_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.
[Illustration: A cell of pond scum. How might it divide to form a long thread made up of cells?]
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_.
[Illustration: The formation of spores in pond scum. _zs_, zygospore; _f_, fusion in progress.]
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.
[Illustration: The formation and growth of a plant embryo. 1, the sperm and egg cell uniting; 2, a fertilized egg; 3, two cells formed by division; 4, four cells formed from two; 5, a many-celled embryo; 6, young plant; _H_, hypocotyl; _P_, plumule; _C_, cotyledons.]
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.
[Illustration: A colony of trilliums, a flowering plant. (Photograph by W.
C. Barbour.)]
[Illustration: Rock fern, _polypody_. Notice the underground stem giving off roots from its lower surface, and leaves (_C_), (_S_), from its upper surface.]
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.
[Illustration: Rockweed, a brown algae, showing its distribution on rocks below highwater mark.]
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.
[Illustration: A moss plant. _G_, the moss body; _S_, the spore-bearing stalk (fruiting body).]
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.
[Illustration: Stages in the development of a fertilized egg into the gastrula stage. Read your text, then draw these stages and name each stage.]
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.
[Illustration: Photograph of a living _vorticella_, showing the contractile stalk and the cilia around the mouth. Compare this figure with that of the paramoecium. Which cell shows greater division of labor?]
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.
[Illustration: Enlarged lengthwise section of the hydra, a very simple animal which shows slight division of labor. _ba_, base; _b_, bud; _m_, mouth; _ov_, ovary; _sp_, spermary.]
[Illustration: Different forms of tissue cells. _C_, bone making cells; _E_, epithelial cells; _F_, fat cells; _L_, liver cells; _M_, muscle cell; _i_, involuntary; _v_, voluntary; _N_, nerve cell; _C B_, cell body; _N.F._, nerve fiber; _T.B._, nerve endings; _W_, colorless blood cells.]
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_.
[Illustration: Part of a sponge, showing how cells perform division of labor. _ect_, ectoderm; _mes_, mesoderm; _end_, endoderm; _c.c._, ciliated cells, which take in food by means of their flagellae or large cilia (_fla_).]
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.
[Illustration: The glasslike skeleton of a _radiolarian_, a protozoan.
(From model at American Museum of Natural History.)]