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W. T. (William Thompson) Sedgwick.

An introduction to general biology online

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cal changes known collectively as digestion y and only after the
completion of this process can all the food be absorbed into the
cii'culation. For this j^urpose the food is taken not into the
body proper, but into a kind of tubular chemical lalioratory
called the alimentary canal through which it slowly passes^
being subjected meanwhile to the action of certain chemical sub-
stances, or reagents, known as digestive ferments. These sub-
stances, which are dissolved in a watery liquid to form the (/iges-
ti/ve fluid, are secreted by the walls of the alimentary tube.
Through their action the solid portions are liquefied and tlie food
is rendered capable of absorption into the proper body.

The alimentary canal is divisible into several ditfcrently con-
structed portions playing different parts in the process of alimtMi-
tation. Going backwards from the mouth these are as follows :

1. The pharynx (Fig. 24, jt>/<), an elongated barrel-shapi'd
pouch extending to about the 6tli somite. Its walls are thick
and muscular, and from their coelomic surface numerous small
muscles radiate on every side to the body-wall. AVhen these
muscles contract, the cavity of the pharynx is exj^anded ; and if
the mouth has been previously applied to any solid object, such
as a leaf or pebble, the pharynx acts upon it like a suction-i>ump.



50



THE BIOLOGY OF AN ANIMAL,



10.



15.




Fig. 24.— Dorsal view of the anterior part of the body of Lumhricus, as it appears
when laid open along the dorsal aspect, ao, aortic arch ; c, crop ; eg, cerebral
ganglia ; c.gl, calciferous glands ; d, dissepiment ; d.i\ dorsal vessel ; g, gizzard ;
CB, oesopnagns ; ph, pharynx ; ps, prostomium ; s.i, stomach-intestine, showing
the lateral pouches; s.r, seminal receptacles; s.v.^, s.v.^, s.v.^, the three pairs of
lateral seminal vesicles.



ORGANS OF ALIMENTATION. 51

In tliis way the animal lays hold of the various ol)jects, nutri-
tious and otherwise, which it devours or draws into its buri-ow.

Embedded in the muscular walls of the pharynx are a
number of small " salivary " glands of wliose function notliing
is definitely known, thougli they doubtless 2)our a digestive fluid
into the pharyngeal cavity.

2. The oesophagus (ce), a slender, thin- walled tube extendino"
from the 6tli to the loth somite. Through this the food is
sw^allowed, being driven slowly along by wavelike (jjeristaltic)
contractions (p. 55). In the region of the lltli and 12t}i
somites are three pairs of small pouches opening at the sides of
the oesophagus. These are the calciferous glands {c.gL). They
contain solid masses of calcium carbonate, and Darwin conjec-
tures that their use is partly to aid digestion by neutralizing tlie
acids generated during the digestion of leaves, and i)erhaps
partly to serve as an outlet for the excess of lime in the body,
especially when worms live in calcareous soil.

3. The crop (<?), about the 16th somite; a thin-walled, sac-
like dilatation of the alimentary canal, which serves as a reser-
voir to receive the swallowed food.

4. The gizzard (g), about the iTth somite; a cylindrical,
firm and muscular portion, lined by a horny membrane. In this
the food is rolled about, squeezed and ground to prepare it for
digestion in the following portion, viz. :

5. The stomach-intestine (s.i.), which corresponds physio-
logically to both the stomach and intestine of higher animals.
This is a straight thin- walled tube, extending from the gizzard
to the anus, without convolutions, not difi'erentiated into stomach
and intestine, and devoid of distinct glandular appendages sui-h
as the liver or pancreas existing in the higher animals. The
digestive fluid is secreted by the w^alls of the alimentary canal
itself, the surface of which is much increased l)y the presence of
lateral pouches or diverticula, one on either side in eacli somite.
In front these are large and conspicuous, l)ut behind they gradu-
ally diminish in size until scarcely percei)tible.

The inner surface of the stomach-intestine is further increased by a
deep inward fold, called the typJiJosole, running longitudinally along the
dorsal median line. The typhlosole is not visible on the exterior, but is
seen by opening the stomach-intestine from the side or below, or upon



52 THE BIOLOGY OF AN ANIMAL.

making a cross-section. It is richly supplied with blood- cessels that pass
down into its cavity from the dorsal vessel (Fig. 39), and its main func-
tion is probably to increase the surface for the absorption of food (cf. the
" spiral valve " in the intestine of sharks.)

The outer surface of the stomach-intestine is covered with pigmented,
yellowish-brown "chloragogue-cells." These were formerly supposed to be
concerned with the secretion of the digestive fluid, and hence are often
called "hepatic cells." This, however, is probably an erroneous interpreta-
tion, and they are now believed to be concerned with the process of excre-
tion (p. 61).

