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 Paramœcium.—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 Paramœcium.—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 paramœcium 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 paramœcia 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 paramœcia shows thousands of tiny whitish bodies collected near the surface of the jar. In the paramœcium, 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.
A paramœcium. c.v., contractile vacuole; f.v., food vacuole;m, mouth; ma.n., macronucleus; mi.n., micronucleus; w.v., water vacuole.
The Structure of Paramœcium.—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 paramœcium 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.
Paramœcium dividing by fission. M, mouth; MAC., macronucleus; MIC., micronucleus. (After Sedgwick and Wilson.)
Reproduction of Paramœcium.—Sometimes a paramœcium 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.
Amœba, with pseudopodia (P.) extended; EC, ectoplasm; END, endoplasm; the dark area (N.) is the nucleus. (From a photograph loaned by Professor G. N. Calkins.)
Amœba.[25]—In order to understand more fully the life of a simple bit of protoplasm, let us take up the study of the amœba, a type of the simplest form of animal life. Unlike the plant and animal cells we have examined, the amœba 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.
Although but a single cell, still the amœba 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 paramœcium, 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.
Amœba, 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 amœba, 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 amœba, they each in turn divide. This is a kind of asexual reproduction.
When conditions unfavorable for life come, the amœba, 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.
Vorticella. e, gullet; n, nucleus; cv, contractile vacuole; a, axis; s, sheath; fv, food vacuole. (From Herrick's General Zoölogy.)
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 amœba 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 paramœcium 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.)
[25] Amœbæ 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 algæ growing in quiet localities. No sure method of obtaining them can be given.
[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.
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.
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.
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.
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.
A colony of trilliums, a flowering plant. (Photograph by W. C. Barbour.)
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.
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 Algæ and the Fungi; the algæ being green, while the fungi have no chlorophyll. |
Rockweed, a brown algæ, 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 algæ and fungi), approximately 55,000 species of fungi, and about 16,000 species of algæ.
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.
Stages in the development of a fertilized egg into the gastrula stage. Read your text, then draw these stages and name each stage.
Photograph of a living vorticella, showing the contractile stalk and the cilia around the mouth. Compare this figure with that of the paramœcium. Which cell shows greater division of labor?
Physiological Division of Labor.—If we compare the amœba and the paramœcium, we find the latter a more complex organism than the former. An amœba may take in food through any part of the body; the paramœcium has a definite gullet; the amœba may use any part of the body for locomotion; the paramœcium has definite parts of the cell, the cilia, fitted for this work. Since the structure of the paramœcium 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.
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.
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.
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 flagellæ 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 coördinated 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 glasslike skeleton of a radiolarian, a protozoan. (From model at American Museum of Natural History.)
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 Amœba or building limy (Foraminifera) or glasslike skeletons (Radiolaria).
Class II. Infusoria (in infusions). Usually active ciliated Protozoa. Examples, Paramœcium, Vorticella.
Class III. Sporozoa (spore animals). Parasitic and usually nonactive. Example, Plasmodium malariæ.
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.
A horny fiber sponge. Notice that it is a colony. One fourth natural size.
III. Cœlenterates.—The hydra and its salt-water allies, the jellyfish, hydroids, and corals, belong to a group of animals known as the Cœlenterata. The word "cœlenterate" (cœlom = body cavity, enteron = food tube) explains the structure of the group. They are animals in which the real body cavity is lacking, the animal in its simplest form being little more than a bag. Some examples are the hydra, shown on page 179, salt-water forms known as hydroids, colonial forms which have part of their life free swimming as jellyfish; sea anemones and coral polyps, tiny colonial hydra like forms which build a living or secreted covering.
Sea anemones. One half natural size. The right hand specimen is expanded and shows the mouth surrounded by the tentacles. The left hand specimen is contracted. (From model at the American Museum of Natural History.)
IV. Worms.—The wormlike animals are grouped into flatworms, roundworms, and segmented or jointed worms.
(a) Flatworms are sometimes parasitic, examples being the tapeworm and liver fluke. They are usually small, ribbon- or leaf-like and flat and live in water.
(b) Roundworms, minute threadlike creatures, are not often seen by the city girl or boy. Vinegar eels, the horsehair worm, the pork worm or trichina and the dread hookworm are examples.
(c) Segmented worms are long, jointed creatures composed of body rings or segments. Examples are the earthworm, the sandworm (known to New York boys as the fishworm), and the leeches or bloodsuckers.
A jointed worm. The sandworm. Slightly reduced.
The common starfish seen from below to show the tube feet. About one half natural size.
V. Echinoderms.—These are spiny-skinned animals, which live in salt water. They are still more complicated in structure than the worms and may be known by the spines in their skin. They show radial symmetry. Starfish or sea urchins are examples.
The crayfish, a crustacean. A, antenna; M, mouth; E, compound stalked eye; Ch, pincher claw; C.P., cephalothorax; Ab, abdomen; C.F., caudal fin. A little reduced.
