Il6 PHYLUM PROTOZOA^THE SIMPLEST ANIMALS 



plasm segregates into little masses, each centred in a 

 nucleus. Each of these nuclei divides by mitosis into two, 

 then into four, and the division of the nucleus is followed 

 by the division of the protoplasmic mass, so that hosts of 

 tiny cells are formed. These become provided with 

 flagella, swim out into the water, leaving behind them the 

 empty test, and there conjugate in pairs, not with one 

 another but with similar " gametes " from another megalo- 

 spheric 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 



Paramaecium, a type of ciliated Infusorians, especially 

 of those which are uniformly covered with short cilia 

 (Holotricha). 



Description. — Specimens of Paramoecium may be 

 readily and abundantly obtained by leaving fragments of 

 hay to soak for some days in a glass of water. A few in- 

 dividuals have been lying dormant about the plant ; they 

 revive and multiply with extraordinary rapidity. They are 

 also abundant in most stagnant pools, and are just visible 

 when a test-tube containing them is held between the 

 eye and the light. Their food consists of small vegetable 

 particles. 



The form is a long oval, with the blunter end in front ; 

 the outer portion of the cell substance is differentiated into 

 a dense rind or cortex, with a delicate external cuticle, 

 perforated by cilia. There is a definite opening, the so- 

 called mouth, which serves for the ingestion of food 

 particles ; and there is also a particular anal spot posterior 

 to the mouth, from which undigested residues are got rid of. 

 The surface is covered with cilia, in regular longitudinal 

 rows ; these serve both for locomotion and for driving 

 food particles towards the mouth. Paramoecium rotates 

 like a rifle bullet as it swims ; its track is not straight, but 

 an open spiral. If it strikes a solid object or enters an 



