Chapter VI: Part 6

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.

III. Coelenterates.--The hydra and its salt-water allies, the jellyfish, hydroids, and corals, belong to a group of animals known as the _Coelenterata_. The word "coelenterate" (_coelom_ = 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.

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.

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.

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_.

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.

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.

Five groups or classes of vertebrates exist. _Fishes_, _Amphibians_, _Reptiles_, _Birds_, and _Mammals_. Let us see how to distinguish one class from another.

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.

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.

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.

CLASSIFICATION OF AMPHIBIA

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.

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.

Birds.--Birds among all other animals are known by their covering of feathers and the presence of wings. The feathers are developed from the skin. These aid in flight, and protect the body from the cold.

The form of the bill in particular shows adaptation to a wonderful degree. A duck has a flat bill for pushing through the mud and straining out the food; a bird of prey has a curved or hooked beak for tearing; the woodpecker has a sharp, straight bill for piercing the bark of trees in search of the insect larvae which are hidden underneath. Birds do not have teeth.

The rate of respiration, of heartbeat, and the body temperature are all higher in the bird than in man. Man breathes from twelve to fourteen times per minute. Birds breathe from twenty to sixty times a minute. Because of the increased activity of a bird, there comes a necessity for a greater and more rapid supply of oxygen, an increased blood supply to carry the material to be used up in the release of energy, and a means of rapid excretion of the wastes resulting from the process of oxidation. Birds are large eaters, and the digestive tract is fitted to digest the food quickly, by having a large crop in which food may be stored in a much softened condition. As soon as the food is part of the blood, it may be sent rapidly to the places where it is needed, by means of the large four-chambered heart and large blood vessels.

The high temperature of the bird is a direct result of this rapid oxidation; furthermore, the feathers and the oily skin form an insulation which does not readily permit of the escape of heat. This insulating cover is of much use to the bird in its flights at high altitudes, where the temperature is often very low. Birds lay eggs and usually care for their young.

CLASSIFICATION OF BIRDS

ORDER I. _Cursores._ Running birds with no keeled breastbone. Examples: ostrich, cassowary.

ORDER II. _Passeres._ Perching birds; three toes in front, one behind. Over one half of all species of birds are included in this order. Examples: sparrow, thrush, swallow.

ORDER III. _Gallinae._ Strong legs; feet adapted to scratching. Beak stout. Examples: jungle fowl, grouse, quail, domestic fowl.

ORDER IV. _Raptores._ Birds of prey. Hooked beak. Strong claws. Examples: eagle, hawk, owl.

ORDER V. _Grallatores._ Waders. Long neck, beak, and legs. Examples: snipe, crane, heron.

ORDER VI. _Natatores._ Divers and swimmers. Legs short, toes webbed. Examples: gull, duck, albatross.

ORDER VII. _Columbinae._ Like Gallinae, but with weaker legs. Examples: dove, pigeon.

ORDER VIII. _Pici._ Woodpeckers. Two toes point forward, two backward, and adaptation for climbing. Long, strong bill.

ORDER IX. _Psittaci._ Parrots, hooked beak and fleshy tongue.

ORDER X. _Coccyges._ Climbing birds, with powerful beak. Examples: kingfisher, toucan, and cuckoo.

ORDER XI. _Macrochires._ Birds having long-pointed wings, without scales on metatarsus. Examples: swift, humming bird, and goatsucker.

Mammals.--Dogs and cats, sheep and pigs, horses and cows, all of our domestic animals (and man himself) have characters of structure which cause them to be classed as mammals. They, like some other vertebrates, have lungs and warm blood. They also have a hairy covering and bear young developed to a form similar to their own,[28] and nurse them with milk secreted by glands known as the _mammary glands_; hence the term "mammal."

Footnote 28: With the exception of the monotremes.

Adaptations in Mammalia.--Of the thirty-five hundred species, most inhabit continents; a few species are found on different islands, and some, as the whale, inhabit the ocean. They vary in size from the whale and the elephant to tiny shrew mice and moles. Adaptations to different habitat and methods of life abound; the seal and whale have the limbs modified into flippers, the sloth and squirrel have limbs peculiarly adapted to climbing, while the bats have the fore limbs modeled for flight.

Lowest Mammals.--The lowest are the monotremes, animals which lay eggs like the birds, although they are provided with hairy covering like other mammals. Such are the Australian spiny anteater and the duck mole.

All other mammals bring forth their young developed to a form similar to their own. The kangaroo and opossum, however, are provided with a pouch on the under side of the body in which the very immature, blind, and helpless young are nourished until they are able to care for themselves. These pouched animals are called _marsupials_.

