92 PHYLUM PROTOZOA—THE SIMPLEST ANIMALS. 
In the allied genus Actinospherium, 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 amceboid nucleated spores which leave the 
