90 PHYLUM PROTOZOA THE SIMPLEST ANIMALS. 



contains very abundant granules, some of which are of 

 a proteid, and others of a fatty nature. According to 

 Professor Ray Lankester, the formation of pseudopodia is 

 due to the outflowing of the central fluid substance at 

 places where the outer pellicle has been temporarily 

 ruptured. In the centre of the cell lies the usually single 

 nucleus, but Amoeba princeps has numerous nuclei. The 

 food consists of minute Algae, such as diatoms, or of 

 vegetable debris. It is surrounded by the finger-like pro- 

 cesses, and engulfed along with drops of water, which form 

 food vacuoles in the cell substance. After the digestible 

 parts of the food have been absorbed, the undigested residue 



FIG. 36. Life history of Amoeba. 



i. Amoeba with pseudopodia ; n., nucleus ; c.? 1 ., contractile vacuole. 2. Division 

 in two. 3. Encystation. 4. Escape of Amoeba from its cyst. 



is got rid of at any point of the protoplasm. One or more 

 contractile vacuoles are visible in the cell substance. They 

 have an excretory function, and serve to get rid of the finer 

 waste products. 



Life history. - - In favourable nutritive conditions the 

 Amoeba grows. At the limit of growth it reproduces by 

 dividing into two. In disadvantageous conditions, such as 

 drought, it may become globular, and, secreting a cell wall 

 or cyst, lie dormant for a time. The cyst w r all is said to be 

 chitinoid. With the return of favourable conditions the 

 Amoeba revives, and, bursting from the cyst with renewed 

 energy, recommences the cell-cycle. The conjugation of 

 two Amcebae has been observed, and spore formation oc- 

 casionally occurs. 



