An introduction to general biology online

The tube is therefore comparable to a drain-pipe in which each cylinder
represents a cell. Its cavity is not intercellular (between the cells, like the
alimentary cavity), but intracellular {vMliin the cells, like a vacuole).

The mode of action of the nephridia is as yet only partially
understood, though there is no doubt regarding their general char-
acter. It is certain that their principal office is to remove from
the body waste nitrogenous matters resulting from the decompo-
sition of proteids ; and there is reason to believe that these waste
matters are passed out either as urea ( [NHJjCO) or as a nearly
related substance, together with a certain quantity of water and
inorganic salts.

Excretion in Lumbricus appears, however, to involve two quite distinct
actions on the part of the nephridia. In the first place the glandular walls
of the tube, which are richly supplied with blood-vessels, elaborate certain
liquid waste substances from the blood and pass them into the cavity of



BBEATUING. 61

the tube. In the second place the ciliated funnels are believed to take up
solid waste particles floating in the ca'lomic fluid and to pass tiiem on into
the tube, whence they are ultimately voided to the exterior together with
the liquid products described above. It is nearly certain that these parti-
cles are derived from the breakii^.g up of " lynipiioid " cells, some of wliicU
may have been phagocytes (p. 53;, floating in the coelomic fluid, and that
most if not all of these cells arise from '' chloragogue cells " set free from
the surface of the blood-vessels and of the intestine.

Respiration. Respiration, or breathing, is a twofold oj^eration,
consisting of the taking in of free oxygen and tlie giving oil of
carbon dioxide by gaseous diffnsion tlirougli the surface of tlie
body. Strictly speaking, this free oxygen must Ije regarded nA
food, while carbon dioxide is to be regarded as one of the excre-
tions. Hence respiration is tril)utary both to alimentation and to
excretion ; but since many animals possess special mechanisms to
carry on respiration, it is convenient and customary to treat of
it as a distinct process.

Respiration is essentially an exchange of gases between tlie
blood and the air, carried on through a delicate membrane lying
between them. The earthworm represents the sim})lest condi-
tions possible, since the exchange takes place all over the body,
precisely as in a plant. Its moist and delicate walls are every-
where traversed bv a fine network of blood-vessels Ivinii: lust
beneath the surface. The oxvffen of the air, either in the
atmosphere or dissolved in w^ater, readily diffuses into the blood
at all points, and carbon dioxide makes its exit in the reverse
direction. Freed of carbon dioxide and enriched with oxygen,
the blood is then carried away by the circulation to the inniT
parts, where it gives up its oxygen to tlie tissues and becomes
once more laden with carbon dioxide.

In higher animals it has been proved that the red cc>loring
matter (haemoglobin) is the especial veliicle for the al)sorptioii
and carriage of the oxygen of tlie blood, entering into a looso
chemical union with it and readily setting it free again under the
appropriate conditions. This is doubtless true in the earthworm
also.

It is interesting to study the various devices by which this function is
performed in different animals. In the earthworm the whole outer surface
is respiratory, and no special respiratory organs exist. In other animals
such organs arise simply by the differentiation of certain regions of the



62 THE BIOLOGY OF AN ANIMAL.

general surface, which then carry on the gaseous exchange for the whole
organism. In many aquatic animals such regions bear filaments or flat
plates or feathery processes known as gills or hrancluce^ which are bathed
by the water containing dissolved air, though in many such animals
respiration takes place to some extent over the general surface as well. In
insects the respiratory surface is confined to narrow tubes {trachece) which
grow into the body from the surface and branch through every part, but
must nevertheless be regarded as an infolded part of the outer surface.
In man and other air-breathing vertebrates the respiratory surface is
mainly confined to the lungs, which are simply localized infoldings of the
•outer surface specially adapted to effect a rapid exchange of gases between
the blood and the air.

It is easy to see why special regions of the outer surface have in higher
animals been set aside for respiration. It is essential to rapid difi'usion
that the respiratory surface should be covered with a thin, moist membrane,
and it is no less essential that many animals should be provided with a
firm outer covering as a protection against mechanical injury or desicca-
tion. Hence the outer surface becomes more less distinctly differentiated
into two parts, viz., a protecting part, the general integument ; and a
respiratory part, which is usually preserved from injury by being folded
into the interior as in the case of lungs or tracheae, or by being covered
with folds of skin as in the gills of fishes, lobsters, etc. This covering or
turning in of the respiratory surfaces brings with it the need of mechanical
arrangements for pumping air or water into the respiratory chamber ; and
thus arise many complicated accessory respiratory mechanisms.

