XIII. SINGLE-CELLED ANIMALS CONSIDERED AS ORGANISMS
_Problems.--To determine:_ _(a) How a one-celled animal is influenced by its environment._ _(b) How a single cell performs its functions._ _(c) The structure of a single-celled animal._
LABORATORY SUGGESTIONS
_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.
REFERENCE BOOKS
ELEMENTARY
Hunter, _Laboratory Problems in Civic Biology_. American Book Company.
Davison, _Human Body and Health_. American Book Company.
Jordan, Kellogg and Heath, _Animal Studies_. D. Appleton and Company.
Sharpe, _Laboratory Manual_, pp. 140-143. American Book Company.
ADVANCED
Calkins, _The Protozoa_. Macmillan Company.
Jennings, _Study of the Lower Organisms_. Carnegie Institution Report.
Parker, _Lessons in Elementary Biology_. The Macmillan Company.
Wilson, _The Cell in Development and Inheritance_. The Macmillan Company.
XIV. DIVISION OF LABOR. THE VARIOUS FORMS OF PLANTS AND ANIMALS
_Problems.--The development and forms of plants._ _The development of a simple animal._ _What is division of labor? In what does it result?_ _How to know the chief characters of some great animal groups._
LABORATORY SUGGESTIONS
_A visit to a botanical garden or laboratory demonstration._--Some of the forms of plant life. Review of essential facts in development of bean or corn embryo.