The other mammals may be briefly classified as follows:--

CLASSIFICATION OF HIGHER MAMMALS

ORDER I. _Edentata._ Toothless or with very simple teeth. Examples: anteater, sloth, armadillo.

ORDER II. _Rodentia._ Incisor teeth chisel-shaped, usually two above and two below. Examples: beaver, rat, porcupine, rabbit, squirrel.

ORDER III. _Cetacea._ Adapted to marine life. Examples: whale, porpoise.

ORDER IV. _Ungulata._ Hoofs, teeth adapted for grinding. Examples: (_a_) odd-toed, horse, rhinoceros, tapir; (_b_) even-toed, ox, pig, sheep, deer.

ORDER V. _Carnivora._ Long canine teeth, sharp and long claws. Examples: dog, cat, lion, bear, seal, and sea lion.

ORDER VI. _Insectivora._ Example: mole.

ORDER VII. _Cheiroptera._ Fore limbs adapted to flight, teeth pointed. Example: bat.

ORDER VIII. _Primates._ Erect or nearly so, fore appendage provided with hand. Examples: monkey, ape, man.

Increasing Complexity of Structure and of Habits in Plants and Animals.--In our study of biology so far we have attempted to get some notion of the various factors which act upon living things. We have seen how plants and animals interact upon each other. We have learned something about the various physiological processes of plants and animals, and have found them to be in many respects identical. We have found grades of complexity in plants from the one-celled plant, bacterium or pleurococcus, to the complicated flowering plants of considerable size and with many organs. So in animal life, from the Protozoa upward, there is constant change, and the change is toward greater complexity of structure and functions. An insect is a higher type of life than a protozoan, because its structure is more complex and it can perform its work with more ease and accuracy. A fish is a higher type of animal than the insect for these same reasons, and also for another. The fish has an internal skeleton which forms a pointed column of bones on the _dorsal_ side (the back) of the animal. It is a vertebrate animal.

The Doctrine of Evolution.--We have now learned that animal forms may be arranged so as to begin with very simple one-celled forms and culminate with a group which contains man himself. This arrangement is called the _evolutionary series_. Evolution means change, and these groups are believed by scientists to represent stages in complexity of development of life on the earth. Geology teaches that millions of years ago, life upon the earth was very simple, and that gradually more and more complex forms of life appeared, as the rocks formed latest in time show the most highly developed forms of animal life. The great English scientist, Charles Darwin, from this and other evidence, explained the theory of evolution. This is the belief that simple forms of life on the earth slowly and gradually gave rise to those more complex and that thus ultimately the most complex forms came into existence.

The Number of Animal Species.--Over 500,000 species of animals are known to exist to-day, as the following table shows.

Protozoa 8,000 Arachnids 16,000
Sponges 2,500 Crustaceans 16,000
Coelenterates 4,500 Mollusks 61,000
Echinoderms 4,000 Fishes 13,000
Flat-worms 5,000 Amphibians 1,400
Roundworms 1,500 Reptiles 3,500
Annelids 4,000 Birds 13,000
Insects 360,000 Mammals 3,500
Myriapods 2,000 -------
Total 518,900

Man's Place in Nature.--Although we know that man is separated mentally by a wide gap from all other animals, in our study of physiology we must ask where we are to place man. If we attempt to classify man, we see at once he must be placed with the vertebrate animals because of his possession of a vertebral column. Evidently, too, he is a mammal, because the young are nourished by milk secreted by the mother and because his body has at least a partial covering of hair. Anatomically we find that we must place man with the apelike mammals, because of these numerous points of structural likeness. The group of mammals which includes the monkeys, apes, and man we call the _primates_.

Although anatomically there is a greater difference between the lowest type of monkey and the highest type of ape than there is between the highest type of ape and the lowest savage, yet there is an immense mental gap between monkey and man.

Instincts.--Mammals are considered the highest of vertebrate animals, not only because of their complicated structure, but because their instincts are so well developed. Monkeys certainly seem to have many of the mental attributes of man.

Professor Thorndike of Columbia University sums up their habits of learning as follows:--

"In their method of learning, although monkeys do not reach the human
stage of a rich life of ideas, yet they carry the animal method of
learning, by the selection of impulses and association of them with
different sense-impressions, to a point beyond that reached by any
other of the lower animals. In this, too, they resemble man; for he
differs from the lower animals not only in the possession of a new
sort of intelligence, but also in the tremendous extension of that
sort which he has in common with them. A fish learns slowly a few
simple habits. Man learns quickly an infinitude of habits that may be
highly complex. Dogs and cats learn more than the fish, while monkeys
learn more than they. In the number of things he learns, the complex
habits he can form, the variety of lines along which he can learn
them, and in their permanence when once formed, the monkey justifies
his inclusion with man in a separate mental genus."