B. Orgaxs of Relation. (For A see p. 49.)

Motor System. The movements of the body have a twofold
purpose. In the first place they enable the animal to alter its
relation to the environment, to move about (locoinotion)^ to seize
and swallow food, and to perform various adaptive actions in
response to changes in the environment. In the second place,
the movements may alter the relation of the various parts of the
body one to another {visceral movements and the like), such as
the movements which propel the blood, drive the food along the
alimentary canal and roll it about (p. 49), those which ex|3el
waste matters from the nephridia, discharge the reproductive
products, etc.

Most of these movements are performed by structures known
as m^uscles^ which consist of elongated cells (fibres) endowed in a
high degree with the power of contractility — i.e., of shortening,
or dra^ving together (cf. p. 27). Ordinary ''muscles" are in



MUSCLES. 63

tlie form of long bands or sheets of parallel Hbres, such as those
that form the body-wall, that move the setse, and dilate tlic
pharynx. Other muscular structures, liowever, do not form dis-
tinct " nmscles," but consist of muscular fi])rcs more or less
irregularly arranged and often intermingled with otlier kinds of
tissue. Of this character are the nniscular walls of tlie contrac-
tile vessels, and of the muscular portions of the nepliridia and
dissepiments. It is clear from the above that the nuiscular sys-
tem is not isolated, but is intimately involved in many organs.

The muscles of the body-wall are arranged in two concentric layers
below the skin. In the outer layer the muscles run around the body, and
are therefore called circular muscles. Those of the inner layers have a
longitudinal course, — i.e., parallel with the long axis of the body, — and
are arranged in a number of different bands. The most important of these
are :

1. The dorsal hands (Fig. 39), one on either side above, in contact at
the median dorsal line, and extending down on either side as far as the
outer row of setae.

2. The ventral hands, on either side the middle ventral line and occupy-
ing the space between the two inner (lower) rows of setaB.

3. The lateral hands, occupying the space on either side between tlie
two rows of setae.

All these vary greatly in different regions of the body, and in some parts
become more or less broken up into subsidiary bands. There is also a
narrow band traversing the space between the two setae of each group.

The setce, which may be reckoned as part of the motor system, are pro-
duced by glandular cells covering their inner ends, and tliey grow con-
stantly from this point, somewhat as hairs grow from the root. After
being fully formed, and after a certain amount of use, the setic are cast
off and replaced by new ones which have meanwhile been forming. lu
each group we find, therefore, setae of different sizes. At their inner ends
they are covered by a common investment of glandular cells wliich appears
as a slight rounded prominence when viewed from within. These prom-
inences are called the setigeroiis glands. When a worm is laid open from
above, the glands are seen in four parallel rows, two of wliich lie on either
side of the nerve-cord (see Fig. 29).

Each group of setae is provided with special retractor or protractor
muscles, and a narrow muscular band passes from the upper to tlie lower
group on each side internal to the body-wall.

Cilia. A second set of motor organs are cilia (their mode of action has
been referred to on p. 31), which are of the utmost importance m the
life of the earthworm. They cover the inner surface of the stomach-intes-
tine (where they doubtless assist in the movements of the food) play the
important part in excretion already described, collect and help to discharge



64 THE BIOLOGY OF AN ANIMAL.

the reproductive elements (p. 74), and assist in the fertilization of the Qg^
(p. 74). Their action, like that of the muscle-fibres, is doubtless due to the
property of contractility., the protoplasm alternately contracting on opposite
sides of the cilium and thus causing its whiplike action.

White Blood-corpuscles. Amoeboid Cells. Lymph-cells. Phagocytes,
Besides muscle-cells and ciliated cells there is a third variety which display
contractility and movement. These are the coelomic corpuscles referred to
above (p. 53). Until recently their function was wholly unknown, but it
is now generally believed that they are the scavengers of the body, devour-
ing the dead tissues or foreign bodies which invade the organism. Whether
they also attack and devour living parasites such as Qregarina and Bacteria
is not yet fully determined. They move their parts much as Amoebae do,
engulfing particles about them by a kind of flux.

Nervous System. Organs of Coordination.

