Page:A Text-book of Animal Physiology.djvu/42

 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 being living protoplasm they transform unorganized material into their own substance; and they grow and reproduce by analogous methods.

But there are sharply defined differences. For the green plant sunlight is essential, in the presence of which its chlorophyl 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 (Torula) 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.

(Amœba).

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 circumstances.

Morphological.—Amœba 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 consistent 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 Amœba. At varying short periods certain parts of its body (pseudopodia) are thrust out and others withdrawn.

Physiological.—Amœba can not live on such food as proves adequate for either Protococcus or Torula, but requires, besides