112 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS 



digestion during the acid phase in Amoebae. The fer- 

 ments secreted attack proteins, carbohydrates including 

 cellulose, but possibly not fats. After the digestible parts 

 of the food have been absorbed, the undigested residue 

 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, and also of the w^ater which must be 

 continually drawn into the cell, whose contents have a 

 higher osmotic pressure than the media in which the 

 Amoebae usually live. 



Life-history. — In favourable nutritive conditions the 

 Amoeba grows. At the limit of growth it reproduces by 



Fig. 46. — Life-history of ^mQ?&a. 



I. Amoeba with pseudopodia ; n., nucleus ; c.v., contractile vacuole. 2. Division 

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



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 wall 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 Amoebae has been observed, and spore-formation 

 occasionally occurs. 



Second Type of Protozoa — Actinophrys 



The Sun-animalcule, Actinophrys sol, is a type of the 



Heliozoa. 



Description. — Like most other Heliozoa, Actinophrys 

 lives in fresh water, floating about or rolling over the 