Introduction. The general office of the nervous system of
organs is to regnlate and coordinate the actions of all the other
parts in such wise that these actions shall form an harmonious
and orderly whole. Through ner\'ous organs the worm receives
from the environment impressions which pass inwards through
the nerves as sensory or afferent unpulses, to the nervous centres ;
and through other nervous organs impulses (efferent or motor)
pass outwards from the centres to the various parts so as to
arouse, modify, or suspend their activities. Tlius tlie animal is
enabled to call forth movements resulting in the two kinds of
adjustments referred to on p. 6i^, viz., {a) adjustments of the
body as a whole to changes in the environment (e. g. , the with-
drawal of the earthworm into its burrow at the approach of day) ;
and ih) adjustments between the parts of the body itself, so that
a change in one part may call forth answering changes in other
parts (e.g., the increased supply of blood to tlie alimentary canal
during digestion, or vigorous movements of the fore end of the
body when the hind end is irritated).

These functions are always performed by one or more nerve-
cells^ which give off long slender branches known as nerve-fibres
usually gathered together in bundles, the nerves^ extending into
all parts of the body. In all higher animals the main bulk of
the nerve- cells are aggregated in definite bodies known as
ganglia.^ out of which, into which, or through which, the nerves
proceed ; and as a matter of convenience it is customary to desig-
nate the most important of these ganglia collectively as the cen-



NERVES AND GANGLIA. i5f)

tral nervous system. The remaining p(jrti«jn, which consists
mainly of nerve-fibres, tliongli it may also contain many nerve-
cells and small sporadic ganglia, is known as the peripheral
Tiervoiis system.

Gener'ol Anatomy of the Nervous System. In the earth-
v^^orm the central system consists of a long series of douhk* gangUa,
metamerically repeated, and connected by nL;rvL*-c(jrds known a«
commissures. The most anterior pair of ganglia, known as the
supra-msophageal or cerebral ganglia, lie on the dorsal ii,si)ect of
the pharynx, a short distance behind the anterior extremity
(Figs. 24, 29), From each of them a slender cord, the circum-
msophageal commissui'e^ passes dow^n at the side of the pharynx
to end in the sub-oesophageal or first ventral ganglion on the
lower side, forming with its fellow a complete ring or pharyn-
geal collar around the alimentary canal. From the sub-(jesoi)lia-
geal ganglion a long double ventral nerve-cord proceeds back weirds
in the middle ventral line. The ventral cord consists of a series
of double ganglia, one to each somite, connected by conunissures
and giving off lateral nerves.^

Internally the cerebral ganglia and the ventral cord (com-
missures as well as ganglia) consist of both nerve-cells and nerve-
fibres as described on p. U-i.

Peripheral Nervous System. To and from the central sys-
tem just described run the nerves wdiich constitute the peripheral
system. These are as follows :

1. A pair of nerves running out on either side of each ven-
tral ffano^lion and lost to \dew amons^ the muscles of the ])ody-
wall.

2. A single nerve proceeding from the ventral commissures
on each side immediately behind the dissepiment to which it is
mainly distributed.

8. A pair of nerves from the sub-03sophageal ganglion.

4. A nerve from each half of the pharyngeal collar just
beyond its divergence from its fellow. (Origin incorrectly
shown.)

5. Two large cerebral nerves, which run forwards from the



* So closely are tlie two halves of the ventral cord uniteil that its double
nature can scarcelv be made out without sections.



66



THE BIOLOGY OF AN ANIMAL.




U.TV.C.

FlQ. 29.— Anterior portion of the earthworm laid open from above, with the alimen-
tary and circulatory systems dissected away, c.c, circum-oesophageal com-
missure ; e.g., cerebral ganglia ; d*% dissepiment : /, funnel of nephridium ; np
nephridium ; o, ovary ; od, oviduct ; p^i, pharynx ; ps, prostomium ; r.s., seminal
receptacle; s.d.^ sperm-duct; s.f., sperm-funnel; s.vi.l.^ lateral seminal vesicle;
t, testis ; r.g., and v.ii.c., ventral nerve-cord.



NERVE-IMPULSES. 67

cerebral ganglia, l)reak up into many brandies, and arc* dis-
tributed to the anterior part of the body.

Besides the main gaiij^jlia of the central system, tliere arc many smaller
ganglia in various parts of the body. Of these the most important are the
pharyngeal ganglia—^ to 5 in niunber— which lie on the wall cf the
pharynx on each side just within the i)haryngeal collar. They are con-
nected with the latter by fine branches, and send minute nerves out upon
the walls of the pharynx. This series of ganglia is often inappropriately
called the sympathetic system.

