_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.
[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.)]
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_.
[Illustration: A horny fiber sponge. Notice that it is a
colony. One fourth natural size.]
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.
[Illustration: 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.)]
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.
[Illustration: A jointed worm. The sandworm. Slightly reduced.]
[Illustration: The common starfish seen from below to show the tube feet.
About one half natural size.]
[Illustration: 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.]
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.
[Illustration: A common snail, a mollusk. (From a photograph by Davison.)]
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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.