THE ORGANS OF RESPIRATION
=How Fishes Breathe.=--The fish breathes the air which is dissolved in
water. It cannot use the oxygen which is a component part of water, nor
can it, as a rule, make use of atmospheric air. The amount of oxygen
required for the low vegetative processes of the fish is comparatively
small. According to Dr. Günther, a man consumes 50,000 times as much
oxygen as a tench. But some fishes demand more oxygen than others.
Some, like the catfish or the loach, will survive long out of water,
while others die almost instantly if removed from their element or if
the water is allowed to become foul. In most cases the temperature
of the blood of the fish is but little above that of the water in
which they live, but in the mackerel and other muscular fishes the
temperature of the body may be somewhat higher.
Some fishes which live in mud, especially in places which become dry
in summer, have special contrivances by which they can make use of
atmospheric air. In a few primitive fishes (Dipnoans, Crossopterygians,
Ganoids) the air-bladder retains its original function of a lung. In
other cases some peculiar structure exists in connection with the
gills. Such a contrivance for holding water above the gills is seen in
the climbing perch of India (_Anabas scandens_) and other members of
the group called Labyrinthici.
In respiration, in fishes generally, the water is swallowed through the
mouth and allowed to pass out through the gill-openings, thus bathing
the gills. In a few of the lower types a breathing-pore takes the place
of the gill-openings.
The gills, or branchiæ, are primarily folds of the skin lining the
branchial cavity. In most fishes they form fleshy fringes or laminæ
throughout which the capillaries are distributed. In the embryos of
sharks, skates, chimæras, lung-fishes, and Crossopterygians external
gills are developed, but in the more specialized forms these do not
appear outside the gill-cavity. In some of the sharks, and especially
the rays, a spiracle or open foramen remains behind the eye. Through
this spiracle, leading from the outside into the cavity of the mouth,
water is drawn downwards to pass outward over the gills. The presence
of this breathing-hole permits these animals to lie on the bottom
without danger of inhaling sand.
[Illustration: FIG. 76.--Gill-basket of Lamprey. (After Dean.)]
=The Gill-structures.=--The three main types of gills among fishes are
the following: (_a_) the purse-shaped gills found in the hagfishes and
lampreys, known as a class as Marsipobranchs, or purse-gills. These
have a number (5 to 12) of sac-like depressions on the side of the
body, lined with gill-fringes and capillaries, the whole supported
by an elaborate branchial basket formed of cartilage. (_b_) The
plate-gills, found among the sharks, rays, and chimæras, thence called
Elasmobranchs, or plate-gills. In these the gill-structures are flat
laminæ, attached by one side to the gill-arches. (_c_) The fringe-gills
found in ordinary fishes, in which the gill-filaments containing
the capillaries are attached in two rows to the outer edge of each
gill-arch. The so-called tuft-gills (Lophobranchs) of the sea-horse
and pipefish are like these in structure, but the filaments are long,
while the arches are very short. In most of the higher fishes a small
accessory gill (pseudobranchia) is developed in the skin of the inner
side of the opercle.
=The Air-bladder.=--The air-bladder, or swim-bladder, must be classed
among the organs of respiration, although in the higher fishes its
functions in this regard are rudimentary, and in some cases it has
taken collateral functions (as a hydrostatic organ of equilibrium, or
perhaps as an organ of hearing) which have no relation to its original
purpose.
[Illustration: FIG. 77.--Weberian apparatus and air-bladder of Carp.
(From Günther, after Weber.)]
The air-bladder is an internal sac possessed by many fishes, but not
by all. It lies in the dorsal part of the abdominal cavity above the
intestines and below the kidneys. In some cases it is closely adherent
to the surrounding tissues. In others it is almost entirely free, lying
almost loose in the cavity of the body. In some cases it is enclosed in
a bony capsule. In the allies of the carp and catfish, which form the
majority of fresh-water fishes, its anterior end is connected through a
chain of modified vertebræ to the ear. Sometimes its posterior end fits
into an enlarged and hollow interhæmal bone. Sometimes, again, a mass
of muscle lies in front of it or is otherwise attached to it. Sometimes
it is divided into two or three parts by crosswise constrictions.