Physiology of the Nervous System. Nerve- impulses,
"What is the origin and nature of a nerve-impulse? lender nor-
mal conditions the impulse is set up as the result of some dis-
turbance, technically called a sthnulus^ acting upon the end of
the fibre. A touch or pressure upon the skin, for example, acts-
as a stinmlus to the nerve-fibres ending near the point touchetl —
that is, it causes nerve-impulses to travel inwards along the fibres
towards the central system. The nerves may be stimulated by
a great variety of agents : — by mechanical disturbance, as in the
case just cited, by heat, electricity, chemical action, and in
special cases by waves of light or of sound, and upon this prop-
erty of tlie nerves depends the power of the worm to receive as-
afferent impulses impressions from the outer world. But, besides
this, nerve-fibres may also be stimulated by physiological clianges
taking place within the nerve-cells, which may thus send out
efferent impulses to the various organs and so control their ac-
tion.

Regarding the precise nature of the nerve-impulse we are ignorant, but
it is probably a chemical or molecular change in the protoplasm, travelling-
rather rapidly along the fibre, like a wave.* We know that tJje nature
of the impulse is not in any way dependent upon the character of the stimu-
lus. The stimulus can only throw the nerve into action ; and this action
is always the same whatever be the stimulus — as the action of a clock
remains the same whether it be driven by a weight or by a spring.

Co-ordination. The activities of tlie various organs are co-
ordinated by a chain of events which in its sinijilest f«nMn is known
as a rejlex action^ and which lies at the bottom of most of
the more complicated forms of nervous action. Its nature is

* In the frog the nervous impulses travel at the rate of about 28 metres j)er
second ; in man it is considerably more rapid.



68



THE BIOLOGY OF AN ANIMAL.




illustrated by the diagram (Fig. 30). Co-ordination be-
tween S and M (two organs) is not effected by a direct nervous

connection, but indirectly
through a nerve-centre, C^
which is a nerve- cell or group
of nerve-cells situated in one
of the ganglia, with which both
S and M are separately con-
nected by nerve-libres. If S
be thrown into action, an affer-
ent mipulse travels to C^ ex-
cites the nerve-centre, and

Fig. 30.— Diagram of simple reflex action. caUSCS an efferent impulse tO

S skin to which stimulus is applied ; a/, ^^.^^^^ ^^^ ^^ j^ ^j^-^j^ -g ^j^^^.^,

the afferent nerve-fibre ; C, nerve-centre ; '

e/, efferent nerve-fibre; M, muscle in hy thrOWll intO actioil alsO, Or

which the efferent fibre ends. . TXi i • xx i.'

IS modmed m respect to actions
^.Iready going on. Thus the actions of S and M are co-ordi-
nated through the agency of C\ the whole chain of events
•constituting a reflex action.

For example, let S be the skin and If a certain group of
muscles. If the skin be irritated, afferent impulses travel in-
wards to nerve-centres in the ganglia (C), which thereupon send
forth efferent impulses to the appropriate muscles. Muscular
contractions result, and the worm draws back from the unwel-
come irritation.

This chain of events involves three distinct actions on the
part of the nervous system which must be carefully distinguished,
viz. : (a) the afferent impulse ; (h) action of the centre ; (c)
the efferent impulse. It must not be supposed that the afferent
Impulse passes unchanged out of the centre as the efferent impulse,
i.e., is simply "reflected," like a ball thrown against a wall, as
the word ' ' reflex ' ' seems to imply. The afferent impulse as such
ends with the nerve-centre, wliich it throws into activity. The
efferent impulse is a new action set up by the agency of the
centre.

There is reason to believe that many if not all nerve-centres
are connected with a number of different afferent and efferent
paths, and also with other centres, as sliown in the diagram
Fig, 31. Efferent impulses may therefore be sent out from




SENSES OF THE EARTH- WORM. 69

tlie centre in various directions, and the precise i)atli cliosen
depends on some unknown-
action taking place in the
centre. The action of the
centre moreover may be
modified by efferent iin})ulses
arriving from other centres,
and thus we can dimly per-
ceive how reflexes may be-
contr oiled and guided, and
how even the most compli-
cated forms of nervous ac-
tivity may be compounded Fio. Sl.-Diagram representing three nerve-
•^ ^ ^^ ^ centres and connections. Arrows represent

out of elements similar to the possible direction of nerve-impulses,
reflex actions ^•^' °^^ afferent path ; e/, one efferent path.

