CHAPTER XXIV

STUDIES IN CRYPTOGAMS

The pupil who has acquired skill in the use of the compound microscope

may desire to make more extended excursions into the cryptogamous

orders. The following plants have been chosen as examples in various

groups. Ferns are sufficiently discussed in the preceding chapter.

BACTERIA

If an infusion of ordinary hay is made in water and allowed to stand,

it becomes turbid or cloudy after a few days, and a drop under the

microscope will show the presence of minute oblong cells swimming in

the water perhaps by means of numerous hair-like appendages, that

project through the cell wall from the protoplasm within. At the

surface of the dish containing the infusion the cells are non-motile

and are united in long chains. Each of these cells or organisms is a

_bacterium_ (plural, _bacteria_). (Fig. 135.)

Bacteria are very minute organisms,--the smallest known,--consisting

either of separate oblong or spherical cells, or of chains, plates,

or groups of such cells, depending on the kind. They possess a

membrane-like wall which, unlike the cell walls of higher plants,

contains nitrogen. The presence of a nucleus has not been definitely

demonstrated. Multiplication is by the fission of the vegetative

cells; but under certain conditions of drought, cold, or exhaustion

of the nutrient medium, the protoplasm of the ordinary cells may

become invested with a thick wall, thus forming an _endospore_ which

is very resistant to extremes of environment. No sexual reproduction

is known.

Bacteria are very widely distributed as parasites and saprophytes in

almost all conceivable places. _Decay_ is largely caused by bacteria,

accompanied in animal tissue by the liberation of foul-smelling

gases. Certain species grow in the reservoirs and pipes of water

supplies, rendering the water brackish and often undrinkable. Some

kinds of _fermentation_ (the breaking down or decomposing of organic

compounds, usually accompanied by the formation of gas) are due to

these organisms. Other bacteria oxidize alcohol to _acetic acid_, and

produce _lactic acid_ in milk and _butyric acid_ in butter. Bacteria

live in the mouth, stomach, intestines, and on the surface of the

skin of animals. Some secrete gelatinous sheaths around themselves;

others secrete sulfur or iron, giving the substratum a vivid color.

Were it not for bacteria, man could not live on the earth, for not

only are they agents in the process of decay, but they are concerned

in certain healthful processes of plants and animals. We have learned

in Chap. VIII how bacteria are related to nitrogen-gathering.

Bacteria are of economic importance not alone because of their effect

on materials used by man, but also because of the _disease-producing

power_ of certain species. _Pus_ is caused by a spherical form,

_tetanus_ or _lock-jaw_ by a rod-shaped form, _diphtheria_ by short

oblong chains, _tuberculosis_ or “_consumption_” by more slender

oblong chains, and _typhoid fever_, _cholera_, and other diseases

by other forms. Many _diseases of animals and plants_ are caused by

bacteria. Disease-producing bacteria are said to be _pathogenic_.

The ability to grow in other nutrient substances than the natural

one has greatly facilitated the study of these minute forms of life.

By the use of suitable culture media and proper precautions, _pure

cultures_ of a particular disease-producing bacterium may be obtained

with which further experiments may be conducted.

Milk provides an excellent collecting place for bacteria coming

from the air, from the coat of the cow and from the milker. Disease

germs are sometimes carried in milk. If a drop of milk is spread on

a culture medium (as agar), and provided with proper temperature,

the bacteria will multiply, each one forming a colony visible to the

naked eye. In this way, the number of bacteria originally contained

in the milk may be counted.

Bacteria are disseminated in water, as the germ of typhoid fever and

cholera; in milk and other fluids; in the air; and on the bodies of

flies, feet of birds, and otherwise.

Bacteria are thought by many to have descended from algæ by the loss

of chlorophyll and decrease in size due to the more specialized

acquired saprophytic and parasitic habit.

ALGÆ

The algæ comprise most of the green floating “scum” which covers the

surfaces of ponds and other quiet waters. The masses of plants are

often called “frog spittle.” Others are attached to stones, pieces of

wood, and other objects submerged in streams and lakes, and many are

found on moist ground and on dripping rocks. Aside from these, all

the plants commonly known as seaweeds belong to this category; these

latter are inhabitants of salt water.

The simplest forms of algæ consist of a single spherical cell, which

multiplies by repeated division or fission. Many of the forms found

in fresh water are filamentous, _i.e._ the plant body consists of

long threads, either simple or branched. Such a plant body is termed

a _thallus_. This term applies to the vegetative body of all plants

that are _not differentiated into stem and leaves_. Such plants are

known as _thallophytes_ (p. 181). All algæ contain chlorophyll, and

are able to assimilate carbon dioxid from the air. This distinguishes

them from the fungi.

_Nostoc._--On wet rocks and damp soil dark, semitransparent irregular

or spherical gelatinous masses about the size of a pea are often

found. These consist of a colony of contorted filamentous algæ

embedded in the jelly-like mass. The chain of cells in the filament

is necklace-like. Each cell is homogeneous, without apparent

nucleus, and blue-green in color, except one cell which is larger

and clearer than the rest. The plant therefore belongs to the group

of _blue-green algæ_. The jelly probably serves to maintain a more

even moisture and to provide mechanical protection. Multiplication is

wholly by the breaking up of the threads. Occasionally certain cells

of the filament thicken to become _resting-spores_, but no other

spore formation occurs.

[Illustration: FIG. 264.--FILAMENT OF OSCILLATORIA, showing one dead

cell where the strand will break.]

_Oscillatoria._--The blue-green coatings found on damp soil and

in water frequently show under the microscope the presence of

filamentous algæ composed of many short homogeneous cells (Fig. 264).

If watched closely, some filaments will be seen to wave back and

forth slowly, showing a peculiar power of movement characteristic

of this plant. Multiplication is by the breaking up of the threads.

There is no true spore formation.

[Illustration: FIG. 265.--_Strand of Spirogyra_, showing the

chlorophyll bands. There is a nucleus at _a_. How many cells, or parts

of cells, are shown in this figure?]

SPIROGYRA.--One of the most common forms of the green algæ is

spirogyra (Fig. 265). This plant often forms the greater part of the

floating green mass (or “frog spittle”) on ponds. The threadlike

character of the thallus can be seen with the naked eye or with a

hand lens, but to study it carefully a microscope magnifying two

hundred diameters or more must be used. The thread is divided into

long cells by cross walls which, according to the species, are either

straight or curiously folded (Fig. 266). The chlorophyll is arranged

in _beautiful spiral bands_ near the wall of each cell. From the

character of these bands the plant takes its name. Each cell is

provided with a _nucleus_ and other _protoplasm_. The nucleus is

suspended near the center of the cell (_a_, Fig. 265) by delicate

strands of protoplasm radiating toward the wall and terminating

at certain points in the chlorophyll band. The remainder of the

protoplasm forms a thin layer lining the wall. The interior of the

cell is filled with cell-sap. The protoplasm and nucleus cannot be

easily seen, but if the plant is stained with a dilute alcoholic

solution of eosin they become clear.

Spirogyra is propagated vegetatively by the breaking off of parts of

the threads, which continue to grow as new plants. Resting-spores,

which may remain dormant for a time, are formed by a process known

as _conjugation_. Two threads lying side by side send out short

projections, usually from all the cells of a long series (Fig. 266).

The projections or processes from opposite cells grow toward each

other, meet, and fuse, forming a connecting tube between the cells.

The protoplasm, nucleus, and chlorophyll band of one cell now pass

through this tube, and unite with the contents of the other cell. The

entire mass then becomes surrounded by a thick cellulose wall, thus

completing the _resting-spore_, or _zygospore_ (_z_, Fig. 266).

[Illustration: FIG. 266.--CONJUGATION OF SPIROGYRA. Ripe zygospores on

the left; _a_, connecting tubes.]

[Illustration: FIG. 267.--STRAND, OR FILAMENT OF ZYGNEMA, freed from

its gelatinous covering.]

_Zygnema_ is an alga closely related to spirogyra and found in

similar places. Its life history is practically the same, but it

differs from spirogyra in having _two star-shaped chlorophyll bodies_

(Fig. 267) in each cell, instead of a chlorophyll-bearing spiral band.

[Illustration: FIG. 268.--THREAD OF VAUCHERIA WITH OÖGONIA AND

ANTHERIDIA.]

_Vaucheria_ is another alga common in shallow water and on damp

soil. The thallus is much branched, but the threads are not divided

by cross walls as in spirogyra. The plants are attached by means of

colorless root-like organs which are much like the root hairs of the

higher plants: these are _rhizoids_. The chlorophyll is in the form

of _grains scattered through the thread_.

Vaucheria has a special mode of asexual reproduction by means of

swimming spores or _swarm-spores_. These are formed singly in a

short enlarged lateral branch known as the _sporangium_. When the

sporangium bursts, the entire contents escape, forming a single large

swarm-spore, which swims about by means of numerous lashes or cilia

on its surface. The swarm spores are so large that they can be seen

with the naked eye. After swimming about for some time they come to

rest and germinate, producing a new plant.

The formation of resting-spores of vaucheria is accomplished by means

of special organs, _oögonia_ (_o_, Fig. 268) and _antheridia_ (_a_,

Fig. 268). Both of these are specially developed branches from the

thallus. The antheridia are nearly cylindrical, and curved toward

the oögonia. The upper part of an antheridium is cut off by a cross

wall, and within it numerous ciliated _sperm-cells_ are formed. These

escape by the ruptured apex of the antheridium. The oögonia are more

enlarged than the antheridia, and have a beak-like projection turned

a little to one side of the apex. They are separated from the thallus

thread by a cross wall, and contain a single large green cell, the

_egg-cell_. The apex of the oögonium is dissolved, and through the

opening the sperm-cells enter. Fertilization is thus accomplished.

After fertilization the egg-cell becomes invested with a thick wall

and is thus converted into a resting-spore, the _oöspore_.

_Fucus._--These are rather large specialized algæ belonging to the

group known as brown seaweeds and found attached by a disk to the

rocks of the seashore just below high tide (Fig. 269). They are firm

and strong to resist wave action and are so attached as to avoid

being washed ashore. They are very abundant algæ. In shape the plants

are long, branched, and multicellular, with either flat or terete

branches. They are olive-brown. Propagation is by the breaking off of

the branches. No zoöspores are produced, as in many other seaweeds;

and reproduction is wholly sexual. The _antheridia_, bearing

_sperm-cells_, and the _oögonia_, each bearing eight _egg-cells_,

are sunken in pits or _conceptacles_. These pits are aggregated in

the swollen lighter colored tips of some of the branches (_s_, _s_,

Fig. 269). The egg-cells and sperm-cells escape from the pits and

fertilization takes place in the water. The matured eggs, or spores,

reproduce the fucus plant directly.

[Illustration: FIG. 269.--FUCUS. Fruiting branches at _s_, _s_. On the

stem are two air-bladders.]

[Illustration: FIG. 270.--NITELLA.]

_Nitella._--This is a large branched and specialized fresh-water alga

found in tufts attached to the bottom in shallow ponds (Fig. 270).

Between the whorls of branches are long _internodes consisting of a

single cylindrical cell_, which is one of the largest cells known in

vegetable tissue. Under the microscope the walls of this cell are

found to be lined with a layer of small stationary chloroplastids,

within which layer the protoplasm, under favorable circumstances,

will be found in motion, moving up one side and down the other (in

rotation). Note the clear streak up the side of the cell and its

relation to the moving current.

FUNGI

Some forms of fungi are familiar to every one. Mushrooms and

toadstools, with their varied forms and colors, are common in fields,

woods, and pastures. In every household the common molds are familiar

intruders, appearing on old bread, vegetables, and even within

tightly sealed fruit jars, where they form a felt-like layer dusted

over with blue, yellow, or black powder. The strange occurrence of

these plants long mystified people, who thought they were productions

of the dead matter upon which they grew, but now we know that a mold,

as any other plant, cannot originate spontaneously; it must start

from something which is analogous to a seed. The “seed” in this case

is a _spore_. A spore may be produced by a _vegetative process_

(growing out from the ordinary plant tissues), or it may be the

result of a _fertilization process_.

_Favorable conditions for the growth of fungi._--Place a piece of

bread under a moist bell jar and another in an uncovered place near

by. Sow mold on each. Note the result from day to day. Moisten a

third piece of bread with weak copper sulfate (blue vitriol) or

mercuric chlorid solution, sow mold, cover with bell jar, note

results, and explain. Expose pieces of different kinds of food in

a damp atmosphere and observe the variety of organisms appearing.

Fungi are saprophytes or parasites, and must be provided with organic

matter on which to grow. They are usually most abundant in moist

places and wet seasons.

[Illustration: FIG. 271.--MUCOR MUCEDO, showing habit.]

[Illustration: FIG. 272.--SPORES OF MUCOR, some germinating.]

_Mold._--One of these molds (_Mucor mucedo_), which is very common on

all decaying fruits and vegetables, is shown in Fig. 271, somewhat

magnified. When fruiting, this mold appears as a _dense mass of long

white hairs_, often over an inch high, standing erect from the fruit

or vegetable on which it is growing.

The life of this mucor begins with a minute rounded spore (_a_,

Fig. 272), which lodges on the decaying material. When the spore

germinates, it sends out a delicate thread that grows rapidly in

length and forms very many branches that soon permeate every part

of the substance on which the plant grows (_b_, Fig. 272). One of

these threads is termed a _hypha_. All the threads together form the

_mycelium_ of the fungus. The mycelium disorganizes the material in

which it grows, and thus the mucor plant (Fig. 271) is nourished. It

corresponds physiologically to the roots and stems of other plants.

When the mycelium is about two days old, it begins to form the

long fruiting stalks which we first noticed. To study them, use a

compound microscope magnifying about two hundred diameters. One of

the stalks, magnified, is shown in _a_, Fig. 274. It consists of a

rounded head, the _sporangium, sp_, supported on a long, delicate

stalk, the _sporangiophore_. The stalk is separated from the

sporangium by a wall which is formed at the base of the sporangium.

This wall, however, does not extend straight across the thread, but

it arches up into the sporangium like an inverted pear. It is known

as the _columella, c_. When the sporangium is placed in water, the

wall immediately dissolves and allows hundreds of spores, which

were formed in the cavity within the sporangium, to escape, _b_.

All that is left of the fruit is the stalk, with the pear-shaped

columella at its summit, _c_. The spores that have been set free by

the breaking of the sporangium wall are now scattered by the wind

and other agents. Those that lodge in favorable places begin to grow

immediately and reproduce the fungus. The others soon perish.

