An introduction to general biology online

The nerves leaving the central system are mixed, i.e., they contain both
sensory and motor fibres.



nc




Fig. 43.— Transverse section of ventral part of the body, showing the nervous con-
nections, n.c, ventral ganglion, giving off a lateral nerve at l.n. ; p.e., peritoneal
epithelium; l.m., longitudinal muscles; ?)i/, hypodermis; s, seta. A single motor
nerve-cell (black) is shown sending a fibre into the nerve towards the left. In
the nerve to the right are sensory fibres proceeding inward from the sensory cells
(black) of the hypodermis, and terminating in branching extremities. (After
Lenhoss6k.)

Sections through the ventral commissures are similar to those through
the ganglia, but the central portion (i.e., that within the sheath) is smaller,
is divided into two distinct parts, and the nerve-cells are less abundant.

Sections through the nerves show them to consist only of parallel fibres
surrounded by a sheath which gradually fades away as the nerves grow
smaller, and finally disappears, the muscular layer first disappearing, and
then the epithelial covering.

With this brief sketch of the histological structure of the
earthworm we conclude our morphological stu(ij of the animal.
Those who desire fuller information on the histology will find a
geneial treatment of it in the work of Claparede, already cited
â– at p. 93. Many later works have been published on the de-
tailed histology.



CHAPTER YII.

THE BIOLOGY OF AN ANIMAL {Continued.)
Physiology of the Earthworm.

In the preceding pages l)rief descriptions of many sjx-cial
physiological phenomena have l)een given in connection with the
detailed descriptions of the primary functions and systems. It
now remains to consider the more general problems of tlie life of
the animal, and especially its relations to the enviromnent, and
the transformations of matter and energy which it elfects.

The Earthworm and its Environment. Tlie earthworm is an
organized mass of living matter occupying a definite position in
space and time, and existing amid certain delinite and charactt-r-
istic physical surroundings which constitute its ^'^ environment.'^

As ordinarily understood the term environment apjilies only
to the immediate surroundings of the animal — to the eartli
through which it burrows, the air and moisture tliat batlie its
surface, and the like. Strictly s^^eaking, however, the environ-
ment includes everything that may in any manner act upon the
organism — that is, the whole universe outside the worm. For
the animal is directly and profoundly affected by rays of light
and heat that travel to it from the sun ; it is extremely sensitive
to the alternations of day and niglit, and tlie seasons of the year;
it is acted on by gravity; and to all these, as well as to more
immediate influences, the animal makes deflnite responses.

We have seen that the body of the eartliworm is a compli-
cated piece of mechanism constructed to perform certain definite
actions. But every one of these actions is in one way or an-
other dependent upon the environment and directly or indirectly
relates to it. At every moment of its existence the organism is
acted on by its environment; at every moment it reacts upon
the environment, maintaining with it a constantly shifting state
of equilibrium which finally gives way only when the life of the
animal draws to a close.

Adaptation of the Organism to its Environment. In its rela-
tions to the environment the earthworm embodies a fundamental

97



98 THE BIOLOGY OF AN ANIMAL.

biological law, viz. , that the living organistn must he adapted to
its environment^ or, in otlier words, that a certain harmony
between organism and envh'oninent is essential to the continu-
ance of life, and any influence which tends to disturb or destroy
this harmony tends to disturb or destroy life. The ada]3tation
may be either passive (structural) or active (functional). Struc-
tural adaptation is well illustrated, for instance, by the general
shape of the body, so well adaj^ted for burrowing through the
earth. Again, the delicate integument gives to the body the
flexibility demanded by the pecuKar mode of locomotion; it
affords at the same time a highly favorable respiratory surface —
a matter of no small importance to the worm in its badly-venti-
lated burrow ; and yet this delicate integument does not lead to
desiccation, because the animal lives alwavs in contact with moist
earth. The alimentary canal, long and complicated, is most
perfectly fitted for working over and extracting nutriment from
the earthy diet. The reproductive organs are a remarkable in-
stance of complex structural adaptation in an animal which on
the whole is of comparatively simple structure.

Functional adaptation is perhaps best shown in the instinctive
actions or "habits" of the worm. Its nocturnal mode of life
(functional adaptation to light) and its ' ' timidity ' ' protect it
from heat, desiccation, from birds and other enemies. In win-
ter or in seasons of drought it burrows deep into the earth.

