraminifera were indeed 

 classified by the older naturalists with the Ammonites. 

 The test forms a close spiral with beautifully chiselled 

 surface; only the last whorl is visible from the outside. 

 The test is made up of a series of chambers which com- 

 municate with one another and with the exterior by fine 

 pores. Granular protoplasm fills up the chambers and 

 forms also a thin layer on the outside. Long slender 

 pseudopodia issue from the openings in the test and are 

 given off also by the external protoplasmic layer. They 

 frequently branch and anastomose with one another, and 

 their granular protoplasm exhibits marked streaming 

 movements. The pseudopodia serve to catch and en- 

 tangle the diatoms and Infusoria on which the Foraminifer 

 feeds. 



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 indi- 

 viduals. 



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 