Sometimes it is constricted longitudinally, and at other times it
has attached to it a complication of supplemental tubes of the same
character as the air-bladder itself. In still other cases it is divided
by many internal partitions into a cellular body, similar to the lung
of the higher vertebrates, though the cells are coarser and less
intricate. This condition is evidently more primitive than that of the
empty sac.
The homology of the air-bladder with the lung is evident. This is often
expressed in the phrase that the lung is a developed air-bladder.
This is by no means true. To say that the air-bladder is a modified
and degenerate lung is much nearer the truth, although we should
express the fact more exactly to say that both air-bladder and lung
are developed from a primitive cellular breathing-sac, originally a
diverticulum from the ventral walls of the oesophagus.
The air-bladder varies in size as much as in form. In some fishes it
extends from the head to the tail, while in others it is so minute
as to be scarcely traceable. It often varies greatly in closely
related species. The common mackerel (_Scomber scombrus_) has no
air-bladder, while in the closely related colias or chub mackerel
(_Scomber japonicus_) the organ is very evident. In other families,
as the rockfishes (_Scorpænidæ_), genera with and those without the
air-bladder are scarcely distinguishable externally. In general, fishes
which lie on the bottom, those which inhabit great depths, and those
which swim freely in the open sea, as sharks and mackerel, lack the
air-bladder. In the sharks, rays, and chimæras there is no trace of an
air-bladder. In the mackerel and other bony fishes without it, it is
lost in the process of development.
The air-bladder is composed of two layers of membrane, the outer
one shining, silvery in color, with muscular fibres, the inner well
supplied by blood-vessels. The gas within the air-bladder must be in
most cases secreted from the blood-vessels. In river fishes it is said
to be nearly pure nitrogen. In marine fishes it is mostly oxygen, with
from 6 to 10 per cent of carbonic-acid gas, while in the deep-sea
fishes oxygen is greatly in excess. In _Lopholatilus_, a deep-sea fish,
Professor R. W. Tower finds 66 to 69 per cent of oxygen. In _Trigla
lyra_ Biot records 87 per cent. In _Dentex dentex_, a shore fish of
Europe, 40 per cent of oxygen was found in the air-bladder. Fifty
per cent is recorded from the European porgy, _Pagrus pagrus_. In a
fish dying from suffocation the amount of carbonic-acid gas (CO2)
is greatly increased, amounting, according to recent researches of
Professor Tower on the weak-fish, _Cynoscion regalis_, to 24 to 29 per
cent. This shows conclusively that the air-bladder is to some degree a
reservoir of oxygen secreted from the blood, to which channel it may
return through a kind of respiration.
The other functions of the air-bladder have been subject to much
question and are still far from understood. The following summary of
the various views in this regard we copy from Professor Tower's paper
on "The Gas in the Swim-bladder of Fishes":
"The function of the swim-bladder of fishes has attracted the attention
of scientists for many centuries. The rôle that this structure plays in
the life of the animal has been interpreted in almost as many ways as
there have been investigators, and even now there is apparently much
doubt as to the true functions of the swim-bladder. Consequently any
additional data concerning this organ are of immediate scientific value.
"Aristotle, writing about the noises made by fishes, states that
'some produce it by rubbing the gill-arches ...; others by means of
the air-bladder. Each of these fishes contains air, by rubbing and
moving of which the noise is produced.' The bladder is thus considered
a sound-producing organ, and it is probable that he arrived at this
result by his own investigations.