A striking instance of adaptation is shown in the care which
is taken to insure the welfare of the embrvo worms. Minute,

•y 7

delicate, and helpless as they are, they develop in safety inside
the tough, leathery capsule (p. 78), floating in a milklike
liquid which is at once their cradle and their food.

Origin of Adaptations. The development of the earthworm
shows that its whole complex bodily mechanism takes origin in a
single cell (p. 74), and that all the remarkable adaptations ex-
pressed in its structure and action are brought about by a gradual
process in the life-history of each individual worm. There is
reason to believe that this is tj^^ical of the ancestral history (de-
scent) of the species as a whole, and that adaptation has been
gradually acquired in the past. We know that environments
change, and that to a certain extent organisms change corre-
spondingly through functional adaptation, provided the change of



NUTRITION OF THE AMMAL. 99

environment be not too sudden or extrenic In utlicr words
the organism possesses a certain i^lastlmtij wliicli enal)let^ it to
adapt itself to graduallj-clianging conditions of the environment.
Now tliere is good reason to believe tliat as envirunmunt
has gradually undergone changes in tlie })ast, organisms have
gradually undergone corresponding clianges of structure. Those
which have become in any way so modilied as tu he most per-
fectly adapted to the changed environment have tended to sur-
vive and leave similarly-adapted descendants. Those wliich
have been less j)erfectlj adapted have tended to die out tlirough
lack of fitness for the environment ; and by this process — called
bj Darwin '' Natural Selection" and by Spencer the "Survival
of the Fittest" — the remarkable adaptations everywhere met
wdth are believed to have been gradually worked out.

It should be observed that Natural Selection does not really explain the
origin of adaptations, but only their persistence and accumulation. The
theory of evolution is not at present such as to enable us to say with cer-
tainty what causes the tirst origin of adaptive variations.

Nutrition. The earthworm does work. It works in travel-
ling about and in forcing its way through the soil ; in seizing,
swallowing, digesting, and absorbing food ; in pumping the
blood; in maintaining the action of cilia; in receiving and send-
ing out nerve-impulses; in growing; in reproducing itself — in
short, in carrying on any and every form of vital action. To
live is to work. Now work involves the expenditure of energy,
and the animal body, like any other machine, while life con-
tinues, requires a continual supply of energy. It is clear from
what has been said on p. 32 that the innnediate source of tho
energy expended in vital action is the working protoplasm itsolf,
which undergoes a destructive chemical change (kataholism or
destructive metabolism) having the nature of an oxidation. I'^i-nni
this it follows on the one hanl that the waste ])ro(lucts of this
action must be ultimately passed out of the body ;us excretions,
and on the other hand that the loss must ukimately ]>e made

good by fresh supplies entering the animal in the form of f 1.

It is further evident that the income must ecpnU the (Uitgo if tiie
animal is merely to hold its own, and must exceed it if the ani-
mal is to grow.



100



THE BIOLOGY OF AN ANIMAL.



Thus it comes about tliat there is a more or less steady flow
of matter and of energy through the hving organism, which is
itself a centre of activity, like a whirlpool (p. 2). The chemical
phenomena accompanying the flow of matter and energy through
the organism are those of nutrition in tlie widest sense. This
term is more often restricted especially to the phenomena accom-
panying the income, while those pertaining to the outgo are
regarded as belonging to excretion. The intermediate processes
directly connected with the life of protoplasm are put together
under the head of metaholism ^ they include both the construc-
tive processes by which protoplasm is built uj) {ayiabolism) and
the destructive processes by which it is broken down (kataholmn)
in the liberation of energy.

Income. It is difticult to determine the exact income of
Lumhricus^ but it may be set down approximately as follows : —

INCOME OF LUMBRICUS.



Matter.


Whence Derived.


1. ProteMs.


From vegetal or animal matters taken in through the mouth.


2. Fats.


From vegetal or animal matters taken in through the mouth.


3. Carbohydrates.


From vegetal or animal matters taken in through the mouth.


4. Water.


Taken in through the mouth, or perhaps to some extent ab-
sorbed through the body- walls.


5. Free oxygen.


Absorbed directly from the atmosphere or ground-air by dif-
fusion through the body-walls. Sometimes from water in
which it is dissolved.


6. Salts.


Various inorganic salts taken along with other food-stuffs.


Energy.




Potential.


In the food.



