Part 13

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.

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.

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.

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. _Demonstration._--Charts or models showing the development of a many-celled animal from egg through gastrula stage. _Demonstration._--Types which illustrate increasing complexity of body form and division of labor. _Museum trip._--To afford pupil a means of identification of examples of principal phyla. This should be preceded by objective demonstration work in school laboratory.

Reproduction in Plants.--Although there are very many plants and animals so small and so simple as to be composed of but a single cell, by far the greater part of the animal and plant world is made up of individuals which are collections of cells living together.

In a simple plant like the pond scum, a string or filament of cells is formed by a single cell dividing crosswise, the two cells formed each dividing into two more. Eventually a long thread of cells is thus formed. At times, however, a cell is formed by the union of two cells, one from each of two adjoining filaments of the plant. At length a hard coat forms around this cell, which has now become a _spore_. The tough covering protects it from unfavorable changes in the surroundings. Later, when conditions become favorable for its germination, the spore may form a new filament of pond scum. In molds, in yeasts, and in the bacteria we also found spores could be formed by the protoplasm of the plant cutting up into a number of tiny spores. These spores are called _asexual_ (without sex) because they are not formed by the union of two cells, and may give rise to other tiny plants like themselves. Still other plants, mosses and ferns, give rise to two kinds of spores, sexual and asexual. All of these collectively are called _spore plants_.

Reproduction in Seed Plants.--Another great group of plants we have studied, plants of varied shapes and sizes, produce seeds. They bear flowers and fruits.

The embryo develops from a single fertilized "egg," growing by cell division into two, four, eight, and a constantly increasing number of cells until after a time a baby plant is formed, which as in the bean, either contains some stored food to give it a start in life, or, as in the corn, is surrounded with food which it can digest and absorb into its own tiny body. We have seen that these young plants in the seed are able to develop when conditions are favorable. Furthermore, the young of each kind of plant will eventually develop into the kind of plant its parent was and into no other kind. Thus the plant world is divided into many tribes or groups.

Plants are placed in Groups.--If we plant a number of peas so that they will all germinate under the same conditions of soil, temperature, and sunlight, the seedlings that develop will each differ one from another in a slight degree.[27] But in a general way they will have many characters in common, as the shape of the leaves, the possession of tendrils, form of the flower and fruit. A _species_ of plants or animals is a group of individuals so much alike in their characters that they might have had the same parents. Individuals of such species differ slightly; for no two individuals are exactly alike.

Footnote 27: NOTE TO TEACHERS.--A trip to the Botanical Garden or to a Museum should be taken at this time.

Species are grouped together in a larger group called a genus. For example, many kinds of peas--the wild beach peas, the sweet peas, and many others--are all grouped in one genus (called _Lathyrus_, or vetchling) because they have certain structural characteristics in common.

Plant and animal genera are brought together in still larger groups, the classification based on general likenesses in structure. Such groups are called, as they become successively larger, _Family_, _Order_, and _Class_. Thus both the plant and animal kingdoms are grouped into divisions, the smallest of which contains individuals very much alike; and the largest of which contains very many groups of individuals, the groups having some characters in common. This is called a system of classification.

Classification of the Plant Kingdom.--The entire plant kingdom has been divided into four sub-kingdoms by botanists:--

1. _Spermatophytes._ { _Angiosperms_, true flowering plants. { _Gymnosperms_, the pines and their allies. 2. _Pteridophytes._ The fern plants and their allies. 3. _Bryophytes._ The moss plants and their allies 4. _Thallophytes._ The Thallophytes form two groups: the Algae and the Fungi; the algae being green, while the fungi have no chlorophyll.

The extent of the plant kingdom can only be hinted at; each year new species are added to the lists. There are about 110,000 species of flowering plants and nearly as many flowerless plants. The latter consist of over 3500 species of fernlike plants, some 16,500 species of mosses, over 5600 lichens (plants consisting of a partnership between algae and fungi), approximately 55,000 species of fungi, and about 16,000 species of algae.

Development of a Simple Animal.--Many-celled animals are formed in much the same way as are many-celled seed plants. A common bath sponge, an earthworm, a fish, or a dog,--each and all of them begin life in the same manner. In a many-celled animal the life history begins with a single cell, the fertilized egg. As in the flowering plant, this cell has been formed by the union of two other cells, a tiny (usually motile) cell; the _sperm_, and a large cell, the _egg_. After the egg is fertilized by a sperm cell, it splits into two, four, eight, and sixteen cells; as the number of cells increases, a hollow ball of cells called the _blastula_ is formed; later this ball sinks in on one side, and a double-walled cup of cells, now called a _gastrula_, results. Practically all animals pass through the above stages in their development from the egg, although these stages are often not plain to see because of the presence of food material (yolk) in the egg.

In animals the body consists of three layers of cells: those of the outside, developed from the outer layer of the gastrula, are called _ectoderm_, which later gives rise to the skin, nervous system, etc.; an inner layer, developed from the inner layer of the gastrula, the _endoderm_, which forms the lining of the digestive organs, etc.; a middle layer, called the _mesoderm_, lying between the ectoderm and the endoderm, is also found. In higher animals this layer gives rise to muscles, the skeleton, and parts of other internal structures.

Physiological Division of Labor.--If we compare the amoeba and the paramoecium, we find the latter a more complex organism than the former. An amoeba may take in food through any part of the body; the paramoecium has a definite gullet; the amoeba may use any part of the body for locomotion; the paramoecium has definite parts of the cell, the cilia, fitted for this work. Since the structure of the paramoecium is more complex, we say that it is a "higher" animal. In the vorticella, a still more complex cell, part of the cell has grown out like a stalk, has become contractile, and acts like muscle.

