An introduction to general biology online

of the protoplasm, and in consequence animals awake from their
sleep and plants put forth their leaves.

But this law is true only within certain limits. Extreme
heat and cold are alike inimical to hfe, and as the temperature
approaches these extremes all forms of vital action gradually or ;>
suddenly cease. The limits are so variable that it is not at
present possible to formulate any exact law which shall include
all known cases. For instance, many organisms are killed at
the freezing-point of water (0° C); but certain forms of life
have withstood a temperature of — 87° C. {— 123° F.), and re-
cent experiments show that frogs and rabbits may be chilled to
an unexpected degree without fatal results.

The upper limit is also inconstant, though less so than the lower.
Most organisms are destroyed at the temperature of boiling
water (100°C.), but the spores of bacteria have been exposed to
a much higher temperature ^vdthout destruction (120"— 125° C).
As a rule, protoplasm is killed by a temperature varying from
40° to 50° C, the immediate cause of death being aj^parently
due to a sudden, coagulation (p. 36) of certain substances in the
protoplasm. Thus^ if a brainless frog be gradually heated,



PROTOPLASM AND PHYSICAL AGENTS. 89

death ensues at about -iO^ C, and tlie body becomes Ptiff and
rigid [rigor' caloris) from tlie coagulation of tlie imi.scle-sub-
stance. The lower forms of animal life agree well with ])lants
in their " fatal temperatures," which in many cases lie between
40° and 50° C.

Lastly, it appears to be true that there is a certain most
favorable or optimum temperature for the protoplasm of each
species of plant and animal, this optimum differing considera])ly
in different sj^ecies. Probably the highest limit occurs among
the birds, where the uniform temperature of the body may be
as high as 40° C. The lowest occurs among the marine i)]ant8
and animals of the Arctic seas, or of great de2)ths, where the
temperature seldom rises more than a degree or two al)ove the
freezing-point. Between these limits there aj^pears to be great
variation, but 35° C. may perhaps be taken as the average op-
timum.

Moisture. Protoplasm always contains a large amount of water, of
which indeed the lifeless portion of living things chiefly consists. (Se
table on p. 34.) All plants and animals are believed to be killed by com-
plete drying, though some of the simpler forms resist partial drying for a
long time, becoming quiescent and reviving again when moistened, some-
times even after the lapse of years. Hence water appears to be an essen-
tial constituent of protoplasm, although, as in the case of mineral matters,
we do not know the nature of its connection with the other elements or
compounds present.

Electricity. It has been shown that many forms of vital action are ac-
companied by electrical disturbances in the protoplasm. It is therefore
not surprising that the application of electricity to living protoplasm sliould
have a marked effect on its actions. If the stimulus be very slight, proto-
plasmic movements are favored. Colorless blood-corpuscles creep more
actively, and ciliary action increases in vigor. Stronger shocks cause a
spasmodic contraction of the protoplasm (tetanus), from which it m.-iy or
may not be able to recover, according to the strength of the shock.

Poisons. Towards certain agents protoplasm is indifferent or seemingly
so, but towards others it behaves in a very remarkable manner. The mat-
ters known as poisons modify or destroy its activity, as is well known from
the familiar effects of arsenic, opium, etc. Disease may also interfere with
its normal activity ; but the consideration of these phases of the subject
belongs to the more exclusively medical sciences, such as toxicology and
pathology.

Other Physical Agents. The more highly specialized forms of proto-
plasm are affected by a great variety of physical agents, such as light,



40 â–  PROTOPLASM AND THE CELL.

sound, pressure, etc., and upon this susceptibility depend many of the
higher manifestations of life. For instance, waves of light or of sound,
acting upon special protoplasmic structures in the eye and ear, call forth
actions which ultimately result in the sensations of sight and hearing.
Similar considerations apply to the senses of smell, taste, and touch ; but
the discussion of all these special modes of protoplasmic action must be
deferred. Enough has been said to show that living organisms (that is,
the protoplasm which is their essential part) are able to respond to many
influences proceeding from the world in which they live. Upon this prop-
erty depend the intimate relations between the organism and its environ-
ment, and the power of adaptability to the environment which is one of the
most marvellous and characteristic properties of living things.