[Illustration: FIG. 273.--MUCOR, showing formation of zygospore on the

right; germinating zygospore on the left.]

[Illustration: FIG. 274.--MUCOR.

_a_, sporangium; _b_, sporangium bursting; _c_, columella.]

The mucor may continue to reproduce itself in this way indefinitely,

but these spores are very delicate and usually die if they do not

fall on favorable ground, so that the fungus is provided with another

means of carrying itself over unfavorable seasons, as winter. This

is accomplished by means of curious _thick-walled resting-spores_

or _zygospores_. The zygospores are formed on the mycelium buried

within the substance on which the plant grows. They originate in the

following way: Two threads that lie near together send out short

branches, which grow toward each other and finally meet (Fig. 273).

The walls at the ends, _a_, then disappear, allowing the contents to

flow together. At the same time, however, two other walls are formed

at points farther back, _b_, _b_, separating the short section, _c_,

from the remainder of the thread. This section now increases in size

and becomes covered with a thick, dark brown wall ornamented with

thickened tubercles. The zygospore is now mature and, after a period

of rest, it germinates, either producing a sporangium directly or

growing out as mycelium.

The zygospores of the mucors form one of the most interesting and

instructive objects among the lower plants. They are, however, very

difficult to obtain. One of the mucors (_Sporodinia grandis_) may be

frequently found in summer growing on toadstools. This plant usually

produces zygospores that are formed on the aërial mycelium. The

zygospores are large enough to be recognized with a hand lens. The

material may be dried and kept for winter study, or the zygospores

may be prepared for permanent microscopic mounts in the ordinary way.

[Illustration: FIG. 275.--YEAST PLANTS.]

_Yeast._--This is a very much reduced and simple fungus, consisting

normally of isolated spherical or elliptical cells (Fig. 275)

containing abundant protoplasm and probably a nucleus, although the

latter is not easily observed. It propagates rapidly by _budding_,

which consists of the gradual extrusion of a wart-like swelling that

is sooner or later cut off at the base by constriction, thus forming

a separate organism. Although simple in structure, the yeast is found

to be closely related to some of the higher groups of fungi as shown

by the method of spore formation. When grown on special substances

like potato or carrot, the contents of the cell may _form spores

inside of the sac-like mother cell_, thus resembling the sac-fungi to

which blue mold and mildews belong. The yeast plant is remarkable on

account of its power to induce alcoholic fermentation in the media in

which it grows.

There are many kinds of yeasts. One of them is found in the common

_yeast cakes_. In the process of manufacture of these cakes, the

yeast cells grow to a certain stage, and the material is then dried

and fashioned into small cakes, each cake containing great numbers of

the yeast cells. When the yeast cake is added to dough, and proper

conditions of warmth and moisture are provided, the yeast grows

rapidly and breaks up the sugar of the dough into carbon dioxid

and alcohol. This is _fermentation_. The gases escape and puff up

the dough, causing the _bread to rise_. In this loosened condition

the dough is baked; if it is not baked quickly enough, _the bread_

“_falls_.” Shake up a bit of yeast cake in slightly sweetened water:

the water soon becomes cloudy from the growing yeasts.

_Parasitic fungi._--Most of the molds are saprophytes. Many other

fungi are parasitic on living plants and animals (Fig. 285). Some

of them have complicated life histories, undergoing many changes

before the original spore is again produced. The _willow mildew_ and

the common _rust of wheat_ will serve to illustrate the habits of

parasitic fungi.

The _willow mildew (Uncinula salicis)_.--This is one of the sac

fungi. It forms white downy patches on the leaves of willows (Fig.

276). These patches consist of numerous interwoven threads that may

be recognized under the microscope as the mycelium of the fungus. The

mycelium in this case lives on the surface of the leaf and nourishes

itself by sending short branches into the cells of the leaf to absorb

food materials from them.

[Illustration: FIG. 276.--COLONIES OF WILLOW MILDEW.]

[Illustration: FIG. 277.--SUMMER-SPORES OF WILLOW MILDEW.]

[Illustration: FIG. 278.--PERITHECIUM OF WILLOW MILDEW.]

[Illustration: FIG. 279.--SECTION THROUGH PERITHECIUM OF WILLOW MILDEW.]

Numerous _summer-spores_ are formed of short, erect branches all over

the white surface. One of these branches is shown in Fig. 277. When

it has grown to a certain length, the upper part begins to segment or

divide into spores which fall and are scattered by the wind. Those

falling on other willows reproduce the fungus there. This process

continues all summer, but in the later part of the season provision

is made to maintain the mildew through the winter. If some of the

white patches are closely examined in July or August, a number of

little black bodies will be seen among the threads. These little

bodies are called _perithecia_, shown in Fig. 278. To the naked eye

they appear as minute specks, but when seen under a magnification of

200 diameters they present a very interesting appearance. They are

hollow spherical bodies decorated around the outside with a fringe

of crook-like hairs. The _resting-spores_ of the willow mildew are

produced in sacs or _asci_ inclosed within the leathery perithecia.

Figure 279 shows a cross-section of a perithecium with the asci

arising from the bottom. The spores remain securely packed in the

perithecia. They do not ripen in the autumn, but fall to the ground

with the leaf, and there remain securely protected among the dead

foliage. The following spring they mature and are liberated by the

decay of the perithecia. They are then ready to attack the unfolding

leaves of the willow and repeat the work of the summer before.

[Illustration: FIG. 280.--SORI CONTAINING TELEUTOSPORES OF WHEAT RUST.]

_The wheat rust._--The development of some of the rusts, as the

common _wheat rust (Puccinia graminis)_, is even more interesting

and complicated than that of the mildews. Wheat rust is also a true

parasite, affecting wheat and a few other grasses. The mycelium

here cannot be seen by the unaided eye, for it consists of threads

which are present within the host plant, mostly in the intercellular

spaces. These threads also send short branches, or _haustoria_ (Fig.

132), into the neighboring cells to absorb nutriment.

[Illustration: FIG. 281.--TELEUTOSPORE OF WHEAT RUST.]

The _resting-spores_ of wheat rust are produced in late summer, when

they may be found in black lines breaking through the epidermis of

the wheat stalk (black-rust stage). They are formed in masses, called

_sori_ (Fig. 280), from the ends of numerous crowded mycelial strands

just beneath the epidermis of the host. The individual spores are

very small and can be well studied only with a microscope of high

power (× about 400). They are brown two-celled bodies with a thick

wall (Fig. 281). Since they are the resting or winter-spores, they

are termed _teleutospores_ (“completed spores”). Usually they do not

fall, but remain in the sori during winter. The following spring each

cell of the teleutospore puts forth a rather stout thread, which

does not grow more than several times the length of the spore and

terminates in a blunt extremity. This germ tube, _promycelium_, now

becomes divided into four cells by cross walls, which are formed from

the top downwards. Each cell gives rise to a short, pointed branch

which, in the course of a few hours, forms at its summit a single

spore called a _sporidium_. This in turn germinates and produces a

mycelium. In Fig. 282 a germinating teleutospore is drawn to show the

promycelium, _p_, divided into four cells, each producing a short

branch with a little _sporidium, s_.

[Illustration: FIG. 282.--GERMINATING TELEUTOSPORE OF WHEAT RUST.]

[Illustration: FIG. 283.--LEAF OF BARBERRY WITH CLUSTER-CUPS.]

[Illustration: FIG. 284.--SECTION THROUGH A CLUSTER-CUP ON BARBERRY

LEAF.]

A most remarkable circumstance in the life history of the wheat rust

is the fact that the mycelium produced by the sporidium _can live

only in barberry leaves_, and it follows that if no barberry bushes

are in the neighborhood the sporidia finally perish. Those which

happen to lodge on a barberry bush germinate immediately, producing a

mycelium that enters the barberry leaf and grows within its tissues.

Very soon the fungus produces a new kind of spores on the barberry

leaves. These are called _æcidiospores_. They are formed in long

chains in little fringed cups, or _æcidia_, which appear in groups on

the lower side of the leaf (Fig. 283). These orange or yellow æcidia

are termed _cluster-cups_. In Fig. 284 is shown a cross-section

of one of the cups, outlining the long chains of spores, and the

mycelium in the tissues.

The æcidiospores are formed in the spring, and after they have been

set free, some of them lodge on wheat or other grasses, where they

germinate immediately. The germ-tube enters the leaf through a

stomate, whence it spreads among the cells of the wheat plant. In

summer one-celled reddish uredospores (“blight spores,” red-rust

stage) are produced in a manner similar to the teleutospores. These

are capable of germinating immediately, and serve to disseminate the

fungus during the summer on other wheat plants or grasses. Late in

the season, teleutospores are again produced, completing the life

cycle of the plant.

[Illustration: FIG. 285.--HOW A PARASITIC FUNGUS WORKS. Anthracnose on

a bean pod entering the bean beneath. (Whetzel.)]

Many rusts besides _Puccinia graminis_ produce different spore forms

on different plants. The phenomenon is called _heterœcism_, and

was first shown to exist in the wheat rust. Curiously enough, the

peasants of Europe had observed and asserted that barberry bushes

cause wheat to blight long before science explained the relation

between the cluster-cups on barberry and the rust on wheat. The true

relation was actually demonstrated, as has since been done for many

other rusts on their respective hosts, by sowing the æcidiospores on

healthy wheat plants and thus producing the rust. The _cedar apple_

is another rust, producing the curious swellings often found on the

branches of red cedar trees. In the spring the teleutospores ooze out

from the “apple” in brownish yellow masses. It has been found that

these attack various fruit trees, producing æcidia on their leaves.

Fig. 285 explains how a parasitic fungus works.

_Puffballs_, _mushrooms_, _toadstools_, _and shelf fungi_.--These

represent what are called the _higher fungi_, because of the size

and complexity of the plant body as well as from the fact that they

seem to stand at the end of one line of evolution. The mycelial

threads grow together in extensive strands in rotten wood or in the

soil, and send out large complex growths of mycelium in connection

with which the spores are borne. These aërial parts are the only

ones we ordinarily see, and which constitute the “mushroom” part

(Fig. 131). Only asexual spores (_basidiospores_) are produced, and

on short stalks (_basidia_) (Fig. 286). In the puffballs the spores

are inclosed and constitute a large part of the “smoke.” In the

mushrooms and toadstools they are borne on _gills_, and in the shelf

fungi (Fig. 134) on the walls of minute pores of the underside. The

mycelium of these shelf fungi frequently lives and grows for a long

time concealed in the substratum before the visible fruit bodies are

sent out. Practically all timber decay is caused by such growth, and

the damage is largely done before the fruiting bodies appear. For

other accounts of mushrooms, see Chap. XIV.

[Illustration: FIG. 286.--PART OF GILL OF THE CULTIVATED MUSHROOM.

_tr_, trama tissue; _sh_, hymenium; _b_, basidium; _st_, sterigma;

_sp_, spore. (Atkinson.)]

LICHENS

Lichens are so common everywhere that the attention of the student

is sure to be drawn to them. They grow on rocks, trunks of trees

(Fig. 287), old fences, and on the earth. They are thin, usually gray

ragged objects, apparently lifeless. Their study is too difficult for

beginners, but a few words of explanation may be useful.

[Illustration: FIG. 287.--LICHEN ON AN OAK TRUNK. (A species of

_Physcia_.)]

Lichens were formerly supposed to be a distinct or separate division

of plants. They are now known to be organisms, each species of which

is a constant association of a fungus and an alga. The thallus is

ordinarily made up of fungous mycelium or tissue within which the

imprisoned alga is definitely distributed. The result is a growth

unlike either component. This association of alga and fungus is

usually spoken of as _symbiosis_, or mutually helpful growth, the

alga furnishing some things, the fungus others, and both together

being able to accomplish work that neither could do independently. By

others this union is considered to be a mild form of parasitism, in

which the fungus profits at the expense of the alga. As favorable to

this view, the facts are cited that each component is able to grow

independently, and that under such conditions the algal cells seem to

thrive better than when imprisoned by the fungus.

Lichens propagate by means of _soredia_, which are tiny parts

separated from the body of the thallus, and consisting of one or more

algal cells overgrown with fungus threads. These are readily observed

in many lichens. They also produce spores, usually ascospores, which

are always the product of the fungus element, and which reproduce the

lichen by germinating in the presence of algal cells, to which the

hyphæ immediately cling.

Lichens are found in the most inhospitable places, and, by means of

acids which they secrete, they attack and slowly disintegrate even

the hardest rocks. By making thin sections of the thallus with a

sharp razor and examining under the compound microscope, it is easy

to distinguish the two components in many lichens.

LIVERWORTS

The liverworts are peculiar flat green plants usually found on wet

cliffs and in other moist, shady places. They frequently occur

in greenhouses where the soil is kept constantly wet. One of the

commonest liverworts is _Marchantia polymorpha_, two plants of which

are shown in Figs. 288, 289. The plant consists of a ribbon-like

thallus that creeps along the ground, becoming repeatedly forked as

it grows. The end of each branch is always conspicuously notched.

There is a prominent midrib extending along the center of each branch

of the thallus. On the under side of the thallus, especially along

the midrib, there are numerous rhizoids which serve the purpose of

roots, absorbing nourishment from the earth and holding the plant in

its place. The upper surface of the thallus is divided into minute

rhombic areas that can be seen with the naked eye. Each of these

areas is perforated by a small breathing pore or stomate that leads

into a cavity just beneath the epidermis. This space is surrounded

by chlorophyll-bearing cells, some of which stand in rows from the

bottom of the cavity (Fig. 290). The delicate assimilating tissue is

thus brought in close communication with the outer air through the

pore in the thick, protecting epidermis.

[Illustration: FIG. 288.

FIG. 289.

PLANTS OF MARCHANTIA.]

[Illustration: FIG. 290.--SECTION OF THALLUS OF MARCHANTIA. Stomate at

_a_.]

At various points on the midrib are little cups containing small

green bodies. These bodies are buds or _gemmæ_ which are outgrowths

from the cells at the bottom of the cup. They become loosened and are

then dispersed by the rain to other places, where they take root and

grow into new plants.

The most striking organs on the thallus of marchantia are the

peculiar stalked bodies shown in Figs. 288, 289. These are termed

archegoniophores and _antheridiophores_ or _receptacles_. Their

structure and function are very interesting, but their parts are so

minute that they can be studied only with the aid of a microscope

magnifying from 100 to 400 times. Enlarged drawings will guide the

pupil.

[Illustration: FIG. 291.--SECTION THROUGH ANTHERIDIOPHORE OF

MARCHANTIA, showing antheridia. One antheridium more magnified.]

