PROTOZOA (One-celled Animals)
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.
[Illustration:
FIG. 9.—AMŒBA PROTEUS, much enlarged.
]
[Illustration:
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).
[Illustration:
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.
[Illustration:
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.
[Illustration:
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.
THE SLIPPER ANIMALCULE OR PARAMECIUM
SUGGESTIONS.—Stagnant water often contains the paramecium as well as
the amœba; 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 ⅓ or ¼ in. objective. Restrict their movements by placing
cotton fibres beneath the cover glass; then examine with ⅕ or ⅙
objective. Otherwise, study figures.
=Shape and Structure.=—The paramecium’s whole body, like the amœba’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 amœba, 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 amœba (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.
[Illustration:
FIG. 17.—VORTICELLA (or bell animalcule), two extended, one withdrawn.
]
=Reproduction.=—This, as in the amœba, 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.
[Illustration:
FIG. 18.—Euglena.
]
We thus see that the paramecium, though of only one cell, _is a much
more complex and advanced animal than the amœba_. 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. 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 _amœba_ or _paramecium_? Name two
anatomical similarities and three differences; four functional
similarities and three differences.
The amœba belongs in the class of protozoans called _Rhizopoda_ “root
footed.”
=Other classes of Protozoans= are the _Infusorians_ (in the broad sense
of the term), which have many waving cilia (Fig. 17) or one whiplike
flagellum (Fig. 18), and the _Foraminifers_, which possess a calcareous
shell pierced with holes (Fig. 19). Much chalky limestone has been
formed of their shells. To which class does the paramecium belong?
Protozoans furnish a large amount of food to the higher animals.