Digestion. Digestion begins even before the food is taken
into the ahnientaiy canal ; before being swallowed, the leaves,
etc., are moistened by digestive fluid poured out from the
mouths of the worms. The main action, however, doubtless goes
on in the anterior part of the stomach-intestine and diminishes
as the food passes backward. It has been proved by experhnent
that the digestive fluid acts on at least two of the three principal
varieties of organic food-stuft's, viz., on proteids and on starch
(carbohydrate), and in so far resembles the pancreatic fluid of
higher animals, which it further resembles in having an alkaline
reaction. Analogy leads us to believe that the digestive fluid
has some action also on fats ; but this has not been proved.

Krukenberg and Fredericq have shown that the digestive fluid of the
earthworm contains at least three ferments ; and according to the former
author these occur only in the stomach-intestine. They are as follows :

1. Peptic ferment^ which has the property in an acid medium of con-
verting proteids into soluble and diffusible peptones ; this is therefore
analogous to the pepsin of the gastric juice in higher forms.

2. TryptiG ferment^ having a similar action on proteids, but only in an
alkaline medium — hence analogous to the trypsin of pancreatic juice.

3. Biastatic ferment., which converts starch into glucose (grape-sugar)
in an alkaline medium — hence analogous to the ptyalin of saliva and the
amylolytic ferment of pancreatic juice.

Absorption. The ferments of the digestive fluid convert the
solid proteids into soluble and diffusible peptones, the starchy
matters into sugar (glucose). These products dissolve in the
liquids present and are then gradually absorbed by the walls of
the intestine as the food passes along the alimentary canal. The
precise mechanism of absorption is not yet thoroughly understood,
but it is probable that much of the nutriment passes by diffusion
(osmosis) into the walls of the stomach-intestine and thence into



ORGANS OF CIRCULATION.



r)3



■>



t^ r\ 1^ ^






the blood for distribution to all parts of the l)ody. The refuse
remaining in the alimentary canal (and which has never been a
part of the body proper) is finally voided throuLdi the anus as
Hastings ovfcBces. This process of " deftecation " must not be
confounded with that of excretion^ which will ]>e desciibed later.

Circulatory System. The food, having been absorbed, is
distributed throughout the body by two devices.

1. Coelomio Gircxdation. The cavity of the ca'lom is tilled
with a colorless fluid (' ' coelomic fluid ' ') which must be regarded as a
kind of lymph or blood. By the contractions of the body-wall, as
the worm crawls about, the cadomic fluid is driven back and forth
through all parts of the coelom,
through irregular openings in the
dissepiments. As the digested
food is absorbed from the stomach-
intestine a considerable part of it is
believed to pass into the coelomic
fluid, and is thus conveyed directly
to the organs which this fluid
bathes. The coelomic fluid is com-
posed of two constituents, viz., a
colorless fluid called the plasma,
and colorless isolated cells or coi'-
jpuscles which float in the plasma,
and are remarkable for the fact
that tliey undergo constant though
slow changes of form. In fact they
closely resemble certain kinds of
Ammhce, and we should certainly
consider them to be such if we
found them occurring free in stag-
nant water. We know, however,
that they live only in the plasiria, and have a connnon origin
with the other cells of the body ; hence we must regard them
not as individual animals, but as constituent cells of the eartli-
worm. The ccelomic fluid is in fact a kind of fisst/f' consisting
of isolated colorless cells floating in a fluid intercellular sul)stan('i'.
These free floating cells are probably the scavengers (j>/i(i(/<irt/f*s)
of the body, devouring and destroying waste matters. 8onio




Fig. 25.— Phagocytes, from tlie cnp-
loinic fluid of the earthworm. .1,
af^glomeration of i»lia>:orytes,
surrounding a foreiRn body; /?,
sintrle uliaLTocyte, with vacuole^i.
(After MetschnikotT.)



54 THE BIOLOGY OF AN ANIMAL.

suppose that they also attack invadmg parasites such as bacteria.

2. Vascular Circulation. Besides the coelomic circulation
there is another and more complicated circulatory apparatus con-
sisting of branching tubes, the hlood-vessels., which form a com-
plicated system ramifying throughout the body. Through these
tubes is driven a red lluid analogous to the red blood of higher
animals, and like it consisting oi plasma and corpuscles^ the latter
being flattened and somewhat spindle-shaped. The red color is
due to a substance, licmnoglohin^ dissolved in the plasma and not (as
in higher forms) contained in the corpuscles, which are colorless.