The _antheridiophores_ are fleshy, lobed disks borne on short stalks

(Fig. 291). The upper surface of the disk shows openings scarcely

visible to the naked eye. However, a section of the disk, such as is

drawn in Fig. 291, shows that the pores lead into oblong cavities

in the receptacle. From the base of each cavity there arises a

thick, club-shaped body, the _antheridium_. Within the antheridium

are formed many sperm-cells which are capable of swimming about in

water by means of long lashes or cilia attached to them. When the

antheridium is mature, it bursts and allows the ciliated sperm cells

to escape.

The _archegoniophores_ are also elevated on stalks (Fig. 289).

Instead of a simple disk, the receptacle consists of nine or

more finger-like rays. Along the under side of the rays, between

delicately fringed curtains, peculiar flask-like bodies, or

_archegonia_, are situated. The archegonia are not visible to the

naked eye. They can be studied only with the microscope (× about

400). One of them much magnified is represented in Fig. 292. Its

principal parts are the long _neck, a_, and the rounded _venter, b_,

inclosing a large free cell--the egg-cell.

[Illustration: FIG. 292.--ARCHEGONIUM OF MARCHANTIA.]

[Illustration: FIG. 293.--ARCHEGONIOPHORE, WITH SPOROGONIA, OF

MARCHANTIA.]

We have seen that the antheridium at maturity discharges its

sperm-cells. These swim about in the water provided by the dew and

rain. Some of them finally find their way to the archegonia and

egg-cells, the latter being fertilized, as pollen fertilizes the

ovules of higher plants.

After fertilization the egg-cell develops into the spore capsule

or _sporogonium_. The mature spore capsules may be seen in Fig.

293. They consist of an oval spore-case on a short stalk, the base

of which is imbedded in the tissue of the receptacle, from which

it derives the necessary nourishment for the development of the

sporogonium. At maturity the sporogonium is ruptured at the apex,

setting free the spherical spores together with numerous filaments

having spirally thickened walls (Fig. 294). These filaments are

called _elaters_. When drying, they exhibit rapid movements by means

of which the spores are scattered. The spores germinate and again

produce the thallus of marchantia.

[Illustration: FIG. 294.--SPORES AND ELATERS OF MARCHANTIA.]

[Illustration: FIG. 295.--POLYTRICHUM COMMUNE. _f_,_f_, fertile plants,

one on the left in fruit, _m_, antheridial plant.]

[Illustration: FIG. 296.--SECTION OF LEAF OF POLYTRICHUM COMMUNE.]

MOSSES (Bryophyta)

If we have followed carefully the development of marchantia, the

study of one of the mosses will be comparatively easy. The mosses

are more familiar plants than the liverworts. They grow on trees,

stones, and on the soil both in wet and dry places. One of the

common larger mosses, known as _Polytrichum commune_, may serve as

an example, Fig. 295. This plant grows on rather dry knolls, mostly

in the borders of open woods, where it forms large beds. In dry

weather these beds have a reddish brown appearance, but when moist

they form beautiful green cushions. This color is due, in the first

instance, to the color of the old stems and leaves, and, in the

second instance, to the peculiar action of the green living leaves

under the influence of changing moisture-conditions. The inner or

upper surface of the leaf is covered with thin, longitudinal ridges

of delicate cells which contain chlorophyll. These cells are shown in

cross-section in Fig. 296, as dots or granules. All the other tissue

of the leaf consists of thick-walled, corky cells which do not allow

moisture to penetrate. When the air is moist the green leaves spread

out, exposing the chlorophyll cells to the air, but in dry weather

the margins of the leaves roll inward, and the leaves fold closely

against the stem, thus protecting the delicate assimilating tissue.

The _antheridia_ and _archegonia_ of polytrichum are borne in groups

at the ends of the branches on different plants (many mosses bear

both organs on the same branch). They are surrounded by involucres

of characteristic leaves termed _perichætia_ or _perichætal leaves_.

Multicellular hairs known as _paraphyses_ are scattered among the

archegonia and antheridia. The involucres with the organs borne

within them are called _receptacles_, or, less appropriately, “moss

flowers.” As in marchantia, the organs are very minute and must be

highly magnified to be studied.

[Illustration: FIG. 297.--SECTION THROUGH A RECEPTACLE OF POLYTRICHUM

COMMUNE, showing paraphyses and antheridia.]

The antheridia are borne in broad cup-like receptacles on the

antheridial plants (Fig. 297). They are much like the antheridia of

marchantia, but they stand free among the paraphyses and are not

sunk in cavities. At maturity they burst and allow the sperm cells

or _spermatozoids_ to escape. In polytrichum, when the receptacles

have fulfilled their function, the stem continues to grow from the

center of the cup (_m_, Fig. 295). The archegonia are borne in other

receptacles on different plants. They are like the archegonia of

marchantia except that they stand erect on the end of the branch.

The _sporogonium_ which develops from the fertilized egg is shown

in _a_, _b_, Fig. 295. It consists of a long, brown stalk bearing

the spore-case at its summit. The base of the stalk is imbedded in

the end of the moss stem by which it is nourished. The capsule is

entirely inclosed by a hairy cap, the _calyptra, b_. The calyptra is

really the remnant of the archegonium, which, for a time, increases

in size to accommodate and protect the young growing capsule. It is

finally torn loose and carried up on the spore-case. The mouth of

the capsule is closed by a circular lid, the _operculum_, having a

conical projection at the center.

The operculum soon drops, or it may be removed, displaying a fringe

of sixty-four teeth guarding the mouth of the capsule. This ring of

teeth is known as the _peristome_. In most mosses the teeth exhibit

peculiar hygroscopic movements; _i.e._ when moist they bend outwards,

and upon drying curve in toward the mouth of the capsule. This

motion, it will be seen, serves to disperse the spores gradually over

a long period of time.

Not the entire capsule is filled with spores. There are no elaters,

but the center of the capsule is occupied by a columnar strand of

tissue, the _columella_, which expands at the mouth into a thin,

membranous disk, closing the entire mouth of the capsule except the

narrow annular chink guarded by the teeth. In this moss the points of

the teeth are attached to the margin of the membrane, allowing the

spores to sift out through the spaces between them.

When the spores germinate they form a green, branched thread, the

_protonema_. This gives rise directly to moss plants, which appear

as little buds on the thread. When the moss plants have sent their

little rhizoids into the earth, the protonema dies, for it is no

longer necessary for the support of the little plants, and the moss

plants grow independently.

_Funaria_ is a moss very common on damp, open soil. It forms green

patches of small fine leaves from which arise long brown stalks

terminated by curved capsules (Fig. 298). The structure is similar to

that of polytrichum, except the absence of plates on the under side

of the leaves, the continuous growth of the stem, the curved capsule,

double peristome, monœcious rather than diœcious receptacles, and

nearly glabrous unsymmetrical calyptra.

[Illustration: FIG. 298.--FUNARIA HYGROSCOPICA.]

EQUISETUMS, OR HORSETAILS (Pteridophyta)

There are about twenty-five species of equisetum, constituting the

only genus of the unique family _Equisetaceæ_. Among these _E.

arvense_ (Fig. 299) is common on clayey and sandy soils.

In this species the work of nutrition and that of spore production

are performed by separate shoots from an underground rhizome. The

fertile branches appear early in spring. The stem, which is 3

to 6 inches high, consists of a number of cylindrical, furrowed

internodes, each sheathed at the base by a circle of scale leaves.

The shoots are of a pale yellow color. They contain no chlorophyll,

and are nourished by the food stored in the rhizome (Fig. 299).

The spores are formed on specially developed fertile leaves or

_sporophylls_ which are collected into a spike or cone at the end of

the stalk (_a_, Fig. 299). A single sporophyll is shown at _b_. It

consists of a short stalk expanded into a broad, mushroom-like head.

Several large _sporangia_ are borne on its under side. The spores

formed in the sporangia are very interesting and beautiful objects

when examined under the microscope (× about 200). They are spherical,

green bodies, each surrounded by two spiral bands attached to the

spore at their intersection, s. These bands exhibit hygroscopic

movements by means of which the spores become entangled, and are held

together. This is of advantage to the plant, as we shall see. All

the spores are alike, but some of the _prothallia_ grow to a greater

size than the others. The large prothallia produce only archegonia

while the smaller ones produce _antheridia_. Both of these organs are

much like those of the ferns, and fertilization is accomplished in

the same way. Since the prothallia are usually diœcious, the special

advantage of the spiral bands, holding the spores together so that

both kinds of prothallia may be in close proximity, will be easily

understood. As in the fern, the fertilized egg-cell develops into an

equisetum plant.

[Illustration: FIG. 299.--EQUISETUM ARVENSE.

_st_, sterile shoot; _f_, fertile shoot showing the spike at _a_; _b_,

sporophyll, with sporangia; _s_, spore.]

The sterile shoots (_st_, Fig. 299) appear much later in the season.

They give rise to repeated whorls of angular or furrowed branches.

The leaves are very much reduced scales, situated at the internodes.

The stems are provided with chlorophyll and act as assimilating

tissue, nourishing the rhizome and the fertile shoots. Nutriment is

also stored in special tubers developed on the rhizome.

Other species of equisetum have only one kind of shoot--a tall, hard,

leafless, green shoot with the spike at its summit. Equisetum stems

are full of silex, and they are sometimes used for scouring floors

and utensils; hence the common name “scouring rush.”

ISOËTES (Pteridophyta)

_Isoëtes_ or quillwort is usually found in water or damp soil on the

edges of ponds and lakes. The general habit of the plant is seen

in Fig. 300, _a_. It consists of a short, perennial stem bearing

numerous erect, quill-like leaves with broad sheathing bases. The

plants are commonly mistaken for young grasses.

[Illustration: FIG. 300.--ISOËTES, showing habit of plant at _a_; _b_,

base of leaf, showing sporangium, velum, and ligule.]

Isoëtes bears two kinds of spores, large roughened ones, the

_macrospores_, and small ones or _microspores_. Both kinds are formed

in _sporangia_ borne in an excavation in the expanded base of the

leaf. The macrospores are formed on the outer and the microspores on

the inner leaves. A sporangium in the base of a leaf is shown at _b_.

It is partially covered by a thin membrane, the _velum_. The minute

triangular appendage at the upper end of the sporangium is called the

_ligule_.

The spores are liberated by the decay of the sporangia. They form

rudimentary prothallia of two kinds. The microspores produce

prothallia with _antheridia_, while the macrospores produce

prothallia with _archegonia_. Fertilization takes place as in the

mosses or liverworts, and the fertilized egg-cell, by continued

growth, gives rise again to the isoëtes plant.

CLUB-MOSSES (Pteridophyta)

The club-mosses are low trailing plants of moss-like looks and

habit, although more closely allied to ferns than to true mosses.

Except one genus in Florida, all our club mosses belong to the

genus _Lycopodium_. They grow mostly in woods, having 1-nerved

evergreen leaves arranged in four or more ranks. Some of them make

long strands, as the ground pine, and are much used for Christmas

decorations. The spores are all _of one kind_ or form, borne in

_1-celled sporangia_ that open on the margin into two valves. The

sporangia are borne in some species (Fig. 301) as small yellow bodies

in the axils of the ordinary leaves near the tip of the shoot; in

other species (Fig. 302) they are borne in the axils of small scales

that form a catkin-like spike. The spores are very numerous, and

they contain an oil that makes them inflammable. About 100 species

of lycopodium are known. The plants grown by florists under the name

of lycopodium are of the genus _Selaginella_, more closely allied to

isoëtes, bearing two kinds of spores (microspores and macrospores).

[Illustration: FIG. 301.--A LYCOPODIUM WITH SPORANGIA IN THE AXILS OF

THE FOLIAGE LEAVES. (_Lycopodium lucidulum_).]

[Illustration: FIG. 302.--A CLUB-MOSS (_Lycopodium complanatum_).]

ANIMAL BIOLOGY

CHAPTER I

THE PRINCIPLES OF BIOLOGY

Biology (Greek, _bios_, life; _logos_, discourse) means the science of

life. It treats of animals and plants. That branch of biology which

treats of animals is called _zoology_ (Gr. _zoon_, animal; _logos_,

discourse). The biological science of _botany_ (Gr. _botane_, plant or

herb) treats of plants.

Living things are distinguished from the not living by a series of

processes, or changes (feeding, growth, development, multiplication,

etc.), which together constitute what is called life. These processes

are called _functions_. Both plants and animals have certain parts

called _organs_ which have each a definite work, or function; hence

animals and plants are said to be organized. For example, men and most

animals have a certain organ (the mouth) for taking in nourishment;

another (the food tube), for its digestion.

Because of its _organization_, each animal or plant is said to be

an organism. Living things constitute the _organic kingdom_. Things

without life and not formed by life constitute the _inorganic_, or

_mineral, kingdom_. Mark I for inorganic and O for organic after the

proper words in this list: granite, sugar, lumber, gold, shellac, sand,

coal, paper, glass, starch, copper, gelatine, cloth, air, potatoes,

alcohol, oil, clay. Which of these things are used for food by animals?

Conclusion?

=Energy in the Organic World.=--We see animals exerting energy; that

is, we see them moving about and doing work. Plants are never seen

acting that way; yet they need energy in order to form their tissues,

grow, and raise themselves in the air.

=Source of Plant Energy.=--We notice that green plants thrive only in

the light, while animal growth is largely independent of light. In

fact, in the salt mines of Poland there are churches and villages below

the ground, and children are born, become adults, and live all their

lives below ground, without seeing the sun. (That these people are not

very strong is doubtless due more to want of fresh air and other causes

than want of sunlight.)

[Illustration: FIG. 1.--SURFACES OF A LEAF, MAGNIFIED.]

[Illustration: FIG. 2.--A LEAF STORING ENERGY IN SUNLIGHT.]

_The need of plants for sunlight shows that they must obtain something

from the sun._ This has been found to be _energy_. This enables them

to _lift_ their _stems_ in growth, and _form_ the various structures

called _tissues_ which make up their stems and leaves. (See Part I,

Chap. XIII.) It is noticed that they take in food and water from the

soil through their roots. Experiments also show that green plants take

in through pores (Fig. 1), on the under side of their leaves, a gas

composed of carbon and oxygen, and called _carbon dioxid_. The _energy

in the sunlight_ enables the plant _to separate out the carbon of the

carbon dioxid_ and build mineral and water and carbon into organic

substances. The oxygen of the carbon dioxid is set free and returns to

the air (Fig. 2). Starch, sugar, oil, and woody fiber are examples of

substances thus formed. Can you think of any fuel not due to plants?