Evolution of Man.--Undoubtedly there once lived upon the earth races of men who were much lower in their mental organization than the present inhabitants. If we follow the early history of man upon the earth, we find that at first he must have been little better than one of the lower animals. He was a nomad, wandering from place to place, feeding upon whatever living things he could kill with his hands. Gradually he must have learned to use weapons, and thus kill his prey, first using rough stone implements for this purpose. As man became more civilized, implements of bronze and of iron were used. About this time the subjugation and domestication of animals began to take place. Man then began to cultivate the fields, and to have a fixed place of abode other than a cave. The beginnings of civilization were long ago, but even to-day the earth is not entirely civilized.

The Races of Man.--At the present time there exist upon the earth five races or varieties of man, each very different from the other in instincts, social customs, and, to an extent, in structure. These are the Ethiopian or negro type, originating in Africa; the Malay or brown race, from the islands of the Pacific; the American Indian; the Mongolian or yellow race, including the natives of China, Japan, and the Eskimos; and finally, the highest type of all, the Caucasians, represented by the civilized white inhabitants of Europe and America.

REFERENCE BOOKS

ELEMENTARY

Hunter, _Laboratory Problems in Civic Biology_, American
Book Company.
Bulletin of U. S. Department of Agriculture, _Division of
Biological Survey_, Nos. 1, 6, 13, 17.
Davison, _Practical Zoology_. American Book Company.
Ditmars, _The Reptiles of New York_. Guide Leaflet 20. Amer.
Mus. of Nat. History.
Sharpe, _A Laboratory Manual in Biology_, pp. 140-150,
American Book Company.
Walker, _Our Birds and Their Nestlings_. American Book
Company.
Walter, H. E. and H. A., _Wild Birds in City Parks_.
Published by authors.

ADVANCED

Apgar, _Birds of the United States_. American Book Company.
Beebe, _The Bird_. Henry Holt and Company.
Ditmars, _The Reptile Book_. Doubleday, Page and Company.
Hegner, _Zoology_. The Macmillan Company.
Hornaday, _American Natural History_.
Jordan and Evermann, _Food and Game Fishes_. Doubleday, Page
and Company.
Parker and Haswell, _Textbook of Zoology_. The Macmillan
Company.
_Riverside Natural History._ Houghton, Mifflin and Company.
Weed and Dearborn, _Relation of Birds to Man_. Lippincott.

XV. THE ECONOMIC IMPORTANCE OF ANIMALS

_Problems.--I. To determine the uses of animals._
_(a) Indirectly as food._
_(b) Directly as food._
_(c) As domesticated animals._
_(d) For clothing._
_(e) Other direct economic uses._
_(f) Destruction of harmful plants and animals._
_--II. To determine the harm done by animals._
_(a) Animals destructive to those used for food._
_(b) Animals harmful to crops and gardens._
_(c) Animals harmful to fruit and forest trees._
_(d) Animals destructive to stored food or clothing._
_(e) Animals indirectly or directly responsible for disease._

LABORATORY SUGGESTIONS

Inasmuch as this work is planned for the winter months the
laboratory side must be largely museum and reference work.
It is to be expected that the teacher will wish to refer to
much of this work at the time work is done on a given group.
But it is pedagogically desirable that the work as planned
should be _varied_. Interest is thus held. Outlines prepared
by the teacher to be filled in by the student are desirable
because they lead the pupil to individual selection of what
seems to _him_ as important material. Opportunity should be
given for laboratory exercises based on original sources.
The pupils should be made to use reports of the U. S.
Department of Agriculture, the Biological Survey, various
States Reports, and others.

Special home laboratory reports may be well made at this
time, for example: determination at a local fish market of
the fish that are cheap and fresh at a given time. Have the
students give reasons for this. Study conditions in the meat
market in a similar manner. Other local food conditions may
also be studied first hand.

USES OF ANIMALS

Indirect Use as Food.--Just as plants form the food of animals, so some animals are food for others. Man may make use of such food directly or indirectly. Many mollusks, as the barnacle and mussel, are eaten by fishes. Other fish live upon tiny organisms, water fleas and other small crustaceans. These in turn feed upon still smaller animals, and we may go back and back until finally we come to the Protozoa and one-celled water plants as an ultimate source of food.

