_Laboratory study._--Study of paramoecium under compound
microscope in its relation to food, oxygen, etc.
Determination of method of movement, turning, avoiding
obstructions, sensitiveness to stimuli. Drawings to
illustrate above points.
_Laboratory demonstration._--Living paramoecium to show
structure of cell. Demonstration with carmine to show food
vacuoles, and action of cilia. Use of charts and stained
specimens to show other points of cell structure. Laboratory
demonstration of fission.
[Illustration: Pleurococcus. A very simple plant cell.]
The Simplest Plants.--We have seen that perhaps the simplest plant would be
exemplified by one of the tiny bacteria we have just read about. A typical
one-celled plant, however, would contain green coloring matter or
chlorophyll, and would have the power to manufacture its own food under
conditions giving it a moderate temperature, a supply of water, oxygen,
carbon dioxide, and sunlight. Such a simple plant is the _pleurococcus_,
the "green slime" seen on the shady sides of trees, stones, or city houses.
This plant would meet one definition of a cell, as it is a minute mass of
protoplasm containing a nucleus. It is surrounded by a wall of a woody
material formed by the activity of the living matter within the cell. It
also contains a little mass of protoplasm colored green. Of the work of the
chlorophyll in the manufacture of organic food we have already learned.
Such is a simple plant cell. Let us now examine a simple animal cell in
order to compare it with that of a plant.
Where to find Paramoecium.--If we examine very carefully the surface of a
hay infusion, we are likely to notice in addition to the scum formed of
bacteria, a mass of whitish tiny dots collected along the edge of the jar
close to the surface of the water. More attentive observation shows us that
these objects move, and that they are never found far from the surface.
The Life Habits of Paramoecium.--If we place on a slide a drop of water
containing some of these moving objects and examine it under the compound
microscope, we find each minute whitish dot is a cell, elongated, oval, or
elliptical in outline and somewhat flattened. This is a one-celled animal
known as the _paramoecium_ or the slipper animalcule (because of its
shape).
Seen under the low power of the microscope, it appears to be extremely
active, rushing about now rapidly, now more slowly, but seemingly always
taking a definite course. The narrower end of the body (the _anterior_)
usually goes first. If it pushes its way past any dense substance in the
water, the cell body is seen to change its shape temporarily as it squeezes
through.
Response to Stimuli.--Many of these little creatures may be found collected
around masses of food, showing that they are attracted by it. In another
part of the slide we may find a number of the paramoecia lying close to the
edge of an air bubble with the greatest possible amount of their surface
exposed to its surface. These animals are evidently taking in oxygen by
osmosis. They are breathing. A careful inspection of the jar containing
paramoecia shows thousands of tiny whitish bodies collected near the
surface of the jar. In the paramoecium, as in the one-celled plants, the
protoplasm composing the cell responds to certain agencies acting upon it,
coming from without; these agencies we call _stimuli_. Such stimuli may be
light, differences of temperature, presence of food, electricity, or other
factors of its surroundings. Plant and animal cells may react differently
to the same stimulus. In general, however, we know that protoplasm is
_irritable_ to some of these factors. To severe stimuli, protoplasm usually
responds by _contracting_, another power which it possesses. We know, too,
that plant and animal cells take in food and change the food to protoplasm,
that is, that they _assimilate_ food; and that they may waste away and
repair themselves. Finally, we know that new plant and animal cells are
_reproduced_ from the original bit of protoplasm, a single cell.
[Illustration: A paramoecium. _c.v._, contractile vacuole; _f.v._, food
vacuole; _m_, mouth; _ma.n._, macronucleus; _mi.n._, micronucleus; _w.v._,
water vacuole.]
The Structure of Paramoecium.--The cell body is almost transparent, and
consists of semifluid protoplasm which has a granular grayish appearance
under the microscope. This protoplasm appears to be bounded by a very
delicate membrane through which project numerous delicate threads of
protoplasm called _cilia_. (These are usually invisible under the
microscope).
The locomotion of the paramoecium is caused by the movement of these cilia,
which lash the water like a multitude of tiny oars. The cilia also send
particles of food into a funnel-like opening, the _gullet_, on one side of
the cell. Once inside the cell body, the particles of food materials are
gathered into little balls within the almost transparent protoplasm. These
masses of food seem to be inclosed within a little area containing fluid,
called a _vacuole_. Other vacuoles appear to be clear; these are spaces in
which food has been digested. One or two larger vacuoles may be found;
these are the _contractile vacuoles_; their purpose seems to be to pass off
waste material from the cell body. This is done by pulsation of the
vacuole, which ultimately bursts, passing fluid waste to the outside. Solid
wastes are passed out of the cell in somewhat the same manner. No breathing
organs are seen, because osmosis of oxygen and carbon dioxide may take
place anywhere through the cell membrane. The nucleus of the cell is not
easily visible in living specimens. In a cell that has been stained it has
been found to be a double structure, consisting of one large and one small
portion, called, respectively, the _macronucleus_ and the _micronucleus_.
[Illustration: Paramoecium dividing by fission. _M_, mouth; _MAC._,
macronucleus; _MIC._, micronucleus. (After Sedgwick and Wilson.)]