=How Animals obtain Energy.=--You have noticed that starch, oil, etc.,

will _burn_, or _oxidize_, that is, _unite with the oxygen of the air_;

thus the sun’s energy, stored in these substances, is changed back to

heat and motion. The oxidation of oil or sugar may occur in a furnace;

it may also occur in the living substance of the active animal.

[Illustration: FIG. 3.--Colorless plants, as MUSHROOMS, give off no

oxygen.

A GREEN LEAF, even after it is cut, gives off oxygen (O) if kept in the

sun.]

Fortunately for the animals the plants oxidize very little of the

substances built up by them, since they do not move about nor need

to keep themselves warm. We notice that animals are constantly using

plant substances for food, and constantly drawing the air into their

bodies. If the sunlight had not enabled the green plant to store up

these substances and set free the oxygen (Fig. 3), animals would have

no food to eat nor air to breathe; hence we may say that the sunlight

is indirectly the source of the life and energy of animals. Mushrooms

and other plants without green matter cannot set oxygen free (Fig. 3).

=Experiment to show the Cause of Burning, or Oxidation.=--Obtain a

large glass bottle (a pickle jar), a short candle, and some matches.

Light the candle and put it on a table near the edge, and cover it with

the glass jar. The flame slowly smothers and goes out. Why is this? Is

the air now in the jar different from that which was in it before the

candle was lighted? Some change must have taken place or the candle

would continue to burn. To try whether the candle will burn again

under the jar without changing the air, slide the jar to the edge of

the table and let the candle drop out. Light the candle and slip it up

into the jar again, the jar being held with its mouth a little over

the edge of the table to receive the candle (Fig. 5). The flame goes

out at once. Evidently the air in the jar is not the same as the air

outside. Take up the jar and wave it to and fro a few times, so as to

remove the old air and admit fresh air. The candle now burns in it with

as bright a flame as at first. So we conclude that the candle will not

continue to burn unless there is a constant supply of fresh air. The

gas formed by the burning is carbon dioxid. It is the gas from which

plants extract carbon. (See Plant Biology, Chap. V.) One test for the

presence of this gas is that it forms a white, chalky cloud in lime

water; another is that it smothers a fire.

=Experiment to show that Animals give off Carbon Dioxid.=--Place a

cardboard over the mouth of a bottle containing pure air. Take a long

straw, the hollow stem of a weed, a glass tube, or a sheet of stiff

paper rolled into a tube, and pass the tube into the bottle through a

hole in the cardboard. Without drawing in a deep breath, send one long

breath into the bottle through the tube, emptying the lungs by the

breath as nearly as possible (Fig. 4). Next invert the bottle on the

table as in the former experiment, afterward withdrawing the cardboard.

Move the bottle to the edge of the table and pass the lighted candle

up into it (Fig. 5). Does the flame go out as quickly as in the former

experiment?

If you breathe through a tube into clear lime water, the water turns

milky. The effect of the breath on the candle and on the lime water

shows that carbon dioxid is continually leaving our bodies in the

breath.

[Illustration: FIG. 4.--Breathing into a bottle.[1]]

[Illustration: FIG. 5.--Testing the air in the bottle.[1]]

[1] From Coleman’s “Physiology for Beginners,” Macmillan Co., N.Y.

=Oxidation and Deoxidation.=--The union of oxygen with carbon and

other substances, which occurs in fires and in the bodies of animals,

is called _oxidation_. The separation of the oxygen from carbon such

as occurs in the leaves of plants is called _deoxidation_. _The first

process sets energy free, the other process stores it up._ Animals

give off carbon dioxid from their lungs or gills, and plants give off

oxygen from their leaves. But plants need some energy in growing, so

oxidation also occurs in plants, but to a far less extent than in

animals. At night, because of the absence of sunlight, no deoxidation

is taking place in the plant, but oxidation and growth continue; _so

at night the plant actually breathes out some carbon dioxid_. The

deepest part of the lungs contains the most carbon dioxid. Why was it

necessary to empty the lungs as nearly as possible in the experiment

with the candle? Why would first drawing a deep breath interfere with

the experiment? Why does closing the draught of a stove, thus shutting

off part of the air, lessen the burning? Why does a “firefly” shine

brighter at each breath? Why is the pulse and breathing faster in a

fever? Very slow in a trance?

=The key for understanding any animal= is to find _how it gets food and

oxygen_, and how it uses the energy thus obtained to grow, move, avoid

its enemies, and get more food. Because it moves, it needs senses to

guide it.

=The key for understanding a plant= is to find _how it gets food and

sunlight_ for its growth. It makes little provision against enemies;

its food is in reach, so it needs no senses to guide it. The plant is

built on the plan of having the nutritive activities _near the surface_

(_e.g._ absorption by roots; gas exchange in leaves). The animal is

built on the plan of having its nutritive activities _on the inside_

(_e.g._ digestion; breathing).

=Cell and Protoplasm.=--Both plants and animals are composed of small

parts called _cells_. Cells are usually microscopic in size. They

have various shapes, as spherical, flat, cylindrical, fiber-like,

star-shaped. The living substance of cells is called _protoplasm_. It

is a stiff, gluey fluid, _albuminous_ in its nature. Every cell has a

denser spot or kernel called a _nucleus_, and in the nucleus is a still

smaller speck called a _nucleolus_. Most cells are denser and tougher

on the outside, and are said to have a _cell wall_, but many cells are

naked, or without a wall. Hence the indispensable part of a cell is

not the wall but the nucleus, and a _cell may be defined as a bit of

protoplasm containing a nucleus_. This definition includes naked cells

as well as cells with walls.

=One-celled Animals.=--There are countless millions of animals and

plants the existence of which was not suspected until the invention

of the microscope several centuries ago. They are one-celled, and

hence microscopic in size. It is believed that the large animals and

plants are descended from one-celled animals and plants. In fact, each

individual plant or animal begins life as a single cell, called an egg

cell, and forms its organs by the subdivision of the egg cell into many

cells. An egg cell is shown in Fig. 6, and the first stages in the

development of an egg cell are shown in Fig. 7.

[Illustration: FIG. 6.--Egg cell of mammal with yolk.]

[Illustration: FIG. 7.--Egg cell subdivides into many cells forming a

sphere (morula) containing a liquid. A dimple forms and deepens to form

the next stage (gastrula).]

The animals to be studied in the first chapter are _one-celled

animals_. To understand them we must learn how they eat, breathe, feel,

and move. They are called _Protozoans_ (Greek _protos_, first; _zoon_,

life). All other animals are composed of many cells and are called

_Metazoans_ (Greek _meta_, beyond or after). The cells composing the

mucous membrane in man are shown in Fig. 8. The cellular structure of

the leaf of a many-celled plant is illustrated in Fig. 1. (See also

Chap. I, Human Biology.)

[Illustration: FIG. 8.--MUCOUS MEMBRANE formed of one layer of cells. A

few cells secrete mucus.]

=Method of Classifying Animals.=--The various animals display

differences more or less marked. The question arises, are not some

of them more closely related than others? We conclude that they are,

since the difference between some animals is very slight, while the

difference between others is quite marked.

To show _the different steps in classifying_ an animal, we will take

an example,--the cow. Even little children learn to recognize a cow,

although individual cows differ somewhat in form, size, color, etc.

The varieties of cows, such as short-horn, Jersey, etc., all form one

_species_ of animals, having the scientific name _taurus_. Let us

include in a larger group the animals closest akin to a cow. We see a

cat, a bison, and a dog; rejecting the cat and the dog, we see that

the bison has horns, hoofs, and other similarities. We include it with

the cow in a _genus_ called _Bos_, calling the cow Bos taurus, and the

bison, Bos bison. The sacred cow of India (Bos indicus) is so like the

cow and buffalo as also to belong in the genus Bos. Why is not the

camel, which, like Bos bison, has a hump, placed in the genus Bos?

(Fig. 390.)

The Old World buffaloes,--most abundant in Africa and India,--the

antelopes, sheep, goats, and several other genera are placed with the

genus Bos in a _family_ called the _hollow horns_.

This family, because of its even number of toes and the habit of

chewing the cud, resembles the camel family, the deer family, and

several other families. These are all placed together in the next

higher systematic unit called an _order_, in this case, the order of

_ruminants_.

The ruminants, because they are covered with hair and nourish the young

with milk, are in every essential respect related to the one-toed

horses, the beasts of prey, the apes, etc. Hence they are all placed in

a more inclusive division of animals, the _class_ called _mammals_.

All mammals have the skeleton, or support of the body, on the inside,

the axis of which is called the vertebral column. This feature also

belongs to the classes of reptiles, amphibians, and fishes. It is

therefore consistent to unite these classes by a general idea or

conception into a great _branch_ of animals called the _vertebrates_.

Returning from the general to the particular by successive steps, state

the branch, class, order, family, genus, and species to which the cow

belongs.

=The Eight Branches or Sub-kingdoms.=--The simplest classification

divides the whole animal kingdom into eight branches, named and

characterized as follows, beginning with the lowest: I. PROTOZOANS.

One-celled. II. SPONGES. Many openings. III. POLYPS. Circular;

cup-like; having only one opening which is both mouth and vent. IV.

ECHINODERMS. Circular; rough-skinned; two openings. V. MOLLUSKS. No

skeleton; usually with external shell. VI. VERMES. Elongate body, no

jointed legs. VII. ARTHROPODS. External jointed skeleton; jointed legs.

VIII. VERTEBRATES. Internal jointed skeleton with axis or backbone.

CHAPTER II

PROTOZOA (One-celled Animals)

THE AMEBA

SUGGESTIONS.--Amebas live on the slime found on submerged stems and

leaves in standing water, or in the ooze at the bottom. Water plants

may be crowded into a glass dish and allowed to decay, and after

about two weeks the ameba may be found in the brown slime scraped

from the plants. An ameba culture sometimes lasts only three days.

The most abundant supply ever used by the writer was from a bottle

of water where some oats were germinating. Use ¹⁄₅ or ¹⁄₆ inch

objective, and cover with a thin cover glass. Teachers who object to

the use of the compound microscope in a first course should require a

most careful study of the figures.

[Illustration: FIG. 9.--AMEBA PROTEUS, much enlarged.]

[Illustration: FIG. 10.--AMEBA.

_cv_, contractile vacuole; _ec_, ectoplasm; _en_, endoplasm; _n_,

nucleus; _ps_, pseudopod; _ps′_, pseudopod forming; ectoplasm protrudes

and endoplasm flows into it.]

=Form and Structure.=--The ameba (also spelled amœba) looks so much

like a clear drop of jelly that a beginner cannot be certain that he

has found one until it moves. It is a speck of protoplasm (Fig. 9),

with a clear outer layer, the _ectoplasm_; and a granular, internal

part, the _endoplasm_. Is there a distinct line between them? (Fig. 10.)

Note the central portion and the slender prolongations or _pseudopods_

(Greek, false feet). Does the endoplasm extend into the pseudopods?

(Fig. 10.) Are the pseudopods arranged with any regularity?

Sometimes it is possible to see a denser appearing portion, called the

_nucleus_; also a clear space, the _contractile vacuole_ (Fig. 10).

=Movements.=--Sometimes while the pseudopods are being extended and

contracted, the central portion remains in the same place (this is

_motion_). Usually only one pseudopod is extended, and the body flows

into it; this is _locomotion_ (Fig. 11). There is a new foot made for

each step.

[Illustration: FIG. 11.--The same ameba seen at different times.]

=Feeding.=--If the ameba crawls near a food particle, the _pseudopod is

pressed against it_, or a depression occurs (Fig. 12), and the particle

is soon embedded in the endoplasm. Often a clear space called a _food

vacuole_ is noticed around the food particle. This is the water that is

taken in with the particle (Fig. 12). The water and the particle are

soon absorbed and assimilated by the endoplasm.

[Illustration: FIG. 12.--THE AMEBA taking food.]

=Excretion.=--If a particle of sand or other indigestible matter is

taken in, _it is left behind_ as the ameba moves on. There is a clear

space called the _contractile vacuole_, which slowly contracts and

disappears, then reappears and expands (Figs. 9 and 10). This possibly

aids in excreting oxidized or useless material.

=Circulation= in the ameba consists of the movement of its protoplasmic

particles. It lacks special organs of circulation.

=Feeling.=--_Jarring_ the glass slide seems to be felt, for it causes

the activity of the ameba to vary. It does not take in for food every

particle that it touches. This may be the beginning of _taste, based

upon mere chemical affinity_. The pseudopods aid in feeling.

=Reproduction.=--Sometimes an ameba is seen dividing into two parts. A

_narrowing_ takes place in the middle; the _nucleus also divides_, a

part going to each portion (Fig. 13). The mother ameba finally divides

into two daughter amebas. Sex is wanting.

[Illustration: FIG. 13.--AMEBA, dividing.]

=Source of the Ameba’s Energy.=--We thus see that the ameba moves

without feet, eats without a mouth, digests without a stomach, feels

without nerves, and, it should also be stated, breathes without lungs,

for _oxygen is absorbed_ from the water _by its whole surface_. Its

_movements require energy_; this, as in all animals, is furnished by

the _uniting of oxygen with the food_. Carbon dioxid and other waste

products are formed by the union; these pass off at the surface of the

ameba and taint the water with impurities.

=Questions.=--Why will the ameba die in a very small quantity of

water, even though the water contains enough food? Why will it die

still quicker if air is excluded from contact with the drop of water?

The ameba never dies of old age. Can it be said to be immortal?

According to the definition of a cell (_Chapter I_), is the ameba a

unicellular or multicellular animal?

=Cysts.=--If the water inhabited by a protozoan dries up, it encysts,

that is, it forms a tough skin called a cyst. Upon return of better

conditions it breaks the cyst and comes out. Encysted protozoans may

be blown through the air: this explains their appearance in vessels of

water containing suitable food but previously free from protozoans.

THE SLIPPER ANIMALCULE OR PARAMECIUM

=Suggestions.=--Stagnant water often contains the paramecium as well

as the ameba; or they may be found in a dish of water containing hay

or finely cut clover, after the dish has been allowed to stand in

the sun for several days. A white film forming on the surface is a

sign of their presence. They may even be seen with the unaided eye as

tiny white particles by looking through the side of the dish or jar.

Use at first a ¹⁄₃ or ¹⁄₄ in. objective. Restrict their movements by

placing cotton fibers beneath the cover glass; then examine with ¹⁄₅

or ¹⁄₆ objective. Otherwise, study figures.