TJie food-stuffs are converted by the animal into the sub-
stance of its own body (protoplasm and all its derivatives), and
they must therefore be the ultimate source of energy. It fol-
lows that the animal takes in energy only in the potential form
(i.e., in the chemical potential between the oxidizable proteids,
carbohydrates and fats, and free oxygen). It is true that the



DIGESTION AND ABSORPTION. lOl

animal may under certain circumstances absorl) kinetic enerirs' in
the form of heat, but this is available only as a condition^ not as
a cause of protoplasmic action. In this inability to use kinetic
energy the earthworm is typical of animals as a wIkjIc.

Of the organic portion of the food proteids are a »iti£ qmv
non^ and in this respect again the worm is a type of aTiimal life
in general. Either the fats or the carbohydrates may be omitted
(though the animal probably thrives best upon a mixed diet in
which both are present), but without 2)roteids no animal, as far
as is known, can long exist.

General History of the Food. Digestion and Absorption.
LxiDibricus takes daily into its alimentary canal a certain amount
of necessary food-stuffs, but these are not really inside the body
so long as they remain in the alimentary canal ; for this is shown
by its development to be only a part of the outer surface folded
in to afford a safe receptacle within which the food may be
worked over. Before the food can be actuallv taken into the
body, or absorhed^ it must undergo certain chemical changes col-
lectively called digestion (cf. p. 49). A very important part
of this process consists in rendering non-diffusible substances dif-
fusible, in order that they may pass through the walls of the
alimentary canal into the blood. Proteids, for exam})le, have
been shown to be non-diffusible (Chap. III). In digestion they
are changed by the fluids of the alimentary canal into peptones
— substances much like proteids, but readily diff"usible. \\\
like manner the non-diffusible starch is chano^ed into diffusible
sugar and becomes capable of absorption. It is higlily j)robable
that all carbohydrates are thus turned into sugar. The fats are
probably converted in part into soluble and diff"usi])le soaps whieh
are readily absorbed, but are mainly enuilsitied and directly passed
into ^tlie cells of the alimentary tract in a finely divided state.
Nothino^, however, is known of this save by analoi^v with hii^her
animals. In all cases digestion takes place outside the ho(fy^ and
is only preliminary to the real entrance of food into the physit>-
logical, or true, interior.

Metabolism. After absorption into the body proper the
incoming matters are distributed by the circulation to the ulti-
mate living units or cells, and are finally taken up by them and
built into their substance. There is reason to believe that each



102 THE BIOLOGY OF AN ANIMAL.

cell takes from the common carrier, the blood, only such ma-
terials as it needs, leading a somewhat independent life as to its
own nutrition. It co-operates with other cells under the direc-
tion of the nervous system (co-ordinating mechanism), but to a
great degree is independent in its clioice of food — just as a sol-
dier in a well-fed army obeys orders for the common good, but
yet takes only what he chooses from the daily ration supplied to
all.

What takes place within the cell upon the entrance of the
food is almost wholly unknown, but somehow the food-matters,
rich in potential energy, are built up into the living substance
probably by a series of constructive processes culminating in pro-
toplasm. Alongside these constructive processes (anabolism) a
contumal destructive action goes on (katabolism) ; for living mat-
ter is decomposed and energy set free in every vital action, and
vitality or life is a continuous process. It nmst not be supposed,
however, that either the synthetic or the destructive process is a
single act. Both probably involve long and complicated chemi-
cal transformations but the precise nature of these changes is at
present almost whqJly unknown. It is certain that the destruc-
tive action is in a general way a process of oxidation effected by
aid of the free oxygen taken in in respiration. We may be
sure, however, that it is not a case of simple combustion (i. e. , the
protoplasm is not " burnt"). It is more probably analogous to
an explosive action, the oxygen first entering into a loose asso-
ciation with complex organic substances in the protoplasm, and
then suddenly combining with them under the appropriate stim-
ulus to form simpler and more highly-oxidized products. Of
the precise nature of the process we are quite ignorant.

Outgo. Just as the income of the animal represents only the
first term in a series of constructive processes, so the outgo is
the last teiTu of a series of destructive actions of which we really
know very little save through their results. The outgo is shown
in the accompanying table.

Both energy and matter leave the cells, and finally leave the
body — the former as heat, work done, or energy still potential
(in urea and other organic matters); the latter as excretions,
wliich diffuse freely outwards through the skin and nephridial
surfaces.