The earthworm is not provided with a special pumping-
organ or heart for the propulsion of the blood, such as we find
in higher animals. In place of this certain of the larger blood-
vessels (viz., the "dorsal vessel" and the ''aortic arches")
have muscular contractile walls, w^hich propel the blood in a con-
stant direction by wave-like contractions that run along the
vessel from one end to the other ("peristaltic" contractions, cf.
p. 51) at regular intervals and thus give rise to a "pulse."
The contractile vessels give off other non-contractile trunks
which divide and subdivide into tubes of extremely small calibre
and having very thin walls. The ultimate branches, known as
capillaries., permeate nearly all the organs and tissues, in which
they form a close network. The stream of blood after passing
through the capillaries is gathered into successively larger vessels
which after a longer or shorter course finally em23ty into the
original contractile trunks and complete the circuit. Thus the
vascular system is a closed system of tubes, and there is reason to
believe that the blood follows a perfectly definite course, though
this is not yet precisely determined.*

We may now consider the arrangement of the principal
tnmks. The largest of them, which is also the most important
of the contractile vessels, is :

a^ The dorsal vessel (Fig. 24, cZ.'y.), a long muscular tube
lying upon the upper side of the alimentary canal. In the liv-
ing worm it may be distinctly seen through the semi-transparent

* It should be noted tbat in the absence of a heart it is difficult to distin
guish between " arteries " and "veins." We may more conveniently distin-
guish " afferent vessels," carrying blood towards the capillaries, and " efferent
vessels," carrying blood away from them.



BLOOD - VESSELS. 55

skin as a dark-red band, wliicli is tolerably strai<^lit wlii-n the
worm is extended, but is made zigzag hy contracti(jn of the bod v.
If it be closely observed, a sort of wavelike contraction is often
seen running from behind forwards. This may be very clearly
observed in a worm stupefied by chloroform, especially if it luw
been laid open along the dorsal side. The dorsal vessel then
ajipears as a deep-red, somewhat twisted, tube running along the
upper side of the alimentary canal. Wavelike contractions
continually start from its hinder end and run ra})i(Ily forwards,
one after another, to the anterior end, where the dorsal vessel
finally breaks up on the pharynx into a large number of branches
(Fig. 24).

The result of these orderly progressive contractions is that
the fluid within the tube is pushed forwards — very much as the
fluid in a rubber tube is forced along when the tube is strip})ed
through the fingers. It is still better illustrated by the action
of the fingers in the operation of milking. This action of the
vessels is a typical example oi jperistaltic contraction,

h. Suh -intestinal vessel. This is a straight vessel which
runs along the middle line on the lower side of the alimentary
canal, parallel to the one just described. It returns to the
hinder part of the body the fluid which has been carried
forwards by the dorsal vessel. On the pharynx it breaks up
into many branches, which receive the fluid from corresponding
branches of the dorsal vessel.

c. Circular or comtnissural vessels., metamerically repeated
trunks which run from the dorsal vessel downwards around the
alimentary canal and ultimately connect with the ventral vessel.
They are of several kinds, of which the most important are as
follows :

1. The aortic arches or circunfioasojphageal vessels, often
known as "hearts," since like the dorsal vessel they are con-
tractile and with the latter furnish the entire propulsive forco
for the circulation. These are five pairs of large vessels en
circuling the oesophagus in somites 7 to 11 inclusive. Tlies*»
vessels pass directly from the dorsal to the ventral vessel, giving
off no branches. During life they perform powerful peristakio
contractions, receiving blood from the dorsal vessel and pumping
it into the sub-intestinal or ventral.



5Q THE BIOLOGY OF AN ANIMAL.

2. Dorso-intestinal vessels^ passing from the dorsal vessel
into the wall of the gut in the region of the stomach-intestine.
Of these vessels there are two or three pairs in each somite.
Thej are thickly covered (like the dorsal vessel in this region)
with pigmented ' ' chloragogue-cells, ' ' so that their red color is
usually not apparent. Unlike the aortic arches these vessels
break up on the wall of the intestine into capillaries which are
continuous with branches from the ventral vessel.

3. Dorso-teguTnentary vessels^ passing from the dorsal vessel
along the dissepiment into the body -wall on each side. Tliese
are small vessels that pass directly around the body to connect
with a longitudinal trunk (" sub-neural ") lying below the ven-
tral nerve-cord (see below), and giving off branches to the body-
wall, dissepiments, and nephridia.