=Shape and Structure.=--The paramecium’s whole body, like the

ameba’s, is only one cell. It resembles a slipper in _shape_, but

the pointed end is the hind end, the _front end_ being rounded (Fig.

14). The paramecium is propelled by the rapid beating of numerous

fine, threadlike appendages on its surface, called _cilia_ (Latin,

eyelashes) (Figs.). The cilia, like the pseudopods of the ameba, are

merely prolongations of the cell protoplasm, but they are permanent.

The separation between the outer _ectoplasm_ and the interior granular

_endoplasm_ is more marked than in the ameba (Fig. 14).

[Illustration: FIG. 14.--PARAMECIUM, showing cilia, _c_.

Two contractile vacuoles, _cv_; the macronucleus, _mg_; two

micronuclei, _mi_; the gullet (_Œ_), a food ball forming and ten food

balls in their course from gullet to vent, _a_.]

[Illustration: FIG. 15.]

=Nucleus and Vacuoles.=--There is a large nucleus called the

macronucleus, and beside it a smaller one called the micronucleus. They

are hard to see. About one third of the way from each end is a clear,

pulsating space (bb. Fig. 15) called the pulsating vacuole. These

spaces contract until they disappear, and then reappear, gradually

expanding. Tubes lead from the vacuoles which probably serve to keep

the contents of the cell in circulation.

[Illustration: FIG. 16.--TWO PARAMECIA exchanging parts of their

nuclei.]

=Feeding.=--A depression, or _groove_, is seen on one side, this serves

as a mouth (Figs.). A _tube_ which serves as a gullet leads from the

mouth-groove to the interior of the cell. The mouth-groove is lined

with cilia which sweep food particles inward. The particles accumulate

in a mass at the inner end of the gullet, become separated from it as a

_food ball_ (Fig. 14), and sink into the soft protoplasm of the body.

The food balls follow a circular course through the endoplasm, keeping

near the ectoplasm.

=Reproduction.=--This, as in the ameba, is by division, the

constriction being in the middle, and part of the nucleus going to each

half. Sometimes two individuals come together with their mouth-grooves

touching and exchange parts of their nuclei (Fig. 16). They then

separate and each divides to form two new individuals.

We thus see that the paramecium, though of only one cell, _is a much

more complex and advanced animal than the ameba_. The tiny paddles, or

cilia, the mouth-groove, etc., have their special duties similar to the

specialized organs of the many-celled animals to be studied later.

[Illustration: FIG. 17.--VORTICELLA (or bell animalcule), two extended,

one withdrawn.]

[Illustration: FIG. 18.--Euglena.]

[Illustration: FIG. 19.--SHELL OF A RADIOLARIAN.]

If time and circumstances allow a prolonged study, several additional

facts may be observed by the pupil, _e.g._ Does the paramecium swim

with the same end always foremost, and same side uppermost? Can it move

backwards? Avoid obstacles? Change shape in a narrow passage? Does

refuse matter leave the body at any particular place? Trace movement of

the food particles.

Draw the paramecium.

Which has more permanent parts, the _ameba_ or _paramecium_? Name

two anatomical similarities and three differences; four functional

similarities and three differences.

The ameba belongs in the class of protozoans called _Rhizopoda_ “root

footed.”

=Other classes of Protozoans= are the _Infusorians_, which have many

waving cilia (Fig. 17) or one whip-like flagellum (Fig. 18), and the

_Foraminifers_ which possess a calcareous shell pierced with holes.

Much chalky limestone has been formed of their shells. These and the

radiolarians, which have flinty shells (Fig. 19), are often placed in

the class rhizopoda. To which class does the paramecium belong?

Protozoans furnish a large amount of food to the higher animals.

=To the Teacher.= If plant, animal, and human biology are to be given

in one year as planned, and full time allowed for practical work, the

portions of the text in small type, as Chapter III, may be omitted

or merely read and discussed. Any two of the three parts forming the

course may be used for a year’s course by using all of the text and

spending more time on practical and field work.

CHAPTER III

SPONGES

SUGGESTIONS.--In many parts of the United States, fresh-water

sponges may, by careful searching, be found growing on rocks and

logs in clear water. They are brown, creamy, or greenish in color,

and resemble more a cushion-like plant than an animal. They have

a characteristic gritty feel. They soon die after removal to an

aquarium.

A number of common small bath sponges may be bought and kept for use

in studying the skeleton of an ocean sponge. These sponges should

not have large holes in the bottom; if so, too much of the sponge

has been cut away. A piece of marine sponge preserved in alcohol or

formalin may be used for showing the sponge with its flesh in place.

Microscopic slides may be used for showing the spicules.

[Illustration: FIG. 21.--FRESH-WATER SPONGE.]

[Illustration: FIG. 22.--SECTION of fresh-water sponge (enlarged).]

The small =fresh-water sponge= (Fig. 21) lacks the more or less

vase-like form typical of sponges. It is a rounded mass growing upon a

rock or log. As indicated by the arrows, where does _water enter_ the

sponge? This may be tested by putting coloring matter in the water near

the living sponge. Where does the _water come out_? (Fig. 22.) Does it

pass through _ciliated chambers_ in its course? Is the _surface_ of the

sponge rough or smooth? Do any of the skeletal _spicules_ show on the

surface? (Fig. 21.) Does the sponge thin out near its edge?

[Illustration: FIG. 23.--EGGS and SPICULES of fresh-water sponge

(enlarged).]

The _egg_ of this sponge is shown in Fig. 23. It escapes from the

parent sponge through the _osculum_, or large outlet. As in most

sponges, the first stage after the egg is ciliated and free-swimming.

=Marine Sponges.=--The _grantia_ (Fig. 24) is one of the simplest of

marine sponges. What is the _shape_ of grantia? What is its length and

diameter? How does the free end differ from the fixed end? Are the

_spicules_ projecting from its body few or many?

[Illustration: FIG. 24.--Grantia.]

Where is the _osculum_, or large outlet? With what is this surrounded?

The osculum opens from a central cavity called the _cloaca_. The canals

from the pores lead to the cloaca.

_Buds_ are sometimes seen growing out from the sponge near its base.

These are young sponges formed asexually. Later they become detached

from the parent sponge.

[Illustration: FIG. 25.--Plan of a sponge.]

=Commercial “Sponge.”=--What part of the complete animal remains in the

bath sponge? _Slow growing sponges_ grow more at the top and form tall,

simple, tubular or vase-like animals. _Fast growing sponges_ grow on

all sides at once and form a complicated system of canals, pores, and

oscula. Which of these habits of growth do you think belonged to the

bath sponge? Is there a large hole in the base of your specimen? If so,

this is because the cloaca was reached in trimming the lower part where

it was attached to a rock. Test the _elasticity_ of the sponge when dry

and when wet by squeezing it. Is it softer when wet or dry? Is it more

elastic when wet or dry? How many _oscula_ does your specimen have? How

many _inhalent pores_ to a square inch? Using a probe (a wire with knob

at end, or small hat pin), try to trace the _canals_ from the pores to

the cavities inside.

Do the _fibers_ of the sponge appear to interlace, or join, according

to any system? Do you see any fringe-like growths on the surface which

show that new tubes are beginning to form? Was the sponge growing

faster at the top, on the sides, or near the bottom?

Burn a bit of the sponge; from the odor, what would you judge of its

composition? Is the inner cavity more conspicuous in a simple sponge or

in a compound sponge like the bath sponge? Is the bath sponge branched

or lobed? Compare a number of specimens (Figs. 26, 27, 28) and decide

whether the common sponge has a typical shape. What features do their

forms possess in common?

[Illustration: FIG. 26.--Bath Sponge.]

[Illustration: FIG. 27.--Bath Sponge.]

[Illustration: FIG. 28.--Bath Sponge.]

Sponges are divided into _three classes_, according as their skeletons

are flinty (silicious), limy (calcareous), or horny.

Some of the _silicious sponges_ have skeletons that resemble spun glass

in their delicacy. Flint is chemically nearly the same as glass. The

skeleton shown in Fig. 29 is that of a glass sponge which lives near

the Philippine Islands.

[Illustration: FIG. 29.--Skeleton of a glass sponge.]

The _horny sponges_ do not have spicules in their skeletons, as

the flinty and limy sponges have, but the skeleton is composed of

interweaving fibers of _spongin_, a durable substance of the same

chemical nature as silk (Figs. 30 and 31).

[Illustration: FIG. 30.--A horny sponge.]

[Illustration: FIG. 31.--Section of horny sponge.]

The _limy sponges_ have skeletons made of numerous spicules of lime.

The three-rayed spicule is the commonest form.

The commercial sponge, seen _as it grows in the ocean_, appears as

a roundish mass with a smooth, dark exterior, and having about the

consistency of beef liver. Several large openings (oscula), from which

the water flows, are visible on the upper surface. Smaller holes

(inhalent pores--many of them so small as to be indistinguishable)

are on the sides. If the sponge is disturbed, the smaller holes, and

perhaps the larger ones, will close.

The outer layer of cells serves as a sort of skin. Since so much of

the sponge is in contact with water, most of the cells do their own

breathing, or absorption of oxygen and giving off of carbon dioxid.

_Nutriment_ is passed on from the surface cells to nourish the rest of

the body.

=Reproduction.=--Egg-cells and sperm-cells are produced by certain

cells along the canals. The egg-cell, after it is fertilized by the

sperm-cell, begins to divide and form new cells, some of which possess

cilia. The embryo sponge passes out at an osculum. By the vibration of

the cilia, it swims about for a while. It afterwards settles down with

the one end attached to the ocean floor and remains fixed for the rest

of its life. The other end develops oscula. Some of the cilia continue

to vibrate and create currents which bring food and oxygen.

The _cilia_ in many species are found only in cavities called ciliated

chambers. (Figs. 22, 32.) There are _no distinct organs_ in the sponge

and there is very little _specialization_ of cells. The ciliated cells

and the reproductive cells are the only specialized cells. The sponges

were for a long time considered as colonies of separate one-celled

animals classed as protozoans. They are, without doubt, many-celled

animals. If a living sponge is cut into pieces, each piece will grow

and form a complete sponge.

=That the sponge is not a colony of one-celled animals=, each like

an ameba, but is a many-celled animal, will be realized by examining

Fig. 32, which shows a bit of sponge highly magnified. A sponge

may be conceived as having developed from a one-celled animal as

follows: Several one-celled animals happened to live side by side;

each possessed a thread-like flagellum (E, Fig. 32) or whip-lash for

striking the water. By lashing the water, they caused a stronger

current (Fig. 25) than protozoans living singly could cause. Thus they

obtained more food and multiplied more rapidly than those living alone.

The habit of working together left its impress on the cells and was

transmitted by inheritance.

[Illustration: FIG. 32.--Microscopic plan of ciliated chamber. Each

cell lining the chamber has a nucleus, a whip-lash, and a collar around

base of whip-lash. _Question_: State two uses of whip-lash.]

Cell joined to cell formed a ring; ring joined to ring formed a tube

which was still more effective than a ring in lashing the water into a

current and bringing fresh food (particles of dead plants and animals)

and oxygen.

=No animals eat sponges=; possibly because spicules, or fibers, are

found throughout the flesh, or because the taste and odor is unpleasant

enough to protect them. Small animals sometimes crawl into them to

hide. One species grows upon shells inhabited by hermit crabs. Moving

of the shell from place to place is an advantage to them, while they

conceal the crab and thus protect it.

=Special Report:= _Sponge “Fisheries.”_ (Localities; how sponges are

taken, cleaned, dried, shipped, and sold.)

CHAPTER IV

POLYPS (CUPLIKE ANIMALS)

THE HYDRA, OR FRESH WATER POLYP

[Illustration: FIG. 33.--A HYDRA.]

SUGGESTIONS.--Except in the drier regions of the United States, the

hydra can usually be found by careful search in fresh water ponds

not too stagnant. It is found attached to stones, sticks, or leaves,

and has a slender, cylindrical body from a quarter to half an inch

long, varying in thickness from that of a fine needle to that of a

common pin. The green hydra and the brown hydra, both very small,

are common species, though hydras are often white or colorless. They

should be kept in a large glass dish filled with water. They may be

distinguished by the naked eye but are not studied satisfactorily

without a magnifying glass or microscope. Place a living specimen

attached to a bit of wood in a watch crystal filled with water, or on

a hollowed slip, or on a slip with a bit of weed to support the cover

glass, and examine with hand lens or lowest power of microscope.

Prepared microscopical sections, both transverse and longitudinal,

may be bought of dealers in microscopic supplies. One is shown in

Fig. 39.

Is the hydra’s =body= round or two-sided? (Fig. 35.) What is its

_general shape_? Does one individual keep the same shape? (Fig. 34.)

How does the length of the threadlike _tentacles_ compare with the

length of the hydra’s body? About how many tentacles are on a hydra’s

body? Do all have the same number of tentacles? Are the tentacles

knotty or smooth? (Fig. 35.) The hydra is usually extended and slender;

sometimes it is contracted and rounded. In which of these conditions

is the base (the foot) larger around than the rest of the body? (Fig.

34.) Smaller? How many _openings_ into the body are visible? Is there

a depression or an eminence at the base of the tentacles? For what is

the _opening_ on top of the body probably used? Why are the tentacles

placed at the top of the hydra’s body? Does the _mouth_ have the most

convenient location possible?

[Illustration: FIG. 34.--Forms assumed by Hydra.]

[Illustration: FIG. 35.--HYDRA (much enlarged).]

The conical projection bearing the _mouth_ is called _hypostome_ (Fig.

34). The mouth opens into the _digestive cavity_. Is this the same as

the general body cavity, or does the stomach have a wall distinct from

the _body cavity_? How far down does the body cavity extend? Does it

extend up into the tentacles? (Fig. 39.)

If a _tentacle is touched_, what happens? Is the body ever bent?

Which is more sensitive, the columnar body or the tentacles? In

searching for hydras would you be more likely to find the tentacles

extended or drawn in? Is the hypostome ever extended or drawn in?

(Fig. 34.)

=Locomotion.=--The round surface, or disk, by which the hydra is

attached, is called its foot. Can you move on one foot without hopping?

The hydra moves by alternately elongating and rounding the foot. Can

you discover other ways by which it moves? Does the hydra always stand

upon its foot?

[Illustration: FIG. 36.--NETTLING CELL.

