114 



PHYLUM PROTOZOA — THE SIMPLEST ANIMALS 



gelatinous cyst. Each loses its pseudopodia and forms a 

 membranous cyst. These cysts become associated in pairs. 

 The nucleus of each cyst divides mitotically and a polar 

 body is extruded from each, after which the nucleus 

 returns to the resting condition. The cysts now fuse in 

 pairs, with complete and intimate union of their nuclei 

 and cell-bodies. The zygote so formed rests for a short 

 period, then divides up into two daughter cysts from which 

 emerge two new individuals of Actinophrys. 



In the allied genus Actinosphceriiim, 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. 48) 

 is a type of Foraminifera 

 with a calcareous perforate 

 shell or test. 



Description. — Polysto- 

 mella 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 entangle 

 the diatoms and Infusoria on which the Foraminifer feeds. 



Fig. 48. — Polystomella, megalo- 

 spheric form, with large central 

 chamber (M.) and one nucleus 

 (iV.).— After. Lister. 