THE ANIMAL AND ITS ENVIRONMENT.



\m



OUTGO OF L^'^^nnIc^s:.



Matter.


Manner oe Exit.


Carbon dioxule (COj).


Mainly by diffusion through the skin.


Water (HjO).


Through the skin, through the nephridia, and in the fwces.


Urea [(NH2)aC0], and
its allies.


Through the nephridia.


Salts.


Dissolved in the water.


Proteids and other
organic matters.


In the substance of the germ-cells, the egg-capsules, and
the contained nutrient fluids.


Energy.




Potential.


A small amount still remaining in urea, in the germ-cells,

etc.


Kinetic.


Work performed. Heat.



Of the daily outgo tlie water, carbon dioxide, and salts are
devoid of energy, but the urea contains a small amount wliicli is
a sheer loss to the animal. "Were the earthworm a perfect ma-
chine it could use this residue of energy by decomposing the urea
into simpler compounds [viz., ammonia (Nil,), carbon dioxide
(CO2), and water (H^O)] ; but it lacks this power, tliough there
are certam organisms {Bacteria) which are able to utilize tlie last
traces of energy in urea (p. l!)7). To the daily outgo must be
added the occasional loss both of matter and of energy sutfercd
in giving rise to ova and spermatozoa, and in providing a certain
amount of food and protection for the next generation.

Interaction of the Animal and the Environment. The action
of the environment upon the animal has already been sutHciently
stated (p. 97). It remains to point out the changes worked by
the animal on the environment. These changes are of two
kinds, mechanical (or physical) and chemical. The most imj)or-
tant of the former is the continual transformation of the soil
which the worms effect, as Darwin showed, by bringing the
deeper layers to the surface, where they are exposed to the at-
mosphere, and also by dragging superficial objects into the bur-
rows. The chemical changes are still more significant. The



104 THE BIOLOGY OF AN ANIMAL.

general effect of the metabolism of the animal is the destruction
by oxidation of organic matter ; that is, matter originally taken
from the environment in the form of complex proteids, fats, and
carbohydrates is returned to it in the form of sunpler and more
highly oxidized substances, of which the most important are car-
bon dioxide and water (both inorganic substances). This action
furthermore is accompanied by a dissipation of energy — that is,
a conversion of potential into kinetic energy.

On the whole, therefore, the action of the animal upon the
environment is that of an oxidizing agent, a reducer of comj^lex
compounds to simpler ones, and a dissipator of energy. And
herein it is typical of animals in general.



CHAPTER YIII.

THE BIOLOGY OF A PLANT.

The Common Brake or Fern.

{Pteris aquilina, Linnaeus.)

Foe the study of a representative vegetal organism some
plant should be chosen which may be readily procured and is
neither very high nor very low in the scale of organization.
Such a plant is a common fern.

Ferns grow generally in damp and shady places, tlioiigli
they are by no means confined to such localities. Some of tlie
more hardy species prefer dry rocks or even bold cliffs, in the
crevices of which they find support ; others live in open tiekls
or forests, and still others on sandy hillsides. In tlie northern
United States there are altogether some fifty species of wild
ferns, but those which are common in any particuhir locaHty are
seldom more than a score in number. Throughout the \vh(>lc
world some four thousand species of ferns are known, l)ut by
far the greater number are found only in tropical regions, where
the climate is best suited to their wants. At an earHer j)eriod
of the earth's history ferns attained a great size, and formed a
conspicuous and important feature of the vegetation. At
present, however, they are for the most part only a few feet in
height. Nearly all are perennial; that is, they may Hve for ;in
indefinite number of years. Most of tliem liave creeping or
subterranean stems; but some of the troj)ical species have eri'ct,
aerial stems, sometimes rising to a height of fifty feet or more
and forming a trunk which is cylindrical, of equal diametei
throughout, and bears leaves only at the sunnnit, like a palm
(tree-ferns).

Of all the ferns perhaps the commonest and most widely

distributed is the " brake" or " eagle-fern," which is known to

botanists as Pteris aquilina^ Linnaeus, or rtevldlum aquUinuni^

105



106 THE BIOLOGY OF A PLANT.

Kulin. This plant is not only common, but of comparatively
simple structure ; it is of a convenient size, and lias been much
studied. It may therefore be taken both as a representative
fern and as a representative of all higher vegetal organisms.