II. discharged, and I. not discharged.]

=Lasso Cells.=--Upon the tentacles (Fig. 35) are numerous cells

provided each with a thread-like process (Fig. 36) which lies coiled

within the cell, but which may be thrown out upon a water flea, or

other minute animal that comes in reach. The touch of the lasso

paralyzes the prey (Fig. 37). These cells are variously called lasso

cells, nettling cells, or thread cells. The thread is hollow and is

pushed out by the pressure of liquid within. When the pressure is

withdrawn the thread goes back as the finger of a glove may be turned

back into the glove by turning the finger outside in. When a minute

animal, or other particle of food comes in contact with a tentacle, how

does the tentacle get the food to the mouth? By bending and bringing

the end to the mouth, or by shortening and changing its form, or in

both ways? (Fig. 34, _C_.) Do the neighboring tentacles seem to bend

over to assist a tentacle in securing prey? (Fig. 34, _C_.)

[Illustration: FIG. 37.--HYDRA capturing a water flea.]

=Digestion.=--The food particles break up before remaining long in the

stomach, and the nutritive part is absorbed by the lining cells, or

endoderm (Fig. 39). The indigestible remnants go out through the mouth.

The hydra is not provided with a special vent. Why could the vent not

be situated at the end opposite the mouth?

=Circulation and Respiration.=--Does water have free access to the body

cavity? Does the hydra have few or nearly all of its cells exposed to

the water in which it lives? From its structure, decide whether it

can breathe like a sponge or whether special respiratory cells are

necessary to supply it with oxygen and give off carbon dioxid. Blood

vessels are unnecessary for transferring oxygen and food from cell to

cell.

[Illustration: FIG. 38.--HYDRAS on pondweed.]

=Reproduction.=--Do you see any swellings upon the side of the

hydra? (Fig. 34, A.) If the swelling is near the tentacles, it is a

_spermary_; if near the base it is an _ovary_. A sperm coalesces with

or fertilizes the ovum after the ovum is exposed by the breaking of the

ovary wall. Sometimes the sperm from one hydra unites with the ovum of

another hydra. This is called _cross-fertilization_. The same term is

applied to the process in plants when the male element, or pollen, of

the flower unites with the ovules, or female element, of the flower

on another plant. The hydra, like most plants and some other animals,

is hermaphrodite, that is to say, both sperms and ova are produced by

one individual. In the autumn, eggs are produced with hard shells to

withstand the cold until spring. Sexual reproduction takes place when

food is scarce. Asexual generation (by budding) is common with the

hydra when food supply is abundant. After the bud grows to a certain

size, the outer layer of cells at the base of the bud constricts and

the young hydra is detached.

=Compare the sponge and the hydra= in the following respects:--many

celled, or one celled; obtaining food; breathing; tubes and cavities;

openings; reproduction; locomotion. Which ranks higher among the

metazoa? The metazoa, or many celled animals, include all animals

except which branch?

[Illustration: FIG. 39.--Longitudinal section of hydra (microscopic and

diagrammatic).]

Figure 39 is a _microscopic view_ of a vertical section of a hydra to

show the =structure of the body wall=. There is an outer layer called

the _ectoderm_, and an inner layer called the _endoderm_. There

is also a thin supporting layer (black in the figure) called the

_mesoglea_. The mesoglea is the thinnest layer. Are the cells larger

in the endoderm or the ectoderm? Do both layers of cells assist in

forming the reproductive bud? The ectoderm cells end on the inside

in contractile tails which form a thin line and have the effect of

muscle fibers. They serve the hydra for its remarkable changes of

shape. When the hydra is cut in pieces, each piece makes a complete

hydra, provided it contains both endoderm and ectoderm.

In what ways does the hydra show “=division of labor=”? Answer this

by explaining the classes of cells specialized to serve a different

purpose. Which cells of the hydra are least specialized? In what

particulars is the plan of the hydra different from that of a simple

sponge? An ingenious naturalist living more than a century ago,

asserted that it made no difference to the hydra whether the ectoderm

or the endoderm layer were outside or inside,--that it could digest

equally well with either layer. He allowed a hydra to swallow a worm

attached to a thread, and then by gently pulling in the thread,

turned the hydra inside out. More recently a Japanese naturalist

showed that the hydra could easily be turned inside out, but he also

found that when left to itself it soon reversed matters and returned

to its natural condition, that =the cells are really specialized= and

each layer can do its own work and no other.

=Habits.=--The hydra’s whole body is a hollow bag, the cavity extending

even into the tentacles. The tentacles may increase in number as the

hydra grows but seldom exceed eight. The hydra has more active motion

than locomotion. It seldom moves from its place, but its tentacles are

constantly bending, straightening, contracting, and expanding. The body

is also usually in motion, bending from one side to another. When the

tentacles approach the mouth with captured prey, the mouth (invisible

without a hand lens) opens widely, showing five lobes or lips, and the

booty is soon tucked within. A hydra can swallow an animal larger in

diameter than itself.

The =endoderm cells= have _ameboid motion_, that is, they extend

pseudopods. They also resemble amebas in the power of _intra-cellular

digestion_; that is, they absorb the harder particles of food and

digest them afterwards, rejecting the indigestible portions. Some of

these cells have _flagella_ (see Fig. 39) which keep the fluid of the

cavity in constant motion.

Sometimes the =hydra moves= after the manner of a small caterpillar

called a “measuring worm,” that is, it takes hold first by the foot,

then by the tentacles, looping its body at each step. Sometimes the

body goes end over end in slow somersaults.

The _length_ of the extended hydra may reach one half inch. When

touched, both tentacles and body contract until it looks to the unaided

eye like a round speck of jelly. This shows _sensibility_, and a few

small star-shaped cells are believed to be _nerve cells_, but the hydra

has not a nervous _system_. Hydras show their liking for light by

moving to the side of the vessel or aquarium whence the light comes.

[Illustration: FIG. 40.--HYDROID COLONY, with nutritive (_P_)

reproductive (_M_) and defensive (_S_) hydranths.]

=The Branch Polyps= (sometimes called _Cœlenterata_).--The hydra is the

_only fresh water representative_ of this great branch of the animal

kingdom. This branch is characterized by its members having only one

opening to the body. The polyps also include the salt water animals

called _hydroids_, _jellyfishes_, and _coral polyps_.

[Illustration: FIG. 41.--“PORTUGUESE MAN-O’-WAR” (compare with Fig.

40). A floating hydroid colony with long, stinging (and sensory)

streamers. Troublesome to bathers in Gulf of Mexico. Notice

balloon-like float.]

=Hydroids.=--Figure 40 shows a _hydroid_, or group of hydra-like

growths, one of which eats and digests for the group, another defends

by nettling cells, another produces eggs. Each hydra-like part of a

hydroid is called a _hydranth_. Sometimes the buds on the hydra remain

attached so long that a bud forms upon the first bud. Thus three

generations are represented in one organism. Such growths show us that

it is not always easy to tell what constitutes an individual animal.

[Illustration: FIG. 42.--The formation of many free swimming

jellyfishes from one fixed hydra-like form. The saucer-like parts (_h_)

turn over after they separate and become like Fig. 43 or 44. Letters

show sequence of diagrams.]

_Hydroids_ may be conceived _to have been developed_ by the failure

of budding hydras to separate from the parent, and by the gradual

formation of the habit of living together and assisting each other.

When each hydranth of the hydroid devoted itself to a special function

of digestion, defense, or reproduction, this group lived longer and

prospered; more eggs were formed, and the habits of the group were

transmitted to a more numerous progeny than were the habits of a group

where members worked more independently of each other.

As the _sponge_ is the first, lowest, and simplest example of the

devotion of _special cells to special purposes_, the hydroid is the

first, lowest, and simplest example of the occurrence of _organs_, that

is of _special parts of the body_ (groups of cells) _set aside for a

special work_.

How many mature hydranths are seen in the hydroid shown in Fig. 40?

Why are the defensive hydranths on the outside of the colony? Which

hydranths have no tentacles? Why not?

=Jellyfish.=--=Alternation of Generations.=--=Medusa.=--With some

species of hydroids, a very curious thing happens.--The _hydranth

that is to produce the eggs falls off_ and becomes independent of the

colony. More surprising still, its appearance changes entirely and

instead of being hydra-like, it becomes the large and complex creature

called _jellyfish_ (Fig. 43). But the _egg of the jellyfish_ produces a

small _hydra-like animal_ which gives rise by budding to a _hydroid_,

and the cycle is complete.

[Illustration: FIG. 43.--A JELLYFISH.]

[Illustration: FIG. 44.--A JELLYFISH (medusa).]

The bud (or reproductive hydranth) of the hydroid does not produce a

hydroid, but a jellyfish; the egg of the jellyfish does not produce a

jellyfish, but a hydroid. This is called by zoologists, _alternation of

generations_. A _complete individual_ is the life from the germination

of one egg to the production of another. So that an “individual”

consists of a hydroid colony fixed in one place together with all the

jellyfish produced from its buds, and which may now be floating miles

away from it in the ocean. Bathers in the surf are sometimes touched

and stung by the long, streamer-like tentacles of the jellyfish. These,

like the tentacles of the hydra, have nettling cells (Fig. 41).

The umbrella-shaped free swimming jellyfish is called a _medusa_ (Fig.

44).

=Coral Polyps.=--Some of the salt water relatives of the hydra

produce buds which remain attached to the parent without, however,

becoming different from the parent in any way. The _coral polyps_ and

_corallines_ are examples of _colonies_ of this kind, possessing a

common stalk which is formed as the process of multiplication goes

on. In the case of coral polyps, the separate animals and the flesh

connecting them secrete within themselves a hard, _limy, supporting

structure known as coral_. In some species, the coral, or stony part,

is so developed that the polyp seems to be inserted in the coral, into

which it withdraws itself for partial protection (Fig. 45).

[Illustration: FIG. 45.--CORAL POLYPS (tentacles, a multiple of _six_).

Notice hypostome.]

The _corallines_ secrete a smooth stalk which affords no protection,

but they also secrete a coating or sheath which incloses both

themselves and the stalk. The coating has apertures through which the

polyps protrude in order to feed when no danger is near (Fig. 46). The

red “corals” used for jewelry are bits of stalks of corallines. The

corallines (Figs. 47, 48) are not so abundant nor so important as the

coral polyps (Figs. 45, 49).

[Illustration: FIG. 46.--RED CORALLINE with crust and polyps (_eight_

tentacles).]

[Illustration: FIG. 47.--SEA FAN (a coralline).]

Colonies of coral polyps grow in countless numbers in the tropical

seas. The coral formed by successive colonies of polyps accumulates

and builds up many islands and important additions to continents. The

Florida “keys,” or islands, and the southern part of the mainland of

Florida were so formed.

[Illustration: FIG. 48.--ORGAN PIPE “Coral” (a coralline).]

The =Sea Anemone=, like the coral polyp, lives in the sea, but like

the fresh water hydra, it _deposits no limy support for its body_.

The anemone is much larger than the hydra and most coral polyps,

many species attaining a height of several inches. It _does not

form colonies._ When its arms are drawn in, it looks like a large

knob of shiny but opaque jelly. Polyps used to be called _zoophytes

(plant-animals)_, because of their flower-like appearance (Figs. 50,

51).

[Illustration: FIG. 49.--UPRIGHT CUT through coral polyp × 4.

_ms_, mouth; _mr_, gullet; _ls, ls_, fleshy partitions (mesenteries)

extending from outer body wall to gullet (to increase absorbing

surface); _s, s_, shorter partitions; _mb, fb_, stony support (of lime,

called coral); _t_, tentacles.]

[Illustration: FIG. 50.--SEA ANEMONE.]

[Illustration: FIG. 51.--SEA ANEMONES.]

CHAPTER V

ECHINODERMS (SPINY ANIMALS)

THE STARFISH

SUGGESTIONS. Since the echinoderms are aberrant though interesting

forms not in the regular line of development of animals, this chapter

may be omitted if it is desired to shorten the course.--The common

starfish occurs along the Atlantic coast. It is captured by wading

along the shore when the tide is out. It is killed by immersion in

warm, fresh water. Specimens are usually preserved in 4 per cent

formalin. Dried starfish and sea urchins are also useful. A living

starfish kept in a pail of salt water will be instructive.

[Illustration: FIG. 52.--Starfish on a rocky shore.]

[Illustration: FIG. 53.--PLAN of starfish; III, madreporite.]

=External Features.=--Starfish are usually brown or yellow. Why? (See

Fig. 52.) Has it a head or tail? Right and left sides? What is the

shape of the _disk_, or part which bears the five arms or _rays_?

(Fig. 53.) Does the body as a whole have symmetry on two sides of a

line (bilateral symmetry), or around a point (radial symmetry)? Do the

separate rays have bilateral symmetry? The _skeleton_ consists of limy

plates embedded in the tough skin (Fig. 54). Is the _skin_ rough or

smooth? Hard or soft? Are the projections (or _spines_) in the skin

long or short? The skin is hardened by the limy plates, except around

the _mouth_, which is at the center of the lower side and surrounded

by a membrane. Which is rougher, the mouth side, (_oral_ side) or the

opposite (_aboral_ side)? Which side is more nearly flat? The _vent_ is

at or near the center of the disk on the aboral surface. It is usually

very small and sometimes absent. Why a vent is not of much use will be

understood after learning how the starfish takes food.

[Illustration: FIG. 54.--LIMY PLATES in portion of a ray.]

[Illustration: FIG. 55.--Starfish (showing MADREPORITE).]

An organ peculiar to animals of this branch, and called the _madreporic

plate_, or _madreporite_, is found on the aboral surface between the

bases of two rays (Fig. 55). It is wartlike, and usually white or red.

This plate is a _sieve_; the small openings keep out sand but allow

water to filter through.

[Illustration: FIG. 56.--WATER tube System of starfish.

_m_, madreporite; _stc_, stone canal; _ap_, ampulla.]

=Movements: the Water-tube System.=--The water, which is filtered

through the perforated madreporite, is needed to supply a _system of

canals_ (Fig. 56). The madreporite opens into a canal called the _stone

canal_, the wall of which is hardened by the same kind of material

as that found in the skin. The stone canal leads to the ring canal

which surrounds the mouth (Fig. 56). The ring canal sends _radial

canals_ into each ray to supply the double row of _tube feet_ found in

the groove at the lower side of each ray (Fig. 57). Because of their

arrangement in rows, the feet are also called _ambulacral_ feet (Latin

_ambulacra_, “forest walks”). There is a water holder (_ampulla_), or

muscular water bulb at the base of each tube foot (Fig. 58). These

contract and force the water into the tube feet and extend them. The

cuplike ends of the tubes cling to the ground by suction. The feet

contain delicate muscles by which they contract and shorten. Thus the

animal pulls itself slowly along, hundreds of feet acting together.