Non-cliff iisihility. Living protoplasm, like most of the various proteid
matters which it yields (p. 36), is indffusihle. It will be seen eventually
that osmotic processes play a leading role in the lives of plants and animals,
since they are in large part the means by which nutriment is conveyed to
the living substance. In view of this fact, the non-diffusibility of proto-
plasm as well as of ordinary proteids is a fact of much significance.

Vegetal and Animal Protoplasm. The protoplasm of plants is es-
sentially identical with that of animals in chemical and physical relations,
and manifests the same fundamental vital properties. But it would mani-
festly be absurd to suppose this identity absolute, for if it were so, plants
and animals would also be identical ; and furthermore, the protoplasm
of every species of plant and animal must differ more or less from the
protoplasm of every other species. What is meant is that the differences
between the many kinds of protoplasm are far less important than the
fundamental resemblances which underlie them.



CHAPTER lY.

THE BIOLOGY OF AN ANIMAL.

The Common Earthworm.

{Lumhricus terrestris, Linnaeus.)

We now advance to a more precise examination of the living
body considered as an individual. It is a familiar fact that
living things fall into two great groups, known as plants and
animals. We shall therefore examine a representative of each
of these grand divisions of the living world, and inquire huw
they resemble each other and how they differ. Any liigher
animal would serve as a type, but the connnon earthworm is a
peculiarly favorable object of study, because of the sim})licity of
its structure, the clearness of its relation to other animals stand-
inty above and below it in the scale of orojanization, and the ease
with which it may be procured and dissected. Earthworms, of
which there are many kinds, are found in all parts of the world,
extending even to isolated oceanic islands. In the United States
there are several species, of which the most connnon are L.
commicnis {Allolobophora miccosa^ Eisen), L. terrestris^ and Z.
fmtichis {Allolopobliora fmtida^ Eisen). The tirst two of these
are found in the soil of gardens, etc., Z. terredv'is l)eing the
larger and stouter species and readily distinguishable by the
flattened shape of the posterior region, Z. fmtldu'^. a smaller
red species, transversely striped, and having a characteristic
odor, occurs in and about compost-heaps.

Mode of Life, etc. Earthworms live in the earth, burrow-
ing through the soil at a depth varying from a few inches to
several feet. Here they pass the daytime, crawling out at
night or after a shower. The burrows proceed at iirst straight

downwards, and then wind about irreorularlv, sometimes reacli-

41



42 THE BIOLOGY OF AN ANIMAL.

ing a deptli of six or eight feet. The eartliworm is a nocturnal
animal, and during the day lies quiet in its burrow near the sur-
face, extended at full length, head uppermost. At night it
becomes very active, and, thrusting the fore end of the body
far out, explores the vicinity in all directions, tliough still clinging
fast, as a rule, to the mouth of the burrow by the hinder end.
In this way tlie worm is able to forage, seizing leaves, pebbles,
and other small objects, and dragging them into the burrow.
Some of these are devoured ; the remainder (including the peb-
bles, etc.) are used to hue the uj^per part of the burrow, and to
plug up its opening when the worm retires for the day. Be-
sides bits of leaves and animal matter, earthworms swallow large
quantities of earth, which is passed slowly through the alimentary
canal, so that any nutritious substances contained in it may be
digested and absorbed. This earth is generally swallowed at a
considerable distance below the surface of the ground, and is
finally voided at the surface near the opening of tJie burrow.
In this way arise the small j^iles of earth (" castings " or feeces)
which every one has seen, especially in the morning, wherever
earthworms abound. Yery large quantities of earth are thus
brought to the surface by earthworms — in some cases, accord-
mg to Darwin's estimates, more than eigliteen tons j^er acre in
a single year. In fact, most soils are continually being w^orked
over by worms; and Darwin has shown that these humble
creatures, in the course of centuries, have helped to bury huge
rocks and the ruins of ancient buildings.^

The earthworm has no ears, eyes, or any other well-marked
organs of special sense. Nevertheless — and this is a point of
great j)hysiological interest — the fore end of the body is sensi-
tive to light ; for if a strong light be suddenly flashed upon this
part of the worm as it lies stretched fortli, it will often ''dash
like a rabbit into its burrow. ' ' The animal has a keen sense of
touch, as may be proved by tickling it; and its sense of taste
nmst be well developed, since the worm- is somew^hat fastidious
in its choice of food. Earthworms appear to be quite deaf, but
possess a distinct, tliough feeble, sense of smell.

