The Amœba
Suggestions.—Amœbas live in the slime found on submerged stems and leaves in standing water, or in the ooze at the bottom. Water plants may be crowded into a glass dish and allowed to decay, and after about two weeks the amœba may be found in the brown slime scraped from the plants. An amœba culture sometimes lasts only three days. The most abundant supply ever used by the writer was from a bottle of water where some oats were germinating. Use ⅕ or ⅙ inch objective, and cover with a thin cover glass. Teachers who object to the use of the compound microscope in a first course should require a most careful study of the figures.
Fig. 9.—Amœba Proteus, much enlarged.
Fig. 10.—Amœba.
cv, contractile vacuole; ec, ectoplasm; en, endoplasm; n, nucleus; ps, pseudopod; ps, pseudopod forming; ectoplasm protrudes and endoplasm flows into it.
Form and Structure.—The amœba looks so much like a clear drop of jelly that a beginner cannot be certain that he has found one until it moves. It is a speck of protoplasm (Fig. [9]), with a clear outer layer, the ectoplasm; and a granular, internal part, the endoplasm. Is there a distinct line between them? (Fig. [10].)
Note the central portion and the slender prolongations or pseudopods (Greek, false feet). Does the endoplasm extend into the pseudopods? (Fig. [10].) Are the pseudopods arranged with any regularity?
Sometimes it is possible to see a denser appearing portion, called the nucleus; also a clear space, the contractile vacuole (Fig. [10]).
Fig. 11.—The same amœba seen at different times.
Movements.—Sometimes while the pseudopods are being extended and contracted, the central portion remains in the same place (this is motion). Usually only one pseudopod is extended, and the body flows into it; this is locomotion (Fig. [11]). There is a new foot made for each step.
Feeding.—If the amœba crawls near a food particle, the pseudopod is pressed against it, or a depression occurs (Fig. [12]), and the particle is soon embedded in the endoplasm. Often a clear space called a food vacuole is noticed around the food particle. This is the water that is taken in with the particle (Fig. [12]). The water and the particle are soon absorbed and assimilated by the endoplasm.
Fig. 12.—The Amœba taking food.
Excretion.—If a particle of sand or other indigestible matter is taken in, it is left behind as the amœba moves on. There is a clear space called the contractile vacuole, which slowly contracts and disappears, then reappears and expands (Figs. [9] and [10]). This possibly aids in excreting oxidized or useless material.
Circulation in the amœba consists of the movement of its protoplasmic particles. It lacks special organs of circulation.
Feeling.—Jarring the glass slide seems to be felt, for it causes the activity of the amœba to vary. It does not take in for food every particle that it touches. This may be the beginning of taste, based upon mere chemical affinity. The pseudopods aid in feeling.
Reproduction.—Sometimes an amœba is seen dividing into two parts. A narrowing takes place in the middle; the nucleus also divides, a part going to each portion (Fig. [13]). The mother amœba finally divides into two daughter amœbas. Sex is wanting.
Fig. 13.—Amœba, Dividing.
Source of the Amœba’s Energy.—We thus see that the amœba moves without feet, eats without a mouth, digests without a stomach, feels without nerves, and, it should also be stated, breathes without lungs, for oxygen is absorbed from the water by its whole surface. Its movements require energy; this, as in all animals, is furnished by the uniting of oxygen with the food. Carbon dioxide and other waste products are formed by the union; these pass off at the surface of the amœba and taint the water with impurities.
Questions.—Why will the amœba die in a very small quantity of water, even though the water contains enough food? Why will it die still quicker if air is excluded from contact with the drop of water?
The amœba never dies of old age. Can it be said to be immortal?
According to the definition of a cell (Chapter I), is the amœba a unicellular or multicellular animal?
Cysts.—If the water inhabited by a protozoan dries up, it encysts, that is, it forms a tough skin called a cyst. Upon return of better conditions it breaks the cyst and comes out. Encysted protozoans may be blown through the air: this explains their appearance in vessels of water containing suitable food but previously free from protozoans.