CHAPTER II

PROTOZOA (One-celled Animals)

THE AMEBA

SUGGESTIONS.--Amebas live on 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 ameba may be found in the brown slime scraped from the plants. An ameba 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 ¹⁄₅ or ¹⁄₆ 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.

[Illustration: FIG. 9.--AMEBA PROTEUS, much enlarged.]

[Illustration: FIG. 10.--AMEBA.

_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 ameba (also spelled 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).

=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.

[Illustration: FIG. 11.--The same ameba seen at different times.]

=Feeding.=--If the ameba 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.

[Illustration: FIG. 12.--THE AMEBA taking food.]

=Excretion.=--If a particle of sand or other indigestible matter is taken in, _it is left behind_ as the ameba 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 ameba 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 ameba 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 ameba 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 ameba finally divides into two daughter amebas. Sex is wanting.

[Illustration: FIG. 13.--AMEBA, dividing.]

=Source of the Ameba’s Energy.=--We thus see that the ameba 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 dioxid and other waste products are formed by the union; these pass off at the surface of the ameba and taint the water with impurities.

=Questions.=--Why will the ameba 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 ameba never dies of old age. Can it be said to be immortal?

According to the definition of a cell (_Chapter I_), is the ameba 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.

THE SLIPPER ANIMALCULE OR PARAMECIUM

=Suggestions.=--Stagnant water often contains the paramecium as well as the ameba; or they may be found in a dish of water containing hay or finely cut clover, after the dish has been allowed to stand in the sun for several days. A white film forming on the surface is a sign of their presence. They may even be seen with the unaided eye as tiny white particles by looking through the side of the dish or jar. Use at first a ¹⁄₃ or ¹⁄₄ in. objective. Restrict their movements by placing cotton fibers beneath the cover glass; then examine with ¹⁄₅ or ¹⁄₆ objective. Otherwise, study figures.

=Shape and Structure.=--The paramecium’s whole body, like the ameba’s, is only one cell. It resembles a slipper in _shape_, but the pointed end is the hind end, the _front end_ being rounded (Fig. 14). The paramecium is propelled by the rapid beating of numerous fine, threadlike appendages on its surface, called _cilia_ (Latin, eyelashes) (Figs.). The cilia, like the pseudopods of the ameba, are merely prolongations of the cell protoplasm, but they are permanent. The separation between the outer _ectoplasm_ and the interior granular _endoplasm_ is more marked than in the ameba (Fig. 14).

[Illustration: FIG. 14.--PARAMECIUM, showing cilia, _c_.

Two contractile vacuoles, _cv_; the macronucleus, _mg_; two micronuclei, _mi_; the gullet (_Œ_), a food ball forming and ten food balls in their course from gullet to vent, _a_.]

[Illustration: FIG. 15.]

=Nucleus and Vacuoles.=--There is a large nucleus called the macronucleus, and beside it a smaller one called the micronucleus. They are hard to see. About one third of the way from each end is a clear, pulsating space (bb. Fig. 15) called the pulsating vacuole. These spaces contract until they disappear, and then reappear, gradually expanding. Tubes lead from the vacuoles which probably serve to keep the contents of the cell in circulation.

[Illustration: FIG. 16.--TWO PARAMECIA exchanging parts of their nuclei.]

=Feeding.=--A depression, or _groove_, is seen on one side, this serves as a mouth (Figs.). A _tube_ which serves as a gullet leads from the mouth-groove to the interior of the cell. The mouth-groove is lined with cilia which sweep food particles inward. The particles accumulate in a mass at the inner end of the gullet, become separated from it as a _food ball_ (Fig. 14), and sink into the soft protoplasm of the body. The food balls follow a circular course through the endoplasm, keeping near the ectoplasm.

=Reproduction.=--This, as in the ameba, is by division, the constriction being in the middle, and part of the nucleus going to each half. Sometimes two individuals come together with their mouth-grooves touching and exchange parts of their nuclei (Fig. 16). They then separate and each divides to form two new individuals.

We thus see that the paramecium, though of only one cell, _is a much more complex and advanced animal than the ameba_. The tiny paddles, or cilia, the mouth-groove, etc., have their special duties similar to the specialized organs of the many-celled animals to be studied later.

[Illustration: FIG. 17.--VORTICELLA (or bell animalcule), two extended, one withdrawn.]

[Illustration: FIG. 18.--Euglena.]

[Illustration: FIG. 19.--SHELL OF A RADIOLARIAN.]

If time and circumstances allow a prolonged study, several additional facts may be observed by the pupil, _e.g._ Does the paramecium swim with the same end always foremost, and same side uppermost? Can it move backwards? Avoid obstacles? Change shape in a narrow passage? Does refuse matter leave the body at any particular place? Trace movement of the food particles.

Draw the paramecium.

Which has more permanent parts, the _ameba_ or _paramecium_? Name two anatomical similarities and three differences; four functional similarities and three differences.

The ameba belongs in the class of protozoans called _Rhizopoda_ “root footed.”

=Other classes of Protozoans= are the _Infusorians_, which have many waving cilia (Fig. 17) or one whip-like flagellum (Fig. 18), and the _Foraminifers_ which possess a calcareous shell pierced with holes. Much chalky limestone has been formed of their shells. These and the radiolarians, which have flinty shells (Fig. 19), are often placed in the class rhizopoda. To which class does the paramecium belong?

Protozoans furnish a large amount of food to the higher animals.

=To the Teacher.= If plant, animal, and human biology are to be given in one year as planned, and full time allowed for practical work, the portions of the text in small type, as Chapter III, may be omitted or merely read and discussed. Any two of the three parts forming the course may be used for a year’s course by using all of the text and spending more time on practical and field work.