"Borelli (De Motu Animalium, 1680) attributed to the air-bladder
a hydrostatic function which enabled the fish to rise and fall in
the water by simply distending or compressing the air-bladder. This
hypothesis, which gives to the fish a volitional control over the
air-bladder--it being able to compress or distend the bladder at
pleasure--has prevailed, to a greater or less degree, from the time
of Borelli to the present. To my knowledge, however, there are no
investigations which warrant such a theory, while, on the other hand,
there are many facts, as shown by Moreau's experiment, which distinctly
contradict this belief. Delaroche (Annales du Mus. d'Hist. Nat.,
tome XIV, 1807-1809) decidedly opposed the ideas of Borelli, and yet
advanced an hypothesis similar to it in many respects. Like Borelli,
he said that the fish could compress or dilate the bladder by means
of certain muscles, but this was to enable the fish to keep the same
specific gravity as the surrounding medium, and thus be able to remain
at any desired depth (and not to rise or sink). This was also disproved
later by Moreau. Delaroche proved that there existed a constant
exchange between the air in the air-bladder and the air in the blood,
although he did not consider the swim-bladder an organ of respiration.
"Biot (1807), Provençal and Humboldt (1809), and others made chemical
analyses of the gas in the swim-bladder, and found 1 to 5 per cent of
CO_{2}, 1 to 87 per cent of O_{2}, and the remainder nitrogen. The most
remarkable fact discovered about this mixture was that it frequently
consisted almost entirely of oxygen, the per cent of oxygen increasing
with the depth of the water inhabited by the fish. The reasons for this
phenomenon have never been satisfactorily explained.
"In 1820 Weber described a series of paired ossicles which he
erroneously called stapes, malleus, and incus, and which connected the
air-bladder in certain fishes with a part of the ear--the atrium sinus
imparis. Weber considered the swim-bladder to be an organ by which
sounds striking the body from the outside are intensified, and these
sounds are then transmitted to the ear by means of the ossicles. The
entire apparatus would thus function as an organ of hearing. Weber's
views remained practically uncontested for half a century, but recently
much has been written both for and against this theory. Whatever the
virtues of the case may be, there is certainly an inviting field for
further physiological investigations regarding this subject, and more
especially on the phenomena of hearing in fishes.
"Twenty years later Johannes Müller described, in certain Siluroid
fishes, a mechanism, the so-called 'elastic-spring' apparatus, attached
to the anterior portion of the air-bladder, which served to aid the
fish in rising and sinking in the water according as the muscles of
this apparatus were relaxed or contracted to a greater or lesser
degree. This interpretation of the function of the 'elastic-spring'
mechanism was shown by Sörensen to be untenable. Müller also stated
that in some fish, at least, there was an exchange of gas between
blood and air-bladder--the latter having a respiratory function--and
regarded the gas in the air-bladder as the result of active secretion.
In _Malapterurus_ (_Torpedo electricus_) he stated that it is a
sound-producing organ.
"Hasse, in 1873, published the results of his investigations on the
functions of the ossicles of Weber, stating that their action was that
of a manometer, acquainting the animal with the degree of pressure that
is exerted by the gases in the air-bladder against its walls. This
pressure necessarily varies with the different depths of water which
the fish occupies. Hasse did not agree with Weber that the ear is
affected by the movements of these ossicles.
"One year later Dufosse described in some fishes an air-bladder
provided with extrinsic muscles by whose vibration sound was produced,
the sound being intensified by the air-bladder, which acted as a
resonator. He also believed that certain species produced a noise by
forcing the gas from the air-bladder through a pneumatic duct.
"At about the same time Moreau published his classical work on the
functions of the air-bladder. He proved by ingenious experiments that
many of the prevailing ideas about the action of the air-bladder were
erroneous, and that this organ serves to equilibrate the body of the
fish with the water at any level. This is not accomplished quickly,
but only after sufficient time for the air in the bladder to become
adjusted to the increase or decrease in external pressure that has
taken place. The fish, therefore, makes no use of any muscles in
regulating the volume of its air-bladder. The animal can accommodate
itself only gradually to considerable changes in depth of water, but
can live equally comfortably at different depths, provided that the
change has been gradual enough. Moreau's experiments also convinced
him that the gas is actually secreted into the air-bladder, and
that there is a constant exchange of gas between it and the blood.
In these investigations he has also noticed that section of the
sympathetic-nerve fibres supplying the walls of the air-bladder hastens
the secreting of the gas into the empty bladder. Since then Bohr has
shown that section of the vagus nerve causes the secretion to cease.