Direct Use as Food. Lower Forms.--The forms of life lower than the Crustacea are of little use directly as food, although the Chinese are very fond of one of the Echinoderms, a holothurian.

Crustacea as Food.--Crustaceans, however, are of considerable value for food, the lobster fisheries in particular being of importance. The lobster is highly esteemed as food, and is rapidly disappearing from our coasts as the result of overfishing. Between twenty and thirty million are yearly taken on the North Atlantic coast. This means a value at present prices of about $15,000,000. Laws have been enacted in New York and other states against overfishing. Egg-carrying lobsters must be returned to the water; all smaller than six to nine inches in length (the law varies in different states) must be put back; other restrictions are placed upon the taking of the animals, in hope of saving the race from extinction. Some states now hatch and care for the young for a period of time; the United States Bureau of Fisheries is also doing much good work, in the hope of restocking to some extent the now almost depleted waters.

Several other common crustaceans are near relatives of the crayfish. Among them are the shrimp and prawn, thin-shelled, active crustaceans common along our eastern coast. In spite of the fact that they form a large part of the food supply of many marine animals, especially fish, they do not appear to be decreasing in numbers. They are also used as food by man, the shrimp fisheries in this country aggregating over $1,000,000 yearly.

Another edible crustacean of considerable economic importance is the blue crab. Crabs are found inhabiting muddy bottoms; in such localities they are caught in great numbers in nets or traps baited with decaying meat. They are, indeed, among our most valuable sea scavengers, although they are carnivorous hunters as well. The young crabs differ considerably in form from the adult. They undergo a complete _metamorphosis_ (change of form). Immediately after molting or shedding of the outer shell in order to grow larger, crabs are greatly desired by man as an article of food. They are then known as "shedders," or soft-shelled crabs.

Mollusks as Food.--Oysters are never found in muddy localities, for in such places they would be quickly smothered by the sediment in the water. They are found in nature clinging to stones or on shells or other objects which project a little above the bottom. Here food is abundant and oxygen is obtained from the water surrounding them. Hence oyster raisers throw oyster shells into the water and the young oysters attach themselves.

In some parts of Europe and this country where oysters are raised artificially, stakes or brush are sunk in shallow water so that the young oyster, which is at first free-swimming, may escape the danger of smothering on the bottom. After the oysters are a year or two old, they are taken up and put down in deeper water as seed oysters. At the age of three and four years they are ready for the market.

The oyster industry is one of the most profitable of our fisheries. Nearly $15,000,000 a year has been derived during the last decade from such sources. Hundreds of boats and thousands of men are engaged in dredging for oysters. Three of the most important of our oyster grounds are Long Island Sound, Narragansett Bay, and Chesapeake Bay.

Sometimes oysters are artificially "fattened" by placing them on beds near the mouths of fresh-water streams. Too often these streams are the bearers of much sewage, and the oyster, which lives on microscopic organisms, takes in a number of bacteria with other food. Thus a person might become infected with the typhoid bacillus by eating raw oysters. State and city supervision of the oyster industry makes this possibility very much less than it was a few years ago, as careful bacteriological analysis of the surrounding water is constantly made by competent experts.

Clams.--Other bivalve mollusks used for food are clams and scallops. Two species of the former are known to New Yorkers, one as the "round," another as the "long" or "soft-shelled" clams. The former (_Venus mercenaria_) was called by the Indians "quahog," and is still so called in the Eastern states. The blue area of its shell was used by the Indians to make wampum, or money. The quahog is now extensively used as food. The "long" clam (_Mya arenaria_) is considered better eating by the inhabitants of Massachusetts and Rhode Island. This clam was highly prized as food by the Indians. The clam industries of the eastern coast aggregate nearly $1,000,000 a year. The dredging for scallops, another molluscan delicacy, forms an important industry along certain parts of the eastern coast.

Fish as Food.--Fish are used as food the world over. From very early times the herring were pursued by the Norsemen. Fresh-water fish, such as whitefish, perch, pickerel, pike, and the various members of the trout family, are esteemed food and, especially in the Great Lake region, form important fisheries. But by far the most important food fishes are those which are taken in salt water. Here we have two types of fisheries, those where the fish comes up a river to spawn, as the salmon, sturgeon, or shad, and those in which fishes are taken on their feeding grounds in the open ocean. Herring are the world's most important catch, though not in this country. Here the salmon of the western coast is taken to the value of over $13,000,000 a year. Cod fishing also forms an important industry; over 7000 men being employed and over $2,000,000 of codfish being taken each year in this country.

Hundreds of other species of fish are used as food, the fish that is nearest at hand being often the cheapest and best. Why, for example, is the flounder so cheap in the New York markets? In what waters are the cod and herring fisheries, sardine, oyster, sponge, pearl oyster? (See chart on page 201.)

