94 PHYLUM PROTOZOA THE SIMPLEST ANIMALS. 



into the water, leaving behind them the empty test, and 

 there conjugate in pairs, not with one another but with 

 similar " gametes " from another megalospheric individual. 

 The " zygote " so formed becomes the initial chamber of 

 a microspheric individual. In a more direct way by 

 fission the megalospheric individual may give rise to 

 another like itself. There is therefore in this complex life 

 history of Polystomella an alternation between a sexual and 

 an asexual generation. 



Fourth Type of Protozoa PARAMCECIUM 



Paramoeci'um, a type of ciliated Infusorians, especially 

 of those which are uniformly covered with short cilia 

 (Holotricha). 



cv 



FIG. 39. ParamcRriumWi longitudinal optical section, 

 and dividing. After Biitschli. 



C.V., contractile vacuole ; MY., longitudinal " myophan " striations ; 

 MA., macronucleus ; MI., one of two micronuclei ; F.V., food 

 vacuole ; CA., a canal in the cytoplasm entering the contractile 

 vacuole which is bursting through the cortex ; TR., trichocysts at 

 the roots of the cilia (CI.) ; MO., " mouth " leading into gullet. In 

 the left-hand figure D.L. is the transverse dividing line ; the dumb- 

 bell-like elongations of the macronucleus (MA.) and micronuclei 

 (MI.) ; P. A., a " potential anus or weak spot," where debris may 

 be got rid of. 



Description. Specimens of Paramcecium may be readily 

 and abundantly obtained by leaving fragments of hay to 