Course of the Blood. The precise course of the blood in
Lumbricus is still in dispute, though its more general features are
known. It is certain that the bulk of the blood passes forward in
the dorsal vessel, downward around the gut through the aortic arches
into the ventral vessel, and thence backwards towards the pos-
terior region. Its path thence into the dorsal vessel is doubtful.
The most probable view is that the blood proceeds from the ven-
tral vessel through ventro-intestinal vessels to the capillaries of
the intestine and thence to the dorsal vessel throuD^h the dorso-
intestinal vessels. It is possible, however, that the return path
is through the dorso-tegumentary vessels and that the dorso-
intestinal carry blood y;"c>m the dorsal vessel to the intestine.

In the foregoing account only the more obvious features of the blood-
vessels have been mentioned, and many important details have been passed
over. The circular vessels of the stomach-intestine can be followed for
only a short distance out from the dorsal vessel, v\rhere they seem to break
up into a large number of small parallel vessels lying close together and
running around to the lower side. The efferent vessels do not directly join
the sub-intestinal, but empty into a sinus or vessel which runs parallel to
tne latter, closely imbedded in the wall of the stomach-intestine. The sub -
intestinal vessel proper is quite separate from the stomach-intestine, and
communicates by short branches (usually two in each somite) with the
vessel lying above it. This maybe clearly seen in the region of the gizzard.
On this there is a variable number of small lateral vessels, which break up
partly into a branching network, and are partly resolved into extremely
fine parallel vessels surrounding the organ. On the crop are three or four
pairs of lateral branches from the dorsal vessel which branch out into a



BLOOD-VESSELS.



57



fine network, but do not break up into parallel vessels as o\\ the gizzard.
In the two somites (13th and 14th) in front of the crop there are usually
two pairs of vessels running around the cesophagiis. In the 11th and 12th
somites a small branch is given otf to each calciferous gland. The most
anterior pair of circular vessels are in the 6th somite, and are very small.
In front of this the dorsal vessel breaks up into the pharyngeal network.
In front of the 11th somite there are three sub-iutestinal vessels. The two
additional vessels lie, one on either side of the primary one and break up
into branches at the sides of the pharynx. The aortic arches empty into
the middle vessel, and at the point of junction there is a communication
with the lateral vessel of the corresponding side.

Besides the dorsal and sub-intestinal vessels there are three other minor
longitudinal trunks (Fig. 26). Two of these are very small, and lie on




56. n..



Fig. 26.— Dorsal view of part of the ventral nerve-cord, showing the arranpromeni of
the vessels of the ventral region, d.% dissepiment; si, sub-intestinal dp ventral
blood-vessel ; s?>./?., siib-neural ; sp.??., supra-neural. The suli-intcstinal recrivrs on
either side the ventro-laterals (r./) from the nepliridia, of which it forms th«' ef-
ferent vessel (e./). The sub-neural is joined on each side by a continuation of the
d<yrso-tegumentary (d.t.); a/, afferent branch to the nephridiura (cf. Fig. 27).

either side above the nerve-cord (p. 66), sending fine branches out from
each ganglion along the lateral nerves. These are the a up ra- neural trunks
(.9.W.). The third longitudinal vessel {suh-neural) lies below the nerve-cord.
(See Fig. 26.) It receives on each side the termination of the dorso-tegii-
mentary vessel {d.t., Fig. 26) which in its course is connected witii the
capillary networks of the body-wall and the dissej)iment, and gives off a
large branch to the nephridium (cf. Fig. 27).



58



THE BIOLOGY OF AN ANIMAL.



Besides the lateral vessels from the sub-neural and supra-neural a pair
of " ventro-lateral " (y.Z., Figs. 26 and 27) are given off in each somite from
the sub-intestinal to the nephridium, probably receiving from it the blood
Rrhich originally entered through a branch of the dorso-tegumentary.




ST 2/: 7



im




JTITV.



Fig. 27.— Nephridia of JjumbiHcns. A showing the regions of the tube, B the vascular
supply. /, II, III, the three principal loops.

.4. /, funnel; n.t, the "narrow tube"; m.t, middle tube; u\t, wide tube: m.p, mus-
cular tube or end-vesicle ; ds, dissepiment. The narrow tube extends from a to g
and is ciliated between a and ft, at c, and from d to c. The middle (ciliated) tube
extends from g to h, the wide tube from h to k, where it opens into the muscular
part ; eX, external opening.

B, Letters as before ; d.t, dorso-tegumentary vessel, bringing blood from the dorsal
vessel, receiving at s a branch from the body- wall, sending an afferent branch to
the nephridium, and finally joining the sub-neural (s.n); v.l, ventro-lateral vessel
carrying the blood from the nephridium to the sub-intestinal or ventral vessel
(s.i) ; v.v, ventral nerve-cord. (After Benham; the direction of the blood-cur-
rents according to Bourne.)