VI. Arthropods.—These animals are distinguished by having jointed body and legs. They form two great groups. The higher forms of the Crustacea have only two regions in the body, a fused head and thorax, called the cephalothorax, and an abdominal region. A second group is the Insecta, of which we know something already. Crustacea breathe by means of gills, which are structures for taking oxygen out of the water, while adult insects breathe through air tubes called trachea.
A common snail, a mollusk. (From a photograph by Davison.)
Two smaller groups of arthropods also exist, the Arachnida, consisting of spiders, scorpions, ticks, and mites, and the Myriapoda, examples being the "thousand leggers" found in some city houses.
VII. Mollusca.—Another large group is the Mollusca. This phylum gets its name from the soft, unsegmented body (mollis = soft). Mollusks usually have a shell, which may be of one piece, as a snail, or two pieces or valves, as the clam or oyster.
The skeleton of a dog; a typical vertebrate.
VIII. The Vertebrates.—All of the animals we have studied thus far agree in having whatever skeleton or hard parts they possess on the outside of the body. Collectively, they are called Invertebrates. This exoskeleton differs from the main or axial skeleton of the higher animals, the latter being inside of the body. The exoskeleton is dead, being secreted by the cells lining the body, while the endoskeleton is, in part at least, alive and is capable of growth, e.g. a broken arm or leg bone will grow together. But a man has certain parts of the skeleton, as nails or hair, formed by the skin and in addition possesses inside bones to which the muscles are attached. Some of the bones are arranged in a flexible column in the dorsal (the back) side of the body. This vertebral column, as it is called, is distinctive of all vertebrates. Within its bony protection lies the delicate central nervous system, and to this column are attached the big bones of the legs and arms. The vertebrate animals deserve more of our attention than other forms of life because man himself is a vertebrate.
The sand shark, an elasmobranch. Note the slits leading from the gills. (From a photograph loaned by the American Museum of Natural History.)
Five groups or classes of vertebrates exist. Fishes, Amphibians, Reptiles, Birds, and Mammals. Let us see how to distinguish one class from another.
The sturgeon, a ganoid fish.
Fishes.—Fishes are familiar animals to most of us. We know that they live in the water, have a backbone, and that they have fins. They breathe by means of gills, delicate organs fitted for taking oxygen out of the water. The heart has two chambers, an auricle and a ventricle. They have a skin in which are glands secreting mucus, a slimy substance which helps them go through the water easily. They usually lay very many eggs.
Classification of Fishes
Order I. The Elasmobranchs. Fishes which have a soft skeleton made of cartilage and exposed gill slits. Examples: sharks, skates, and rays.
Order II. The Ganoids. Fishes which once were very numerous on the earth, but which are now almost extinct. They are protected by platelike scales. Examples: gars, sturgeon, and bowfin.
A bony fish.
Order III. The Teleosts, or Bony Fishes. They compose 95 per cent of all living fishes. In this group the skeleton is bony, the gills are protected by an operculum, and the eggs are numerous. Most of our common food fishes belong to this class.
Order IV. The Dipnoi, or Lung Fishes. This is a very small group. In many respects they are more like amphibians than fishes, the swim bladder being used as a lung. They live in tropical Africa, South America, and Australia, inhabiting the rivers and lakes there.
Newt. (From a photograph loaned by the American Museum of Natural History.) About natural size.
Characteristics of Amphibia.—The frog belongs to the class of vertebrates known as Amphibia. As the name indicates (amphi, both, and bia, life), members of this group live both in water and on land. In the earlier stages of their development they take oxygen into the blood by means of gills. When adult, however, they breathe by means of lungs. At all times, but especially during the winter, the skin serves as a breathing organ. The skin is soft and unprotected by bony plates or scales. The heart has three chambers, two auricles and one ventricle. Most amphibians undergo a complete metamorphosis, or change of form, the young being unlike the adults.
Order I. Urodela. Amphibia having usually poorly developed appendages. Tail persistent through life. Examples: mud puppy, newt, salamander.
Order II. Anura. Tailless Amphibia, which undergo a metamorphosis, breathing by gills in larval state, by lungs in adult state. Examples: toad and frog.
The leopard frog, an amphibian.
Characteristics of Reptilia.—These animals are characterized by having scales developed from the skin. In the turtle they have become bony and are connected with the internal skeleton. Reptiles always breathe by means of lungs, differing in this respect from the amphibians. They show their distant relationship to birds in that their large eggs are incased in a leathery, limy shell.
Classification of Reptiles
Order I. Chelonia (turtles and tortoises). Flattened reptiles with body inclosed in bony case. No teeth or sternum (breastbone). Examples: snapping turtle, box tortoise.
Order II. Lacertilia (lizards). Body covered with scales, usually having two-paired appendages. Breathe by lungs. Examples: fence lizard, horned toad.
Order III. Ophidia (snakes). Body elongated, covered with scales. No limbs present. Examples: garter snake, rattlesnake.
Order IV. Crocodilia. Fresh-water reptiles with elongated body and bony scales on skin. Two-paired limbs. Examples: alligator, crocodile.