12 ANIMAL PHYSIOLOGY. 
extensions of its protoplasm (cilia) through the cell wall; or 
the cell wall may disappear entirely. Finally, the motile form, 
withdrawing its cilia and clothing itself with a cellulose coat, 
‘becomes globular and passes into a quiescent state again. 
Much of this part of its history is common to lowly animal 
forms. 
Conclusions.—It will be seen that there is much in common 
in the life-history of Torula and Protococcus. By virtue of be- 
ing living protoplasm they transform unorganized material into 
their own substance; and they grow and reproduce by analo- 
gous methods. 
But there are sharply defined differences. For the green 
plant sunlight is essential, in the presence of which its chloro- 
phyl prepares the atmosphere for animals by the removal of 
carbonic anhydride and the addition of oxygen, while for 
Torula neither this gas nor sunlight is essential. 
Moreover, the fungus (Yorula) demands a higher kind of 
food, one more nearly related to the pabulum of animals; and 
is absolutely independent of sunlight, if not actually injured 
by it; not to mention the remarkable process of fermentation. 
UNICELLAR ANIMALS. 
THE PROTEUS ANIMALCULE (Ameaba). 
In order to illustrate animal life in its simpler form we 
choose the above-named creature, which is nearly as readily 
obtainable as Protococcus and often under the same circum- 
stances, 
Morphological A mba is a microscopic mass of transparent 
protoplasm, about the size of the largest of the colorless blood- 
corpuscles of cold-blooded animals, with a clearer, more con- 
sistent outer zone (ectosarc), (although without any proper cell 
wall), and a more fluid, granular inner part. A clear space 
(contractile vesicle, vacuole) makes its appearance at intervals in 
the ectosarc, which may disappear somewhat suddenly. This 
appearance and vanishing have suggested the term pulsating 
or contracting vesicle. Both a nucleus and nucleolus may be 
seen in Ameeba, At varying short periods certain parts of its 
body ( pseudopodia) are thrust out and others withdrawn. 
Physiological—_ Amceba can not live on such food as proves 
adequate for either Protococcus or Torula, but requires, besides 