Moreau noticed in one fish (_Trigla_) having an air-bladder supplied
with muscles that the latter served to make the air-bladder produce
sound.
"Again, in 1885, the Weberian mechanism was brought to our attention
with a new function attributed to it by Sagemehl who stated that this
mechanism exists not for any auditory purposes, nor to tell the fish
at what level of the water it is swimming, but to indicate to the fish
the variations in the atmospheric pressure. Sörensen tersely contrasts
the views of Hasse and Sagemehl by saying that 'Hasse considers the
air-bladder with the Weberian mechanism as a manometer; Sagemehl
regards it as a barometer.' The theory of Sagemehl has, naturally
enough, met with little favor. Sörensen (1895) held that there is but
little evidence for attributing to the air-bladder the function of a
lung. It is to be remembered, however, that, according to Sörensen's
criterion no matter what exchange of gases takes place between blood
and air-bladder, it cannot be considered an organ of respiration,
'unless its air is renewed by mechanical respiration.'
"Sörensen also refutes, from anatomical and experimental grounds, the
many objections to Weber's theory of the function of the ossicles. He
would thus attribute to the air-bladder the function of hearing; indeed
in certain species the only reason for the survival of the air-bladder
is that 'the organ is still of acoustic importance; that it acts as a
resonator.' This idea, Sörensen states, is borne out by the anatomical
structure found in _Misgurnus_ and _Chlarias_, which resembles the
celebrated 'Colladon resonator.' This author attributes to the
air-bladder with its 'elastic spring' and various muscular mechanisms
the production of sound as its chief function."
=Origin of the Air-bladder.=--In the more primitive forms, and probably
in the embryos of all species, the air-bladder is joined to the
oesophagus by an air-duct. This duct is lost entirely in the adult
of all or nearly all of the thoracic and jugular fishes, and in some
of the abdominal forms. The lancelets, lampreys, sharks, rays, and
chimæras have no air-bladder, but in the most primitive forms of true
fishes (Dipnoans and Crossopterygians), having the air-bladder cellular
or lung-like, the duct is well developed, freely admitting the external
air which the fish may rise to the surface to swallow. In most fishes
the duct opens into the oesophagus from the dorsal side, but in the
more primitive forms it enters from the ventral side, like the windpipe
of the higher vertebrates. In some of the Dipnoans the air-bladder
divides into two parts, in further resemblance to the true lungs.
=The Origin of the Lungs.=--The following account of the function of
the air-bladder and of its development and decline is condensed from an
article by Mr. Charles Morris:[5]
"If now we seek to discover the original purpose of this organ, there
is abundant reason to believe that it had nothing to do with swimming.
Certainly the great family of the sharks, which have no bladder, are at
no disadvantage in changing their depth or position in the water. Yet
if the bladder is necessary to any fish as an aid in swimming, why not
to all? And if this were its primary purpose, how shall we explain its
remarkable variability? No animal organ with a function of essential
importance presents such extraordinary modifications in related species
and genera. In the heart, brain, and other organs there is one shape,
position, and condition of greatest efficiency, and throughout the
lower forms we find a steady advance towards this condition. Great
variation, on the other hand, usually indicates that the organ is
of little functional importance, or that it has lost its original
function. Such we conceive to be the case with the air-bladder. The
fact of its absence from some and its presence in other fishes of
closely related species goes far to prove that it is a degenerating
organ; and the same is shown by the fact that it is useless in some
species for the purpose to which it is applied in others. That it had,
at some time in the past, a function of essential importance there
can be no question. That it exists at all is proof of this. But its
modern variations strongly indicate that it has lost this function and
is on the road towards extinction. Larval conditions show that it had
originally a pneumatic duct as one of its essential parts, but this
has in most cases disappeared. The bladder itself has in many cases
partly or wholly disappeared. Where preserved, it seems to be through
its utility for some secondary purpose, such as an aid in swimming or
in hearing. That its evolution began very long ago there can be no
question; and the indications are that it began long ago to degenerate,
through the loss of its primitive function.
"What was this primitive function? In attempting to answer this
question we must first consider the air-bladder in relation to the
fish tribe as a whole. No shark or ray possesses the air-bladder.