The tube feet, for their own protection, may contract and retire into

the groove, the water which extended them being sent back into the

ampulla. This system of water vessels (or water-vascular system) of the

echinodermata is characteristic of them; _i.e._ is not found elsewhere

in the animal kingdom. The grooves and the plates on each side of them

occupy the _ambulacral areas_. The rows of spines on each side of the

grooves are freely movable. (What advantage?) The spines on the aboral

surface are not freely movable.

[Illustration: FIG. 57.--Starfish, from below; tube feet extended.]

[Illustration: FIG. 58.--SECTION OF ONE RAY and central portion of

starfish.

_f1_, _f2_, _f3_, tube feet more or less extended; _au_, eye spot;

_k_, gills; _da_, stomach; _m_, madreporite; _st_, stone canal; _p_,

ampulla; _ei_, ovary.]

=Respiration.=--The _system of water vessels serves the additional

purpose_ of bringing water containing oxygen into contact with various

parts of the body, and the starfish was formerly thought to have no

special respiratory organs. However there are holes in the aboral wall

through which the folds of the delicate lining membrane protrude. These

are now supposed to be _gills_ (_k_, Fig. 58).

=The nervous system= is so close to the aboral surface that much of

it is visible without dissection. Its chief parts are a _nerve ring_

around the mouth, which sends off a _branch_ along each ray. These

branches may be seen by separating the rows of tube feet. They end in a

pigmented cell at the end of each ray called the _eye-spot_.

=The food= of starfish consists of such animals as crabs, snails, and

oysters. When the prey is too large to be taken into the mouth, the

starfish _turns its stomach inside out_ over the prey (Fig. 59). After

the shells separate, the stomach is applied to the soft digestible

parts. After the animal is eaten, the stomach is retracted. This odd

way of eating is very economical to its digestive powers, for _only

that part of the food which can be digested and absorbed is taken into

the body_. Only the lower part of the stomach is wide and extensible.

The upper portion (next to the aboral surface) is not so wide. This

portion receives the secretion from five pairs of digestive glands,

a pair of which is situated in each ray. Jaws and teeth are absent.

(Why?) The vent is sometimes wanting. Why?

[Illustration: FIG. 59.--Starfish eating a sea snail.

_b_, stomach everted.]

=Reproduction.=--There is a pair of ovaries at the base of each ray of

the female starfish (Fig. 58). The spermaries of the male have the same

position and form as the ovaries, but they are lighter colored, usually

white.[2]

[2] The sperm cells and egg cells are poured out into the water by

the adults, and the sperm cell, which, like all sperm cells, has a

vibratory, tail-like flagellum to propel it, reaches and fertilizes the

egg cell.

=Regeneration after Mutilation.=--If a starfish loses one or more rays,

they are replaced by growth. Only a very ignorant oyster-man, angry at

the depredations of starfish upon his oyster beds, would chop starfish

to pieces, as this only serves to multiply them. This power simulates

multiplication by division in the simplest animals.

=Steps in Advance of Lower Branches.=--The starfish and other

echinodermata have a more developed nervous system, sensory organs, and

digestion, than forms previously studied; most distinctive of all, they

have a body cavity distinct from the food cavity. The digestive glands,

reproductive glands, and the fluid which serves imperfectly for blood,

are in the body cavity. There is no heart or blood vessels. The motions

of the stomach and the bending of the rays give motion to this fluid in

the body cavity. It cannot be called blood, but it contains white blood

corpuscles.

[Illustration: FIG. 60.--Young starfish crawling upon their mother.

(Challenger Reports.)]

The starfish when first hatched is an actively swimming bilateral

animal, but it soon becomes starlike (Fig. 60). The limy plates of the

starfish belong neither to the outer nor inner layer (endoderm and

ectoderm) of the body wall, but to a third or middle layer (mesoderm);

for echinoderms, like the polyps, belong to the three-layered animals.

In this its skeleton differs from the shell of a crawfish, which is

formed by the hardening of the skin itself.

=Protective Coloration.=--Starfish are brown or yellow. This makes them

inconspicuous on the brown rocks or yellow sands of the seashore. This

is an example of protective coloration.

THE SEA URCHIN

=External Features.=--What is the _shape_ of the body? What kind of

_symmetry_ has it? Do you find the oral (or mouth) surface? The aboral

surface? Where is the body flattened? What is the shape of the spines?

What is their use? How are the tube feet arranged? Where do the rows

begin and end? Would you think a sea urchin placed upside down in

water, could right itself less or more readily than a starfish? What

advantage in turning would each have that the other would not have? The

name sea urchin has no reference to a mischievous boy, but means sea

hedgehog (French _oursin_, hedgehog), the name being suggested by its

spines.

=Comparison of Starfish and Sea Urchin.=--The water system of the sea

urchin, consisting of madreporite, tubes, and water bulbs, or ampullæ,

is similar to that of the starfish. The tube feet and locomotion are

alike. There is no need for well-developed respiratory organs in either

animal, as the whole body, inside and out, is bathed in water. The

method of reproduction is the same.

[Illustration: FIG. 61.--A SEA URCHIN crawling up the glass front wall

of an aquarium (showing mouth spines and tube feet).]

[Illustration: FIG. 62.--A SEA URCHIN with spines removed, the limy

plates showing the knobs on which the spines grew.]

[Illustration: FIG. 63.--SECTION OF SEA URCHIN with soft parts removed,

showing the jaws which bear the teeth protruding in Fig. 62.]

The starfish eats animal food. The food of the sea urchin is almost

exclusively vegetable, hence it needs teeth (Fig. 62, 63); its food

tube is longer than that of a starfish, just as the food tube of a

sheep, whose food digests slowly, is much longer than that of a dog.

[Illustration: FIG. 64.--THE SEA OTTER, an urchin with mouth (_o_) and

vent (_A_) on same side of body.]

The largest species of sea urchins are almost as big as a child’s

head, but this size is unusual. The spines are mounted on knobs, and

the joint resembles a ball-and-socket joint, and allow as wide range

of movement. Some sea urchins live on sandy shores, other species live

upon the rocks. The sand dollars are lighter colored. (Why?) They are

usually flatter and have lighter, thinner walls, for there is danger of

sinking into the sand. The five-holed sand cake or sand dollar has its

weight still further diminished by the holes, which also allow it to

rise more easily through the water. The flattened lower surface of both

starfish and sea urchin causes the body to remain still while the tube

feet are stretching forward for another step.

OTHER ECHINODERMS

The =sea cucumbers=, or =holothurians=, resemble the sea urchin in

many respects, but their bodies are elongated, and the limy plates are

absent or very minute. The mouth is surrounded by tentacles (Fig. 65).

[Illustration: FIG. 65.--SEA CUCUMBERS.]

[Illustration: FIG. 66.--A BRITTLE STAR.]

The =brittle stars= resemble the starfish in form, but their rays are

very slender, more distinct from the disk, and the tube feet are on the

edges of the rays, not under them (Fig. 66).

The =crinoids= are the most ancient of the echinoderms. (Figs. 67,

68.) Their fossils are very abundant in the rocks. They inhabited

the geological seas, and it is believed that the other echinoderms

descended from them. A few now inhabit the deep seas. Some species

are fixed by stems when young, and later break away and become

free-swimming, others remain fixed throughout life.

The four classes of the branch echinoderms are Starfish (_asteroids_),

Sea urchins (_echinoids_), Sea cucumbers (_holothurians_), and Sea

lilies (_crinoids_).

[Illustration: FIG. 67.--CRINOID, arms closed.]

[Illustration: FIG. 68.--DISK OF CRINOID from above, showing mouth in

center and vent near it, at right (arms removed).]

Comparative Review

Make a table like this as large as the page of the notebook will allow,

and fill in without guessing.

===================+=========+========+=========+=========+=========

| AMEBA | SPONGE | HYDRA | CORAL | STARFISH

| | | | POLYP |

-------------------+---------+--------+---------+---------+---------

| | | | |

Is body round, two-| | | | |

sided, or irregular| | | | |

| | | | |

-------------------+---------+--------+---------+---------+---------

| | | | |

What organs of | | | | |

sense | | | | |

| | | | |

-------------------+---------+--------+---------+---------+---------

| | | | |

Openings into body | | | | |

| | | | |

| | | | |

-------------------+---------+--------+---------+---------+---------

| | | | |

Hard or supporting | | | | |

parts of body | | | | |

| | | | |

-------------------+---------+--------+---------+---------+---------

| | | | |

How food is taken | | | | |

| | | | |

| | | | |

-------------------+---------+--------+---------+---------+---------

| | | | |

How move | | | | |

| | | | |

| | | | |

-------------------+---------+--------+---------+---------+---------

| | | | |

How breathe | | | | |

| | | | |

| | | | |

===================+=========+========+=========+=========+=========

CHAPTER VI

WORMS

SUGGESTIONS:--Earthworms may be found in the daytime after a heavy

rain, or by digging or turning over planks, logs, etc., in damp

places. They may be found on the surface at night by searching with a

lantern. Live specimens may be kept in the laboratory in a box packed

with damp (not wet) loam and dead leaves. They may be fed on bits of

fat meat, cabbage, onion, etc., dropped on the surface. When studying

live worms, they should be allowed to crawl on damp paper or wood. An

earthworm placed in a glass tube with rich, damp soil, may be watched

from day to day.

=External Features.=--Is the body _bilateral_? Is there a _dorsal_ and

_ventral_ surface? Can you show this by a test with a live worm? Do you

know of an animal with dorsal and ventral surface, but not bilateral?

[Illustration: FIG. 69.--An Earthworm.]

Can you make out a head? A head end? A neck? Touch the head and test

whether it can be made to crawl backwards. Which end is more tapering?

Is the mouth at the tip of the head end or on the upper or lower

surface? How is the _vent_ situated? Its shape? As the worm lies on

a horizontal surface, is the body anywhere flattened? Are there any

_very_ distinct divisions in the body? Do you see any _eyes_?

=Experiment= to find whether the worm is sensitive (1) to _touch_,

(2) to _light_, (3) to strong _odors_, (4) to irritating liquids.

Does it show a sense of _taste_? The experiments should show whether

it avoids or seeks a bright light, as a window; also whether any

parts of the body are especially sensitive to touch, or all equally

sensitive. What effect when a bright light is brought suddenly near

it at night?

Is _red blood_ visible through the skin? Can you notice any

_pulsations_ in a vessel along the back? Do all earthworms have the

same number of _divisions_ or rings? Compare the size of the rings or

segments. Can it crawl faster on glass or on paper?

[Illustration: FIG. 70.--MOUTH AND SETÆ.]

[Illustration: FIG. 71.--EARTHWORM, mouth end above.]

A magnifying glass will show on most species tiny bristle-like

projections called _setæ_. How are the setæ arranged? (_d_, Fig. 70.)

How many on one ring of the worm? How do they point? Does the worm feel

smoother when it is pulled forward or backward between the fingers?

Why? Are setæ on the lower surface? Upper surface? The sides? What is

the use of the setæ? Are they useful below ground? Does the worm move

at a uniform rate? What change in form occurs as the front part of the

body is pushed forward? As the hinder part is pulled onward? How far

does it go at each movement? At certain seasons a broad band, or ring,

appears, covering several segments and making them seem enlarged (Fig.

71). This is the _clitellum_, or _reproductive girdle_. Is this girdle

nearer the mouth or the tail?

=Draw= the exterior of an earthworm.

=Dorsal and Ventral Surfaces.=--The earthworm always crawls with the

same surface to the ground; this is called the _ventral_ surface, the

opposite surface is the _dorsal_ surface. This is the first animal

studied to which these terms are applicable. What are the ventral and

dorsal surfaces of a fish, a frog, a bird, a horse, a man?

=The name “worm”= is often carelessly applied to various crawling

things in general. It is properly applied, however, only to

_segmented animals without jointed appendages_. Although a

caterpillar crawls, it is not a worm for several reasons. It has six

jointed legs, and it is not a developed animal, but only an early

stage in the life of a moth or butterfly. A “grubworm” also has

jointed legs (Fig. 167). It does not remain a grub, but in the adult

stage is a beetle. A worm never develops into another animal in the

latter part of its life; its setæ are not jointed.

[Illustration: FIG. 72.--FOOD TUBE of earthworm. (Top view.)]

[Illustration: FIG. 73.--FOOD TUBE AND BLOOD VESSELS of earthworm

showing the ring-like hearts. (Side view.)]

=The Food Tube.=--The earthworm has no teeth, and the food tube, as

might be inferred from the form of the body, is simple and straight. On

account of slight variation in size and structure, its parts are named

the pharynx (muscular), gullet, crop, gizzard (muscular), and the long

intestine extending through the last three fourths of its body (Fig.

72). The functions of the parts of the food tube are indicated by their

names.

[Illustration: FIG. 74.]

=Circulation.=--There is a _large dorsal_ blood vessel above the food

tube (Fig. 73). From the front portion of this tube arise several large

tubular rings or “hearts” which are contractile and serve to keep the

blood circulating. They lead to a _ventral vessel_ below the food

tube (Fig. 74). The blood is red, but the _coloring matter_ is in the

liquid, not in the blood cells.

=Nervous System.=--Between the ventral blood vessels is a _nerve cord_

composed of two strands (see Fig. 75). There is a slight swelling, or

_ganglion_, on each strand, in each segment (Fig. 75). The strands

separate near the front end of the worm, and a branch goes up each side

of the gullet and enters the two pear-shaped _cerebral ganglia_, or

“brain” (Fig. 75).

[Illustration: FIG. 75.--GANGLIA NEAR MOUTH and part of nerve chain of

earthworm.]

=Food.=--The earthworm eats earth containing organic matter, the

inorganic part passing through the vent in the form of circular casts

found in the morning at the top of the earthworm’s hole. What else does

it eat?

The earth worm needs no teeth, as it excretes through the mouth an

_alkaline fluid_ which softens and partly digests the food before it

is eaten. When this fluid is poured out upon a green leaf, the leaf at

once turns brown. The starch in the leaf is also acted upon. The snout

aids in pushing the food into the mouth.

=Kidneys.=--Since oxidation is occurring in its tissues, and impurities

are forming, there must be some way of _removing impurities from the

tissues_. The earthworm does not possess one-pair organs like the

kidneys of higher animals to serve this purpose, but it has numerous

pairs of small tubular organs called _nephridia_ which serve the

purpose. Each one is simply a tube with several coils (Fig. 76). There

is a pair on the floor of each segment (Fig. 76). Each nephridium has

an inner open end within the body cavity, and its outer end opens by a

pore on the surface between the setæ (Fig. 78). The nephridia absorb

waste water from the liquid in the _celom_, or body cavity surrounding

the food tube, and convey it to the outside.