* Darwin, Vegetable Mould and Earthworms. Appleton, N. Y., 1882. See
also White's Natural History ofSelborne, Index, references to " Earthworms.'*



AN TEIW- POSTERIOR DIFFEREyilA TION.



43



Gj:neral Mokpiiology.

Attention will first Le directed to certain features of tlie
BODY seemingly of little importance, but really full of meaning
when compared with like features in other
animals higher or lower in the scale of
organization.

Antero- posterior Differentiation. The
body (Fig. 21) has an elongated cylindrical
form, tapering to a hlnnt point at one end,
obtusely rounded and fattened at the other.
As a rule, the pointed end moves for-
wards in locomotion, and the mouth opens
near it. For these and otlier reasons
the pointed end might be called the head-
end, and the other tlie tail-end. But the
worm has really neither head nor tail, and SO- y- — '
hence the two ends may better be distin-
guished as the fore end and the hinder end,
or still better as anterior and posterior.
And in scientific language the fact that the
worm has anterior and posterior ends
which differ from each other is stated by
saying that it shows antero-posterior differ-
entiation. This simple fact acquires great ^n
importance hi the light of comparative
biology; for it may be shown that the
antero-posterior differentiation of the earth-
worm, insignificant as it seems, is only the
begining of a series of important modifica-
tions extending upwards through more and
more complex stages to culminate in man
himself.

Fio, 21.— Enlarged view of the anterior and posterior
parts of the body of an earthworm as seen from the
ventral aspect, ajj, anus ; c, clitellum ; g.p., glandular
prominences on the 'lWi\ somite ; »i, mouth ; <u\, exter-
nal openings of the oviducts ; p.x., prostomium ; s, set^v ;
s.r., openings of the seminal receptacles ; .x.r/., external
openings of the sperm-ducts. The form of the body
varies greatly in life according to the state of expan-
sion. The specimen here shown is from an alcoholic
preparation. (Slightly enlarged.) Q/l




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44 THE BIOLOGY OF AN ANIMAL.

Dorso-ventral Differentiation. In living or well-preserved spe-
cimens, the body is not perfectly cylindrical, but is somewhat
flattened, particularly near the posterior end, and has a slightly
prismatic four-sided form. One of the flattened sides, slightly
darker in color than the other, is habitually turned upwards, and
is therefore called the back, the opposite or lower side, commonly
turned downwards, being the belly. For the sake of accuracy,
however, biologists are wont to speak of the dorsal asjject (back)
and venti^al asjyect (belly) of the body ; and the fact that an animal
has a back and belly differing from each other in structure or
function, or both, as in the earthworm, is expressed by saying
that the body exhibits dorso-ventral differentiation. Tliis, like
antero-posterior differentiation, is very feebly expressed in the
external features, though clearly marked in tlie arrangement of
the internal parts of the earthworm. In higher animals it
becomes one of the most consjiicuous features of the body.

Bilateral Symmetry. When the body is j^laced in the natural
position, with the ventral aspect downwards, a vertical plane
passing longitudinally through the middle will divide it into
exactly similar right and left halves. Tliis similarity is called
two-sided likeness, or hilateral symmetry. Though not very
obvious externally, this symmetry characterizes the arrangement
of all the internal parts ; and it may be gradually traced up-
wards in higher animals, until it becomes as striking and perfect
as in the human body.

Thus a very superficial examination reveals in the earth-
worm two fundamental laws of organization, viz., dfferentia-
tion or the law of difference, and symmetry or the law of like-
ness. And these laws are of interest for the reason among
many others that earthworms, like other organisms, have as a
race had a history, have co7ne to he by a gradual process (cf.
J). 99). And biology must strive to answer the questions ho^v and
why certain parts have become symmetrical and others differ-
entiated. Without entering into a full discussion of the ques-
tion at this point, it may be said that the main cause of sym-
metry or differentiation has probably been likeness or unlikeness
of fanction, or of relation to the environment. Earthworms
show antero-posterior and dorso-ventral differentiation, because
the anterior and posterior extremities, or the dorsal and ventral



METAMERISM. 45

aspects, have l)een differently used and exposed to different con-
ditions of environment. And on the other liand the (jri^anisni is
hilaterally synnnetrical, because tlie two sides have been similarlv
used and have been exposed to like conditions of enviroinneiit.