In some few sharks, indeed, there is a diverticulum of the pharynx
which may be a rudimentary approach to the air-bladder; but this is
very questionable. The conditions of its occurrence in the main body
of modern fishes, the Teleostean, we have already considered. But in
the most ancient living orders of fishes it exists in an interesting
condition. In every modern Dipnoan, Crossopterygian, and Ganoid the
air-bladder has an effective pneumatic duct. This in the Ganoids
opens into the dorsal side of the oesophagus, but in the Dipnoans and
Crossopterygians, like the windpipe of lung-breathers, it opens into
the ventral side. In the Dipnoans, also survivors from the remote
past, the duct not only opens ventrally into the oesophagus, but the
air-bladder does duty as a lung. Externally it differs in no particular
from an air-bladder; but internally it presents a cellular structure
which nearly approaches that of the lung of the batrachians. There
are three existing representatives of the Dipnoans. One of these, the
Australian lung-fish (_Neoceratodus_) has a single bladder, which,
however, is provided with breathing-pouches having a symmetrical
lateral arrangement. It has no pulmonary artery, but receives branches
from the _arteria coeliaca_. In the other two forms, _Lepidosiren_ and
_Protopterus_, the kindred 'mudfishes' of the Amazon basin and tropical
Africa, the bladder or lung is divided into two lateral chambers, as in
the land animals, and is provided with a separate pulmonary artery.
"The opinion seems to have been tacitly entertained by physiologists
that this employment of the air-bladder by the Dipnoans as a lung
is a secondary adaptation, a side issue from its original purpose.
It is more likely that this is the original purpose, and that its
degeneration is due to the disappearance of the necessity of such
a function. As regards the gravitative employment of the bladder,
the Teleostean fishes, to which this function is confined, are
of comparatively modern origin; while the Dipnoans are surviving
representatives of a very ancient order of fishes, which flourished
in the Devonian age of geology, and in all probability breathed air
then as now; and the Crossopterygians and Ganoids, which approach them
in this particular, are similarly ancient in origin, and were the
ancestors of the Teleosteans. The natural presumption, therefore, is
that the duty which it subserved in the most ancient fishes was its
primitive function.
"The facts of embryology lend strong support to this hypothesis. For
the air-bladder is found to arise in a manner very similar to the
development of the lung. They each begin as an outgrowth from the
fore part of the alimentary tract, the only difference being that the
air-bladder usually rises dorsally and the lung ventrally. The fact
already cited, that the pneumatic duct is always present in the larval
form in fishes that possess a bladder, is equally significant. All the
facts go to show that the introduction of external air into the body
was a former function of the air-bladder, and that the atrophy of the
duct in many cases, and the disappearance of the bladder in others, are
results of the loss of this function.
"Such an elaborate arrangement for the introduction of air into the
body could have, if we may judge from analogy, but one purpose, that
of breathing, to which purpose the muscular and other apparatus
for compressing and dilating the bladder, now seemingly adapted to
gravitative uses, may have been originally applied. The same may
be said of the great development of blood-capillaries in the inner
tunic of the bladder. These may now be used only for the secretion
of gas into its interior, but were perhaps originally employed in
the respiratory secretion of oxygen. In fact all the circumstances
mentioned--the similarity in larval development between the bladder and
lung, the larval existence of the pneumatic duct, the arrangements for
compressing and dilating the bladder, and the capillary vessels on its
inner tunic--point to the breathing of air as its original purpose.
"It is probable that the Ganoid, as well as the Dipnoan, air-bladder is
to some extent still used in breathing. The Dipnoans have both lungs
and gills, and probably breathe with the latter in ordinary cases, but
use their lungs when the inland waters in which they live become thick
and muddy, or are charged with gases from decomposing organic matter.