[Illustration: FIG. 76.--TWO PAIRS OF NEPHRIDIA.]

=Respiration.=--The skin of the earthworm is moist, and the blood

capillaries approach so near to the surface of the body that the oxygen

is constantly passing in from the air, and carbon dioxid passing out;

hence it is constantly breathing through all parts of its skin. _It

needs no lungs_ nor special respiratory organs of any kind.

[Illustration: FIG. 77.--Sperm (_sp_) and egg glands (_es_) of

earthworm.]

=Reproduction.=--When one individual animal produces both sperm cells

and egg cells, it is said to be hermaphrodite. This is true of the

earthworm. The egg cell is always fertilized, however, not by the

sperm cells of the same worm, but by sperm cells formed by another

worm. The openings of these ova or _egg glands_ consist of two pairs

of small pores found on the ventral surface of the fourteenth and

fifteenth segments in most species (see Fig. 77). There are also two

pairs of small _receptacles_ for temporarily holding the _foreign

sperm cells_. One pair of the openings from these receptacles is found

(with difficulty) in the wrinkle behind the ninth segment (Fig. 77),

and the other pair behind the tenth segment. The _sperm glands_ are

in front of the ovaries (Fig. 77), but the _sperm ducts_ are longer

than the _oviducts_, and open behind them (Figs. 77, 78). The worms

exchange sperm cells, but not egg cells. The reproductive girdle, or

_clitellum_, already spoken of, forms the case which is to hold the

eggs (see Fig. 71). When the sperm cells have been exchanged, and the

ova are ready for fertilization, the worm draws itself backward from

the collarlike case or clitellum so that it slips over the head. As it

passes the fifteenth and sixteenth segments, it collects the ova, and

as it passes the ninth and tenth segments, it collects the sperm cells

previously received by touching another worm. The elastic, collar-like

clitellum closes at the ends after it has slipped over the worm’s head,

forming a _capsule_. The ova are _fertilized in this capsule_, and some

of them hatch into worms in a few days. These devour the eggs which do

not hatch. The eggs develop into complete but very small worms before

the worms escape from the capsule.

[Illustration: FIG. 78.--Side view showing setæ, nephridia pores, and

reproductive openings.]

=Habits.=--The earthworm is omnivorous. It will eat bits of meat as

well as leaves and other vegetation. It has also the advantage, when

digging its hole, of _eating the earth_ which must be excavated.

Every one has noticed the fresh “casts” piled up at the holes in the

morning. As the holes are partly filled by rains, the casts are most

abundant after rains. The chief _enemies_ of the earthworm are moles

and birds. The worms _work at night_ and retire so early in the morning

that it takes a very early bird to catch a worm. Perhaps the nearest

to an intelligent act the earthworm accomplishes is to _conceal the

mouth of its hole_ by plugging it with a pebble or bit of leaf. They

_hibernate_, going below danger of frost in winter. In dry weather they

burrow several feet deep.

=The muscular coat= beneath, and much thicker than the skin, consists

of two layers: an outer _layer runs around the body_ just beneath the

skin, and an inner, thicker _layer of fibers runs lengthwise_. The worm

crawls by shortening the longitudinal muscles. As the bristles (_setæ_)

point backward, they prevent the front part of the body from slipping

back, so the hinder part is drawn forward. Next, the circular muscles

contract, and the bristles preventing the hind part from slipping back,

the fore portion is pushed forward. Is the worm thicker when the hinder

part is being pulled up or when the fore part is being thrust forward?

Does the earthworm pull or push itself along, or does it do both?

Occasionally it travels backward, _e.g._ it sometimes goes backward

into its hole. Then the bristles are directed forward.

The right and left halves of the body are counterparts of each other,

hence the earthworm is _bilaterally symmetrical_. The lungs and gills

of animals must always be kept moist. The worm _cannot live long in dry

air_, for respiration in the skin ceases when it cannot be kept moist,

and the worm smothers. Long immersion in water is injurious to them,

perhaps because there is far less oxygen in water than in the air.

Darwin wrote a book called “Vegetable Mold and Earthworms.” He

estimated that there were fifty thousand earthworms to the acre on

farm land in England, and that they bring up eighteen tons of soil in

an acre each year. As the acids of the food tube act upon the mineral

grains that pass through it, the earthworm renders _great aid in

forming soil_. By burrowing it makes the soil more _porous_ and brings

up the subsoil.

Although without eyes, the worm is sensitive to light falling upon its

anterior segments. When the light of a lantern suddenly strikes it at

night, it crawls quickly to its burrow. Its sense of touch is so keen

that it can detect a light puff of breath. Which of the foods kept in a

box of damp earth disappeared first? What is indicated as to a sense of

taste?

Why is the bilateral type of structure better adapted for development

and higher organization than the radiate type of the starfish? The

earthworm’s body is a double tube; the hydra’s body is a single tube;

which plan is more advantageous, and why? Would any other color do just

as well for an earthworm? Why, or why not?

The _sandworm_ (Nereis) lives in the sand of the seashore, and swims

in the sea at night (Fig. 79). It is more advanced in structure than

the earthworm, as it has a distinct head (Fig. 80), eyes, two teeth,

two lips, and several pairs of antennæ, and two rows of muscular

projections which serve as feet. It is much used by fishermen for

bait. If more easily obtained, it may be studied instead of the

earthworm.

[Illustration: FIG. 79.--SAND WORM × ²⁄₃ (Nereis).]

=There are four classes in the branch Vermes=: 1) the _earthworms_,

including sandworms and leeches; 2) the _roundworms_, including

trichina, hairworms, and vinegar eels; 3) _flatworms_, including

tapeworm and liver fluke; 4) _rotifers_, which are mere specks in size.

[Illustration: FIG. 80.--HEAD OF SANDWORM (enlarged).]

The =tapeworm= is a flatworm which has lost most of its organs on

account of its parasitic life. Its egg is picked up by an herbivorous

animal when grazing. The embryo undergoes only partial development

in the body of the herbivorous animal, _e.g._ an ox. The next stage

will not develop until the beef is eaten by a carnivorous animal, to

whose food canal it attaches itself and soon develops a long chain of

segments called a “tape.” Each segment absorbs fluid food through its

body wall. As the segments at the older end mature, each becomes full

of germs, and the segments become detached and pass out of the canal,

to be dropped and perhaps picked up by an herbivorous animal and repeat

the life cycle.

The =trichina= is more dangerous to human life than the tapeworm.

It gets into the food canal in uncooked pork (bologna sausage, for

example), multiplies there, migrates into the muscles, causing great

pain, and encysts there, remaining until the death of the host. It is

believed to get into the bodies of hogs again when they eat rats, which

in turn have obtained the cysts from carcasses.

=Summary of the Biological Process.=--An earthworm is _a living machine

which does work_ (digging and crawling; seizing, swallowing, and

digesting food; pumping blood; growing and reproducing). To do the work

it must have a continual _supply of energy_. The energy for its work

is set free by the protoplasm (in its microscopic cells) undergoing

a destructive chemical change (_oxidation_). The waste products from

the breaking down of the protoplasm must be continually removed

(_excretion_). The broken-down protoplasm must be continually replaced

if life is to continue (the income must exceed the outgo if the animal

is still growing). The microscopic cells construct more protoplasm out

of food and oxygen (_assimilation_) supplied them by the processes of

nutrition (eating, digesting, breathing, circulating). This protoplasm

in turn oxidizes and releases more energy to do work, and thus the

cycle of life proceeds.

CHAPTER VII

CRUSTACEANS

CRAWFISH

SUGGESTIONS.--In regions where crawfish are not found, a live crab

may be used. Locomotion and behavior may be studied by providing a

tub of water, or better, a large glass jar such as a broad candy jar.

For suggestions on study of internal structure, see p. 58.

=Habitat.=--Do you often see crawfish, or crayfish, moving about, even

in water where they are known to be abundant? What does your answer

suggest as to the time when they are probably most active?

Why do you never see one building its chimney, even where crawfish

holes are abundant? Is the chimney always of the same color as the

surface soil? Are the crawfish holes only of use for protection? In

what kind of spots are crawfish holes always dug? Why? What becomes of

crawfish when the pond or creek dries up? How deep are the holes? How

large are the lumps of mud of which the chimney is built? How does it

get them out of the hole? Why is the mud built into a chimney instead

of thrown away? (What would happen to a well with its mouth no higher

than the ground?) Why are crawfish scarce in rocky regions, as New

England?

How does the color of the crawfish compare with its surroundings? Is

its color suited to life in clear or muddy water? Define protective

coloration.

=Habits.=--Does the crawfish walk better in water or out of it? Why?

Does it use the legs with the large claws to assist in walking? Do

the swimmerets (under the abdomen) move fast or slow? (Observe it

from below in a large jar of clear water.) What propels it backward?

Forward? Does the crawfish move at a more uniform rate when swimming

backward or forward? Why? In which way can it swim more rapidly? Do

the big legs with claws offer more resistance to the water while it

is swimming backward or forward? How does it hold the tail after the

stroke, while it is darting backward through the water? Hold a crawfish

with its tail submerged and its head up. Can the tail strike the water

with much force? Allow it to grasp a pencil: can it sustain its own

weight by its grip?

=Feeding.=--Offer several kinds of food to a crawfish that has not

been alarmed or teased. Does it prefer bread, meat, or vegetables?

How does it get the food to its mouth? Does it eat rapidly or slowly?

Does it tear the food with the big pincers? Can it gnaw with the small

appendages near the mouth?

=Breathing.=--Does the crawfish breathe with gills or lungs? Place a

few drops of ink near the base of the hind legs of a crawfish resting

quietly in shallow water. Where is the ink drawn in? Where does it come

out? To explain the cause and purpose of this motion, place a crawfish

in a large glass jar containing water, and see the vibratory motion of

the parts under the front portion of the body. There is a gill paddle,

or gill bailer, under the shell on each side of the body that moves at

the same rate.

=Senses.=--Crawfish are best caught with a piece of meat or beef’s

liver tied to a string. Do they always lose hold as soon as they are

lifted above the water? What do you conclude as to the alertness of

their senses? Does the covering of its body suggest the possession of a

delicate or dull sense of touch?

Of what motions are the _eyes_ capable? Touch one of the eyes. The

result? Can a crawfish see in all directions? To test this, place a

crawfish on a table and try whether you can move to a place where you

can see the crawfish without seeing its eyes. What are the advantages

and disadvantages of having the eyes on stalks?

[Illustration: FIG. 81.--CRAWFISH (dorsal surface).]

[Illustration: FIG. 82.]

Touch the body and the several appendages of the crawfish. Where does

it seem most sensitive to _touch_? Which can reach farther, the antennæ

or the big claws? Why are short feelers needed as well as long ones?

Make a loud and sudden noise without jarring the crawfish. Is it

affected by _sound_?

=External Anatomy= (Figs. 81, 82, 83, 84).--Is the body of the crawfish

rounded out (convex) everywhere, or is any part of its surface either

flat or rounded in (concave)? What _color_ has the crawfish? Is this

color of any use to the crawfish?

[Illustration: FIG. 83.--LATERAL VIEW OF CRAWFISH.]

[Illustration: FIG. 84.--FOURTH ABDOMINAL SEGMENT OF CRAWFISH with

swimmeret.]

Make out the two distinct regions or _divisions of the body_ (Fig. 81).

The anterior (front) region is called the head-chest or cephalothorax,

and the posterior (rear) region is called the tail. Which region is

larger? Why? Which is flexible? Why?

Is the _covering_ of the body hard or soft? What is the advantage

of such a covering? What are its disadvantages? How is the covering

modified at the joints to permit motion?

=Tail.=--How many joints, or segments, on the tail? (Figs. 81, 83.)

Does the hard covering of each segment slip under or over the segment

behind it when the abdomen is straight? Does this lessen friction while

swimming forward?

Is there a pair of _swimmerets_ to each segment of the abdomen? (Figs.

82, 86.) Notice that each swimmeret has a main stalk (protopod), an

outer branch (exopod), and an inner branch (endopod) (Fig. 84). Are the

stalk and the branches each in one piece or jointed? The middle part of

the tail fin is called the telson. By finding the position of the vent,

decide whether the food tube goes into the telson (Fig. 82). Should it

be called an abdominal segment? Are the side pieces of the tail fin

attached to the telson or to the sixth segment? Do these side pieces

correspond to swimmerets? Do they likewise have the Y-shaped structure?

(Fig. 86.)

If the swimmerets on the first abdominal segment are large, the

specimen is a male. If they are small, it is a female. Which sex is

shown in Fig. 82? Fig. 86?

=Carapace.=--The covering of the head chest (cephalothorax) is called

the carapace. Has it free edges? The _gills_ are on the sides of the

body and are covered by the carapace (Fig. 87). The projection in front

is called the _rostrum_, meaning beak. Does the rostrum project beyond

the eyes? There is a transverse groove across the carapace which may be

said to divide the head from the abdomen. Where does this groove end at

the sides?

[Illustration: FIG. 85.--1, mandible; 2, 3, maxillæ; 4, 5, 6,

maxillipeds.]

=Legs.=--How many legs has the crawfish? How many are provided with

large claws? Small claws? Is the outer claw hinged in each of the large

grasping pincers? The inner claw?

[Illustration: FIG. 86.--CRAWFISH (ventral surface).]

=Appendages for Taking Food.=--If possible to watch a living crawfish

eating, notice whether it places the food directly into the mouth with

the large claws. Bend the large claws under and see if they will reach

the mouth.

Attached just in front of the legs the crawfish has three pairs of

finger-like appendages, called foot jaws (maxillipeds), with which it

passes the food from the large pincers to its mouth (Figs. 85, 86).

They are in form and use more like fingers than feet. In front of the

foot jaws are two pairs of thin jaws (maxillæ) and in front of the

thin jaws are a pair of stout jaws (mandibles) (Fig. 85). Do the jaws

move sidewise or up and down? Which of the jaws has a jointed finger

(palp) attached to it? Do all of the appendages for taking food have

both exopod and endopod branches on a basal stalk or protopod? Which of

the appendages have a scalloped edge? How would you know from looking

at the crawfish that it is not merely a scavenger? Why are there no

pincers on the hind feet?

[Illustration: FIG. 87.--Gill cover removed and gills exposed. _Mp_,

gill bailer.]

=Sense Organs.=--Find the _antennæ_, or long feelers (Figs. 82, 90).

Are the antennæ attached above or below the eyes? (Fig. 87.)