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—PaRAM@CIUM 
Paramecium, a type of ciliated Infusorians, especially 
of those which are uniformly covered with short cilia 
(Holotricha). 
Fic. 39.—Paramecium.—After Biitschli. 
ad. Adult form, showing cilia, ‘‘ mouth,” contractile vacuoles, ete. 
div. Transverse division. 
con. Conjugation. 
Description.—Specimens of Paramecium may be readily 
and abundantly obtained by leaving fragments of hay to 
soak for some days in a glass of water. A few individuals 
have been lying dormant about the plant; they revive and 
multiply with extraordinary rapidity. They are also 