There is reason to believe that in the earthworm each ven-
tral ganglion presides over the somite to which it belongs, and
is probably in the main a collection of reflex centres from wlmse
action the element of consciousness is absent. But there is also
some reason to believe that the cerebral ganglia occupy a higlier
position, since they probably receive the nerves of sight, taste,
and smell, besides those of touch, while the ventral ganglia re-
ceive only those of touch. Exj^eriment has shown furtlier tluit
the cerebral ganglia exercise to a certain limited extent a cou-
trollinfi^ action over those of the ventral chain bv means of im-
pulses sent backwards through the commissures, though tliis
action is far less conspicuous here than in higher metameric ani-
mals such as the insects.*

The Sensitive System. (Organs of Sense.) The sensitive
system is distinguished from the nervous system as a matter of
convenience of description, since most of the iiigher animald
possess definite "sense-organs" which receive stimuH and thntw
into action the sensory nerves proceeding from tlieni. Ahlioiigh
the earthworm possesses the "senses" of tuueh, taste, sight,
and smell, it has no special organs for these senses apart from
the general integument covering the surface of the Itodv. and

* For a fuller discussion the student is referred to special works on IMivsi-
ology.



70 THE BIOLOGY OF AN ANIMAL.

hence can hardly be said to possess any proper sensory system.
We do not know, moreover, whether the so-called "sensations"
of the earthworm are really states of consciousness as in ourselves,
for we do not even know whether earthworms possess any form
of consciousness. When, therefore, we speak of the earthworm
as possessing the ' ' sense ' ' of touch or of sight we mean simply
that some of the nerves terminating in the skin may be stimu-
lated l)y mechanical means or by rays of light, without necessa-
rily implying that the worm actually feels or sees as we feel and
see.

It has recently been shown that the skin contains many cells each of
wliich gives off a single nerve-fibre that may be traced directly into the
ventral nerve-cord. These "sensory cells " may be regarded as "end-
organs" through which the stimuli are conveyed to the fibres. It has also
been shown that tliese cells are aggregated in minute groups thickly scat-
tered over the surface of the body. Each of these groups may be regarded
as a simple form of sense-organ.

The sense of touch extends over the whole surface of the
body. That of taste is probably located in the cavity of the
month and pharnyx; the location of the sense of S7nell is un-
known. Darwin's experiments have shown that the earth-
worm's feeble sense of sight is confined to the anterior end of
the ])ody. It is probable that the nerves of sight, taste, and
smell enter the cerebral ganglia alone, while those of touch run
to other ganglia as well.

Systems of (Organs of) Support, Connection, Protection, etc.
The structure and mode of life of many animals are such as to
require some solid support to the soft parts of the body. Such
supporting structures are, for instance, the bones of vertebrata,
the hard outer shell of the lobster or beetle, and the coral
which forms the skeleton of a polyp. The earthworm has,
however, nothing of the sort, and it is obvious that a hard sup-
porting-organ would be not only useless, but even detrimental.
The power of creeping and burrowing through the earth depends
upon great flexibility and extensibility of the body; and with
this the presence of a skeleton might be incomjmtible.

The connecting system consists simply of various tissues by
which the different organs are bound firmly together. These
can only be seen upon microscopical examination. The most
important of them is known as connective tissue.



DEFENCES OF THE EARTHWORM. 71

As to protective structures, tlie eartliworin is prol)a])ly one of
the most defenceless of animals. Nevertheless there are certain
structures which are clearly for this purpose. The cuticle wliicli
covers the surface is a thin but tough memljraiie wliidi ju-otects
the delicate skin from direct contact with hard oljjects. It
passes into the mouth and lines the alimentary canal as far down
as the beo^inninc: of the stomach-intestine. In the irizzard.
where food is ground up, the cuticle is prodigicjusly thick and
tough, and must form a very effective protection for tlie soft
tissues beneath it. The main defence of the animal lies, how-
ever, not in any special armor, but in those instincts which lead
it to lie hidden in the earth during the day and to venture forth
only in the comparative safety of darkness.



CHAPTER Y.

THE BIOLOGY OF AN ANIMAL {Continued).

The Earthworm.
Reproduction. Embryology.

Reproduction. The life of every organic species runs in