Reproduction of Paramoecium.--Sometimes a paramoecium may be found in the
act of dividing by the process known as _fission_, to form two new cells,
each of which contains half of the original cell. This is a method of
_asexual_ reproduction. The original cell may thus form in succession many
hundreds of cells in every respect like the original parent cell.
[Illustration: Amoeba, with pseudopodia (_P._) extended; _EC_, ectoplasm;
_END_, endoplasm; the dark area (_N._) is the nucleus. (From a photograph
loaned by Professor G. N. Calkins.)]
Amoeba.[25]--In order to understand more fully the life of a simple bit of
protoplasm, let us take up the study of the _amoeba_, a type of the
simplest form of animal life. Unlike the plant and animal cells we have
examined, the amoeba has no fixed form. Viewed under the compound
microscope, it has the appearance of an irregular mass of granular
protoplasm. Its form is constantly changing as it moves about. This is due
to the pushing out of tiny projections of the protoplasm of the cell,
called _pseudopodia_ (false feet). The locomotion is accomplished by a
streaming or flowing of the semifluid protoplasm. The pseudopodia are
pushed forward in the direction which the animal is to go, the rest of the
body following. In the central part of the cell is the nucleus. This
important organ is difficult to see except in cells that have been stained.
Footnote 25: Amoebae _may_ be obtained from the hay infusion,
from the dead leaves in the bottom of small pools, from the
same source in fresh-water aquaria, from the roots of
duckweed or other small water plants, or from green algae
growing in quiet localities. No _sure_ method of obtaining
them can be given.
Although but a single cell, still the amoeba appears to be aware of the
existence of food when it is near at hand. Food may be taken into the body
at any point, the semifluid protoplasm simply rolling over and engulfing
the food material. Within the body, as in the paramoecium, the food becomes
inclosed within a fluid space or vacuole. The protoplasm has the power to
take out such material as it can use to form new protoplasm or give energy.
Circulation of food material is accomplished by the constant streaming of
the protoplasm within the cell.
[Illustration: Amoeba, showing the changes which take place during division
of the cell. The dark body in each figure is the nucleus; the transparent
circle, the contractile vacuole; the large granular masses, the food
vacuoles. Much magnified.]
The cell absorbs oxygen from the water by osmosis through its delicate
membrane, giving up carbon dioxide in return. Thus the cell "breathes"
through any part of its body covering.
Waste nitrogenous products formed within the cell when work is done are
passed out by means of the contractile vacuole.
The amoeba, like other one-celled organisms, reproduces by the process of
fission. A single cell divides by splitting into two others, each of which
resembles the parent cell, except that they are of less bulk. When these
become the size of the parent amoeba, they each in turn divide. This is a
kind of asexual reproduction.
When conditions unfavorable for life come, the amoeba, like some one-celled
plants, encysts itself within a membranous wall. In this condition it may
become dried and be blown through the air. Upon return to a favorable
environment, it begins life again, as before. In this respect it resembles
the spore of a plant.
[Illustration: Vorticella. _e_, gullet; _n_, nucleus; _cv_, contractile
vacuole; _a_, axis; _s_, sheath; _fv_, food vacuole. (From Herrick'_s
General Zoology_.)]
The Cell as a Unit.--In the daily life of a one-celled animal we find the
single cell performing all the general activities which we shall later find
the many-celled animal is able to perform. In the amoeba no definite parts
of the cell appear to be set off to perform certain functions; but any part
of the cell can take in food, can absorb oxygen, can change the food into
protoplasm, and excrete the waste material. The single cell is, in fact, an
organism able to carry on the business of living almost as effectually as a
very complex animal.
Complex One-celled Animals.--In the paramoecium we find a single cell, but
we find certain parts of the cell having certain definite functions: the
cilia are used for locomotion; a definite part of the cell takes in food,
while the waste passes out at another definite spot. In another one-celled
animal called _vorticella_, part of the cell has become elongated and is
contractile. By this stalk the little animal is fastened to a water plant
or other object. The stalk may be said to act like a muscle fiber, as its
sole function seems to be movement; the cilia are located at one end of the
cell and serve to create a current of water which will bring food particles
to the mouth. Here we have several parts of the cell, each doing a
different kind of work. This is known as _physiological division of labor_.
Habitat of Protozoa.--Protozoa are found almost everywhere in shallow
water, especially close to the surface. They appear to be attracted near to
the surface by the supply of oxygen. Every fresh-water lake swarms with
them; the ocean contains countless myriads of many different forms.
Use as Food.--They are so numerous in lakes, rivers, and the ocean as to
form the food for many animals higher in the scale of life. Almost all fish
that do not take the hook and that travel in schools, or companies,
migrating from one place to another, live partly on such food. Many feed on
slightly larger animals, which in turn eat the Protozoa. Such fish have on
each side of the mouth attached to the gills a series of small structures
looking like tiny rakes. These are called the _gill rakers_, and aid in
collecting tiny organisms from the water as it passes over the gills. The
whale, the largest of all mammals, strains protozoans and other small
animals and plants out of the water by means of hanging plates of whalebone
or baleen, the slender filaments of which form a sieve from the top to the
bottom of the mouth.
Protozoa cause Disease.--Protozoa of certain kinds play an important part
in causing malaria, yellow fever, and other diseases, as we shall see
later.[26] (See page 217.)
Footnote 26: Teachers may find it expedient to take up the
study of protozoan diseases at this point.