Amphibia and Reptiles as Food.--Frogs' legs are esteemed a delicacy. Certain reptiles are used as food by people of other nationalities, the Iguana, a Mexican lizard, being an example. Many of the sea-water turtles are of large size, the leatherback and the green turtle often weighing six hundred to seven hundred pounds each. The flesh of the green turtle and especially of the diamond-back terrapin, an animal found in the salt marshes along our southeastern coast, is highly esteemed as food. Unfortunately for the preservation of the species, these animals are usually taken during the breeding season when they go to sandy beaches to lay their eggs.

Birds as Food.--Birds, both wild and domesticated, form part of our food supply. Unfortunately our wild game birds are disappearing so fast that we should not consider them as a source of food. Our domestic fowls, turkey, ducks, etc., form an important food supply and poultry farms give lucrative employment to many people. Eggs of domesticated birds are of great importance as food, and egg albumin is used for other purposes,--clarifying sugars, coating photographic papers, etc.

Mammals as Food.--When we consider the amount of wealth invested in cattle and other domesticated animals bred and used for food in the United States, we see the great economic importance of mammals. The United States, Argentina, and Australia are the greatest producers of cattle. In this country hogs are largely raised for food. They are used fresh, salted, smoked as ham and bacon, and pickled. Sheep, which are raised in great quantities in Australia, Argentina, Russia, Uruguay, and this country, are one of the world's greatest meat supplies.

Goats, deer, many larger game animals, seals, walruses, etc., give food to people who live in parts of the earth that are less densely populated.

Domesticated Animals.-- When man emerged from his savage state on the earth, one of the first signs of the beginning of civilization was the domestication of animals. The dog, the cow, sheep, and especially the horse, mark epochs in the advance of civilization. Beasts of burden are used the world over, horses almost all over the world, certain cattle, as the water buffalo, in tropical Malaysia; camels, goats, and the llama are also used as draft animals in some other countries.

Man's wealth in many parts of the world is estimated in terms of his cattle or herds of sheep. So many products come from these sources that a long list might be given, such as meats, milk, butter, cheese, wool, or other body coverings, leather, skins, and hides used for other purposes. Great industries are directly dependent upon our domesticated animals, as the making of shoes, the manufacture of woolen cloth, the tanning industry, and many others.

Uses for Clothing.--The manufacture of silk is due to the production of raw silk by the silkworm, the caterpillar of a moth. It lives upon the mulberry and makes a cocoon from which the silk is wound. The Chinese silkworm is now raised to a slight extent in southern California. China, Japan, Italy, and France, because of cheaper labor, are the most successful silk-raising countries.

The use of wool gives rise to many great industries. After the wool is cut from the sheep, it has to be washed and scoured to get out the dirt and grease. This wool fat or lanoline is used in making soap and ointments. The wool is next "carded," the fibers being interwoven by the fine teeth of the carding machine or "combed," the fibers here being pulled out parallel to each other. Carded wool becomes woolen goods; combed wool, worsted goods. The wastes are also utilized, being mixed with "shoddy" (wool from cloth cuttings or rags) to make woolen goods of a cheap grade.

Goat hair, especially that of the Angora and the Cashmere goat, has much use in the clothing industries. Camel's hair and alpaca are also used.

Fur.--The furs of many domesticated and wild animals are of importance. The Carnivora as a group are of much economic importance as the source of most of our fur. The fur seal fisheries alone amount to many millions of dollars annually. Otters, skunks, sables, weasels, foxes, and minks are of considerable importance as fur producers. Even cats are now used for fur, usually masquerading under some other name. The fur of the beaver, one of the largest of the rodents or gnawing mammals, is of considerable value, as are the coats of the chinchilla, muskrats, squirrels, and other rodents. The fur of the rabbit and nutria are used in the manufacture of felt hats. The quills of the porcupines (greatly developed and stiffened hairs) have a slight commercial value.

Conservation of Fur-bearing Animals Needed.--As time goes on and the furs of wild animals become scarcer and scarcer through overkilling, we find the need for protection and conservation of many of these fast-vanishing wild forms more and more imperative. Already breeding of some fur-bearing animals has been tried with success, and cheap substitutes for wild animal skins are coming more and more into the markets. Black-fox breeding has been tried successfully in Prince Edward Island, Canada, $2500 to $3000 being given for a single skin. Skunk, marten, and mink are also being bred for the market. Game preserves in this country and Canada are also helping to preserve our wild fur-bearing animals.