Excretory System. It is the office of the excretory system to
remove from the body proper the waste matters ulthnately re-



ORGANS OF EXCRETION. NEPllRIDIA.



m







fi'J'.:



suiting from the breaking down of living tissue. Tliis docs not
mean the passing away of the refuse of digestion throiigli tlie
anus (defaecation, p. 53), for sueh matters have never been
absoi'bed and therefore have never really been witliin tlic IkkIv
proper. Excretion means the removal from the body of matter
which has really formed a part of its substance, l)ut lias been
used up and is no longer alive. In higher animals this function
is performed chieily by the kidneys, the lungs, and the skin, the
waste matters passing off in the urine, the breath, and the sweat.
In the earthworm it is principally performed by small organs
called nephridia^ of which here are two in each somite, excej)t-
ing the lirst three or four (Fig. 29).

Each nepln-idium (Fig. 27) consists of a long convoluted
tube, attached to the hinder face of a
•dissepiment, and lying in the coilom at
the side of the alimentary canal. At
one end the tube passes through the ^>^:/;}::;'lii^v;;:- .;
body -wall and opens to the exterior by a ^^B^;^:^^^
minute pore situated between the outer * ' "**"
and inner rows of setie (p. 46). The
other end of the tube passes through the
dissepiment very near to the point
where this is penetrated by the nerve -
cord (p. ^^)^ and opens by a broad,
funnel-like expansion into the cavity of ^^^ ^^ _.^ nephridiai funnel

the next somite in front (/", Fig. 27). much enlarKed, showing the
rr^i • J* j^i J" 1 1 j_i • cilia, the betfinnin^; of tl>o

The margms of the funnel and the nmer ^.^^^,^^^^^ ^,^,^^^ (,)^ ,,,,j the
surface of the upper part of the tube are o^ter sheath («).
densely covered with powerful cilia (Fig. 28), whose action tend.«=i
to produce a current setting from the coelom into the fuimel and
through the nephridium to the exterior.

The coils of the nephridium arc disposed in three principal loops (I, II,
III in Fig. 27). The tube itself comprises five very distinct regions, as
follows :

1. 1\\Q funnel ov nej^hrostome ; much flattened from above downwards,
with the opening reduced to a horizontal chink. It is composed of beau-
tiful ciliated cells set like fan-rays around its edge. It leads into

2. The " narrow tube''' {n.t.), a very delicate thin-walled contorted tubo
extending from the nephrostome through the first loop and a part of the
second. In certain parts of its course (a to 6, at c, and from d to e) this




60 THE BIOLOG Y OF AN ANIMAL.

■*

tube contains cilia which are arranged in two longitudinal bands on the
inner surface. At g it passes into the

3. " Middle tube'''' {m.t.) (g toh), extending straight through the second
loop, of greater diameter, ciliated throughout, and with pigmented walls.
At h it opens into the

4. " Wide tube'''' (w.t.). This is of still greater calibre, with granular
glandular walls and without cilia. It extends through the second loop
(from h to i, II) into and through the first from i to J, and finally into the
third, opening at k into the

5. Muscular part or duct (m.p.) which forms the third loop and opens to-
the exterior at ex. This, the widest part of the entire nephridium, has
muscular w^alls and forms a kind of sac or reservoir like a bladder, in
which the excreted matter may accumulate and from which it may be
passed out to the exterior.

The various parts of the nephridium are held together Vjy connective
tissue (p. 90), and are covered with a rich network of blood-vessels, the
arrangement of which is shown in Fig. 27, B. The smaller vessels usually
show numerous pouchlike dilatations which must serve to retard the flow
of blood somewhat. The vessels supplying the nephridium are connected
(Fig. 27, B) on the one hand with the sub-intestinal vessel through the
ventro-lateral trunks {v. I.) ; on the other hand with the sub-neural (s.n.) and
dorsal vessels, through the dorso-tegumentary (d.t.). The course of the
blood is somewhat doubtful. According to the view here adopted (cf. p. 56)
the blood proceeds from the dorso-tegumentary trunk to the nephridia and
thence through the ventro lateral to the sub-intestinal, as shown by the
arrows in the figure. Benham (from whom the figures are copied) adopts,
the reverse view. The development of the nephridium shows that its
ciliated and glandular portions arise from a solid cord of disk-shaped cells
which afterwards becomes tubular by the hollowing out of its axial portion.


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Online LibraryW. T. (William Thompson) SedgwickAn introduction to general biology → online text (page 6 of 20)