Metamerism. Another general feature of tlie eartliwoi'in is
of great importance in view of the conditions existing in other
animals, including the higher forms. The Ijody is marked off
by transverse grooves into a series of similar parts like the joints
of a band)00 lishing-rod, or like the joints of lingers (Fig. 21).
These parts are called inetameres^ or more often somitex, and
the body is consequently said to have a Qnetameric structnre, or
to exhibit metamerism. From the outside, the somites a})]iear to
be ]3i'oduced simply by regular folds in the skin, like the
wrinkles between the joints of our fingers. But as the wrinkles
of the lingers are only the external expression of a more funda-
mental jointed structure within, so the external fohls sei>arating
the somites, represent an internal division into successive parts,
which affects all the organs of the body, and is a result of some
of the most important phenomena of development.

The explanation of metamerism or ^'â– serial symmetry' is one of the
most difficult problems of morphology. But it will be seen fartlier on that
metamerism, so clearly and simply expressed in the earthworm, can be
traced upward in ever- increasing complexity to the highest forms of life,
and suggests some of the most interesting and fundamental problems with
which biology — and especially morphology — has to deal. Indeed, the
comparative study of the anatomy of most higher animals consists very
largely in tracing out the manifold transformations of their complicated
somites, which under many disguises can be recognized as fundamentally
like the simpler somites of the earthworm.

Modifications of the Somites. The somites differ considerably

«

in different parts of the body. The extreme anterior end is
formed by a smoothly-rounded knob called the j^rostofniinfi,
which is shown by its mode of development not to be a true
somite. It forms a kind of overhanging upper lip to the mouthy
which lies just behind it on the ventral aspect. Behind the
mouth is the first somite, in the form of a ring,"^ interrupted
above by a backward 2:)rolongation of the prostomium.

* In numbering the somites the prostomium must never be reckoned, the
first somite being heJiind the mouth.



46



THE BIOLOGY OF AN ANIMAL.



The somites from the 1st to the 2Tth are rather broad,
and gradually increase in size. A variable number cf the
somites lying between the 7th and 19th are often swollen on
the ventral side, forming the so-called capsulogenous glands.
Between the 2Sth and 35th (the number and position vary-
ing shghtly in different specimens) the somites are swollen
above and on the sides, and the folds between them arc
scarcely defined except on the ventral aspect. Taken together,
they form a broad, conspicuous girdle called the clitelluiiv
(Fig. 21, c), wdiose function is to secrete the capsule in which
the eo;2;s are laid, and also a nutritive milk-like lluid for the use
of the developing embryos. (The clitellum is not present in
immature specimens.) Behind the clitellum the somites are
narrower, somewhat four-sided in cross-section, and iiattened
from above downwards. This flattening sometimes becomes
very conspicuous towards the posterior end. Towards the very
last they decrease in size rather abruptly, and they end in the
anal somite, which is perforated by a vertical slit, the amis
(Fig. 21, ail). All the somites are perforated by small openings
leading into the interior of the body, and forming the outlets of
numerous organs ; the position of these openings will be de-
scribed in treating of the organs. Each somite, excepting the

anterior two or three and the last,
gives insertion to four groups of
short and minute bristles or setce.,
wliich are arrano^ed in four lono^i-
tudinal rows aloncr the bodv. Two
of these rows run along the ventral
aspect, tw^o are more upon the
sides. The setse extend outwards
from the interior of the bodv,
where they are supplied with small
muscles by which they can be
turned somewhat either forwards or backwards, and can also be
protruded or ^vithdrawn (Fig. 22). The setsB are of great use
in locomotion. When pointed backwards they support the worm
as it crawls forwards ; when they are turned forwards the worm
can creep backwards. They are of interest, therefore, as repre-
senting an extremely simple and primitive limb-like organ. :




Fig. 22.— Diagram to iUustrate the
action of the setae. The dotted
outline represents the position of
the seta and its muscles when
bent in the opposite direction, m,
muscles ; s, seta ; %i\ body- wall.



GENERAL PLAN OF TUE BODY.