As we look higher in the scale of life, we invariably find that certain parts of a plant or animal are set apart to do certain work, and only that work. Just as in a community of people, there are some men who do rough manual work, others who are skilled workmen, some who are shopkeepers, and still others who are professional men, so among plants and animals, wherever _collections_ of cells live together to form an organism, there is division of labor, some cells being fitted to do one kind of work, while others are fitted to do work of another sort. This is called physiological division of labor.

As we have seen, the higher plants are made up of a vast number of cells of many kinds. Collections of cells alike in structure and performing the same function we have called a _tissue_. Examples of animal tissues are the highly contractile cells set apart for movement, _muscles_; those which cover the body or line the inner parts of organs, the skin, or _epithelium_; the cells which form secretions or _glands_ and the sensitive cells forming the _nervous_ tissues.

Frequently several tissues have certain functions to perform in conjunction with one another. The arm of the human body performs movement. To do this, several tissues, as muscles, nerves, and bones, must act together. A collection of tissues performing certain work we call an _organ_.

In a simple animal like a sponge, division of labor occurs between the cells; some cells which line the pores leading inward create a current of water, and feed upon the minute organisms which come within reach, other cells build the skeleton of the sponge, and still others become eggs or sperms. In higher animals more complicated in structure and in which the tissues are found working together to form organs, division of labor is much more highly specialized. In the human arm, an organ fitted for certain movements, think of the number of tissues and the complicated actions which are possible. The most extreme division of labor is seen in the organism which has the most complex actions to perform and whose organs are fitted for such work, for there the cells or tissues which do the particular work do it quickly and very well.

In our daily life in a town or city we see division of labor between individuals. Such division of labor may occur among other animals, as, for example, bees or ants. But it is seen at its highest in a great city or in a large business or industry. In the stockyards of Chicago, division of labor has resulted in certain men performing but a single movement during their entire day's work, but this movement repeated so many times in a day has resulted in wonderful accuracy and speed. Thus division of labor obtains its end.

Organs and Functions Common to All Animals.--The same general functions performed by a single cell are performed by a many-celled animal. But in the many-celled animals the various functions of the single cell are taken up by the organs. In a complex organism, like man, the organs and the functions they perform may be briefly given as follows:--

(1) The organs of _food taking_: food may be taken in by individual cells, as those lining the pores of the sponge, or definite parts of a food tube may be set apart for this purpose, as the mouth and parts which place food in the mouth.

(2) The organs of _digestion_: the food tube and collections of cells which form the glands connected with it. The enzymes in the fluids secreted by the latter change the foods from a solid form (usually insoluble) to that of a _fluid_. Such fluid may then pass by osmosis, through the walls of the food tube into the blood.

(3) The organs of _circulation_: the tubes through which the blood, bearing its organic foods and oxygen, reaches the tissues of the body. In simple animals, as the sponge and hydra, no such organs are needed, the fluid food passing from cell to cell by osmosis.

(4) The organs of _respiration_: the organs in which the blood receives oxygen and gives up carbon dioxide. The outer layer of the body serves this purpose in very simple animals; gills or lungs are developed in more complex animals.

(5) The organs of _excretion_: such as the kidneys and skin, which pass off nitrogenous and other waste matters from the body.

(6) The organs of _locomotion_: muscles and their attachments and connectives; namely, tendons, ligaments, and bones.

(7) The organs of _nervous control_: the central nervous system, which has control of coordinated movement. This consists of scattered cells in low forms of life; such cells are collected into groups and connected with each other in higher animals.

(8) The organs of _sense_: collections of cells having to do with the reception and transmission of sight, hearing, smell, taste, touch, pressure, and temperature sensations.

(9) The organs of _reproduction_: the sperm and egg-forming organs.

Almost all animals have the functions mentioned above. In most, the various organs mentioned are more or less developed, although in the simpler forms of animal life some of the organs mentioned above are either very poorly developed or entirely lacking. But in the so-called "higher" animals each of the above-named functions is assigned to a certain organ or group of organs. The work is done better and more quickly than in the "lower" animals. Division of labor is thus a guide in helping us to determine the place of animals in the groups that exist on the earth.

The Animal Series.--We have found that a one-celled animal can perform certain functions in a rather crude manner. Man can perform these same functions in an extremely efficient manner. Division of labor is well worked out, extreme complexity of structure is seen. Between these two extremes are a great many groups of animals which can be arranged more or less as a series, showing the gradual evolution or development of life on the earth. It will be the purpose of the following pages to show the chief characteristics of the great groups of the animal kingdom.

I. Protozoa.--Animals composed of a single cell, reproducing by cell division.

The following are the principal classes of Protozoa, examples of which we may have seen or read about:--

CLASS I. _Rhizopoda_ (Greek for _root-footed_). Having no fixed form, with pseudopodia. Either naked as _Amoeba_ or building limy (_Foraminifera_) or glasslike skeletons (_Radiolaria_).

CLASS II. _Infusoria (in infusions)._ Usually active ciliated Protozoa. Examples, _Paramoecium_, _Vorticella_.

CLASS III. _Sporozoa (spore animals)._ Parasitic and usually nonactive. Example, _Plasmodium malariae_.

II. Sponges.--Because the body contains many pores through which water bearing food particles enters, these animals are called _Porifera_. They are classed according to the skeleton they possess into limy, glasslike, and horny fiber sponges. The latter are the sponges of commerce. With but few exceptions sponges live in salt water and are never free swimming.