Habitat, Name, etc. The brake occurs widely distributed in
the United States, under a great variety of conditions; e.g., in
loose pine groves, especially in sandy regions ; in open wood-
lands amongst the other undergrowth ; on hillside pastures and
in thickets — indeed almost everywhere, exce23t in very wet or
very dry places. It appears to be equally common elsewhere ;
for, according to Sir W. J. Hooker, Pteris aquilina grows
' ' all round the world, both within the tropics and in the north
and south temperate zones. ... In Lapland it just j^asses
within the Arctic circle, ascending in Scotland to 2000 feet,
in the Cameroon Mountains to 7000 feet, in Abyssinia to 8000
or 9000 feet, in the Himalayas to about 8000 feet." {Sy7ioj)sis
Filicuin?)

"Pteris {jirepL^^ the common Greek name iov fern)^ signify-
ing wing or feather, well accords with the apj^earance of Pteris
aquilina., the most common and most generally distributed of
European ferns. It is possible that this fern may rank as the
most universally distributed of all vegetable productions, extend-
ing its dominion from west to east over continents and islands in
a zone reaching from Northern Europe and Siberia to New
Zealand, where it is represented by, and perhaps identical with,
the well-known Pteris esculenta. The rhizome of our plant
like that of the latter is edible, and though not employed in
Great Britain as food, powdered and mixed with a small quan-
tity of barley-meal it is niade into a kind of gruei called gojio.^
in use among the poorer inhabitants of the Canary Islands." —
(Sowerby.)

The specific name aquilina (ciquila^ eagle) and a popular
name, "eagle-fern," in Germany, etc., have come from a
fanciful likeness of the dark tissue seen in a transverse section
of the leaf-stalk to the figure of an outspread eagle. The same
figure has, however, been compared to an oak-tree, and has also
given rise to the name of " devil' s-foot fern," from its alleged
resemblance to "the impression of the deil's foot," etc., etc.

Tlie popular designation of this plant as ' ' the l)rake ' ' testi-



THE PLANT BODY. 107

fies to its great abundance; for a brake is a dense tliickct (.r
undergrowth — as for example a cane '* brake."

When fullj grown (P'ig. 44; the connnon brake has a leafy
top supported by a polished, dark-colored, erect stem, whicli in
New England rises to a height of from one to four feet above
the ground. In this climate, however, it appears to be some-
what undersized, for it grows to a height of fourteen feet in
the Andes,* and in Australia attains to twice the height of a
man, forming a dense undergrowth beneath tree-ferns 4n-l(n)
feet high.f In Great Britain it is from six inches to nine feet
high (Sowerbj), or even larger in exceptional cases. ''In drv
gravel it is usually present, but of small size; while in tliick
shady woods having a moist and rich soil it attains an enormous
size, and may often be seen climbing up, as it were, among the
lower branches and underwood, resting its dehcate pinnules
on the little twigs, and hanging gracefully over them."
(Newman.)

General Morphology of the Body.

The body of the fern, like that of the earthworm, consists
of cells, grouped to form tissues and organs. Their arrange-
ment, however, differs widely from that in the animal, for tlie
plant-body is a nearly solid mass, and there are no extended
internal cavities enclosing internal organs. The organs of the
plant are for the most part external, and arise by local modifica-
tions of the general mass. Like many higher plants the body
of the fern consists of an axis or stem-bearing branches, from
which arise leaves. The fern differs form ordinary trees, how-
ever, in the fact that the stem, with its branches, lies horiz(»ntal
beneath the surface of the ground. Only the leaves (fronds)
rise into the air. (Fig. 44.) It is convenient to describe the
body of the brake, accordingly, as consisting of two very (b*f-
ferent parts — one green and leaHike, which rises above the
ground; the other black and rootlike, lying buried in the soil.
These will henceforth be spoken of as the aerial and the muhr-
groicnd parts.

The underground part lies at a depth of an inch to a foot

* Hooker, I. c.

\ Kroue, Botaii. Jahrcshericht, 1876 (4), 346.




Fig. 44.— The Brake (Pteris aquiUna), showing part of the underground stem (r.?J>
and two leaves, one (?'), of the present year, in full development; the other
d'), of the past year, dead and withered, a.b, apical bud at the extremity of a
branch which bears the stumps of leaves of preceding years and numerous-
roots; P, mature active leaf ; P, dead leaf of preceding year ; l.m, lamina of leaf ;