90 PHYLUM PROTOZOA——-THE SIMPLEST ANIMALS, 
along with drops of water, which form food vacuoles in the 
cell substance, Into these vacuoles digestive ferments flow. 
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. 
Life history.k—In favourable nutritive conditions the 
Ameeba 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 wall is said to be 
chitinoid. With the return of favourable conditions the 
Fic. 35.—Life history of Ame@éa. 
«. Ameeba with pseudopodia ; ., nucleus; c.v., contractile vacuole. ~. Division 
intwo. 3. Encystation. 4. Escape of Ameeba from its cyst. 
Ameeba revives, and, bursting from the cyst with renewed 
energy, recommences the cell-cycle. The conjugation of 
two Amoebe 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 
bottom. It is spherical and minute, measuring at most 
o’o5 mm. in diameter. Long stiff pseudopodia radiate 
out from the body. A clear axial filament runs up each 
pseudopodium, and the small organisms on which Actino- 