47



Plan of the Body. The Ludy of the earthworm (Fi(r. 23),
like tliat of all higher aniniaLs, c(jiisists of two tubes, one (al)
within the other and separated from it by a considerable space
or cavity {coe). The inner tube is the allment'ir;/ canal,, open-
ing in front by tlie mouth and l^ehind by the anuH ', the outer
tube is the body- wall, and its cavity is the hodtj-cavity or Ciduni,




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Fig. 23. — A, diagram of the earthworm as seen in a longitudinal section of the body,
showing the two tubes, the coelom, and the dissepiments. JB, diagram of cross-
section : (7/, alimentary tube; an, anus; Cfr, ccelom; »i, mouth. C, diagram
showing the arrangement of some of the principal orgaus : //i, moutli ; an, anus ;
al, alimentary canal; ds, dissepiments; d.r., dorsal blood-vessel; r, ventral or
sub-intestinal vessel ; c.r., circular vesselb ; ?j, nephridia or excretary organs; <•.(/.,
cerebral ganglia ; r.(/., ventral chain of ganglia; r>.(/., oviduct; o.d., ovary. The
arrows indicate the course of the circulation of the blood.



The coelom is not, however, a free continuous space extending
from end to end, but is divided transversely by a series of tliin
muscular partitions, the dlssejnments, into a series of nearlv
closed chambers traversed by the alimentary canal, Eacli (m.iu-
partment corresponds to one somite, the dissepiments ])eing
opposite the external furrows mentioned on p. 45. All tlie
organs of the body are originally developed from tlie walls of
these chambers, and some of them (e.g., tlie organs of excretion)
project into the cavities of the cliambers, that is into the cadom.



48 THE BIOLOGY OF AN ANIMAL,

In the median dorsal line of eacli somite (excepting the first
two or three) is a minute pore (the dorsal j^ore) which perfo-
rates the body-wall and thus places the coelom in connection
with the exterior."^ Other pores that pass through the body-
wall into the cayities of yarious organs will be described fur-
ther on.

Organs of the Animal Body. Systems of Organs. The body of
the earthworm consists essentially of protoplasm, and in order that
so large a mass of liying matter may continue to exist and carry
on the ordinary life of an earthworm it must be able to obtain
a sufficient supply of food; to digest and absorb it, and dis-
tribute it to all parts of the body ; to build up new protoj^lasm
and remoye waste. It must be sensitiye to external and internal
influences ; capable of motion and locomotion. Aboye all, each
part must act with reference to, and in harmony with, eyery
other part, so that the organism may not be merely an aggregate
of organs, but one body acting as a unit or a whole.

T\\Q^Q functions are fulfilled by the organs, respectiyely, of

ALIMENTATION, DIGESTION, ABSORPTION, CIRCULATION, EXCRETION,

SENSATION, MOTION, and COORDINATION. All of tlicsc minister to
the welfare of the indiyidual. The REPRODucTiyE function, on
the other hand, and its corresponding organs, serye to perpet-
uate the species, thus ministering rather to the race than to the
indiyidual.

Sets of organs deyoted to the same function constitute syS'
terns I as the alimeiitary system^ the circulatory system^ etc.
Those wdiich are more immediately concerned with the income
and outgo of matter — namely, the alimentary, digestiye, absorp-
tiye, circulatory, and excretory systems — are sometimes called the
"vegetative systems or systems of nutrition '^ while those which
haye to do more immediately with the relation of the body to
its enyironment, rather than the indiyidual itself, are called syS'
terns of relation. Examples of the latter are the systems of
organs of support, motion (including locomotion), sensation, and
coordination ; and eyen the reproductiye system, as relating chiefly
to other indiyiduals, finds a place here.

* If living worms be irritated they will often extrude a milky fluid from,
these pores, but the use of the latter is not well understood.



ALIMENTAHY SYSTEM. 49

A. Systems of Nutritive Organs : their Special Mor-
puoLOGY AND Physiology. (ForlJsecp. ()2.)

Alimentary System (Organs of Alimentation). P^artli-wornis
feed mainly upon leaves or decaying vegetalile matter, l»ut
will also eagerly devour meat, fat, and other animal siih-
stances. They also swallow large quantities of earth from
which they extract not only any organic materials that it may
contain, but probably also moisture and a small amount of \;ii-i-
ous salts. The most essential and characteristic i)ai-t (jf their
food is derived from vegetal or animal matter in the form of
various organic compounds, of which the most important are
jproteids (protoplasm, albumen, etc.), carhoh yd rates (starch,
cellulose), di\\(\.fats. These materials are used by the animal in
the manufacture of new protoplasm to take the place (â–ºf tliat
which has been used up. It is, however, impossible for the ani-
mal to build these materials directly into the substance (jf its
own body. They must first undergo certain preparatory chemi-