The Ganoid fishes to some extent breathe the air. In _Polypterus_ the
air-bladder resembles the Dipnoan lung in having lateral divisions and
a ventral connection with the oesophagus, while in _Lepisosteus_ (the
American garpike) it is cellular and lung-like. This fish keeps near
the surface, and may be seen to emit air-bubbles, probably taking in
a fresh supply of air. The American bowfin, or mudfish (_Amia_), has
a bladder of the same lung-like character, and has been seen to come
to the surface, open its jaws widely, and apparently swallow a large
quantity of air. He considers that both _Lepisosteus_ and _Amia_
inhale and exhale air at somewhat regular intervals, resembling in this
the salamanders and tadpoles, 'which, as the gills shrink and the lungs
increase, come more frequently to the surface for air.'
"As the facts stand there is no evident line of demarcation between the
gas-containing bladders of many of the Teleosteans, the air-containing
bladders of the others and the Ganoids, and the lung of the Dipnoans,
and the indications are in favor of their having originally had the
same function, and of this being the breathing of air.
"If now we ask what were the conditions of life under which this
organ was developed, and what the later conditions which rendered
it of no utility as a lung, some definite answer may be given. The
question takes us back to the Devonian and Silurian geological
periods, during which the original development of the bladder
probably took place. In this era the seas were thronged with fishes
of several classes, the Elasmobranchs among others, followed by the
Dipnoi and Crossopterygians. The sharks were without, the Dipnoans
and Crossopterygians doubtless with, an air-bladder--a difference in
organization which was most likely due to some marked difference in
their life-habits. The Elasmobranchs were the monarchs of the seas,
against whose incursions the others put on a thick protective armor,
and probably sought the shallow shore waters, while their foes held
chief possession of the deeper waters without.
"We seem, then, to perceive the lung-bearing fishes, driven by their
foes into bays and estuaries, and the waters of shallow coasts,
ascending streams and dwelling in inland waters. Here two influences
probably acted on them. The waters they dwelt in were often thick
with sediment, and were doubtless in many instances poorly aerated,
rendering gill-breathing difficult. And the land presented conditions
likely to serve as a strong inducement to fishes to venture on shore.
Its plant-life was abundant, while its only animal inhabitants seem to
have been insects, worms, and snails. There can be little doubt that
the active fish forms of that period, having no enemies to fear on the
land, and much to gain, made active efforts to obtain a share of this
vegetable and animal food. Even to-day, when they have numerous foes to
fear, many fishes seek food on the shore, and some even climb trees
for this purpose. Under the conditions of the period mentioned there
was a powerful inducement for them to assume this habit.
"Such conditions must have strongly tended to induce fishes to breathe
the air, and have acted to develop an organ for this purpose. In
addition to the influences of foul or muddy water and of visits to
land may be named that of the drying-out of pools, by which fishes
are sometimes left in the moist mud till the recurrence of rains, or
are even buried in the dried mud during the rainless season. This is
the case with the modern Dipnoi, which use their lungs under such
circumstances. In certain other fresh-water fishes, of the family
Ophiocephalidæ, air is breathed while the mud continues soft enough
for the fish to come to the surface, but during the dry period the
animal remains in a torpid state. These fishes have no lungs, but
breathe the air into a simple cavity in the pharynx, whose opening is
partly closed by a fold of the mucous membrane. Other Labyrinthici,
of similar habits, possess a more developed breathing organ. This
is a cavity formed by the walls of the pharynx, in which are thin
laminæ, or plates, which undoubtedly perform an oxygenating function.
The most interesting member of this family is _Anabas scandens_, the
climbing perch. In this fish, which not only leaves the water, but is
said to climb trees, the air-breathing organ is greatly developed. The
labyrinthici, moreover, have usually large air-bladders. As regards the
occasional breathing of air by fishes, even in species which do not
leave the water, it is quite common, particularly among fresh-water
species. Cuvier remarks that air is perhaps necessary to every kind of
fish; and that, particularly when the atmosphere is warm, most of our
lacustrine species sport on the surface for no other purpose.
"It is not difficult to draw a hypothetical plan of the development
of the air-bladder as a breathing organ. In the two families of
fishes just mentioned, whose air-bladders indicate that they once
possessed the air-breathing function and have lost it, we perceive
the process of formation of an air-breathing organ beginning over
again under stress of similar circumstances. The larval development of
the air-bladder points significantly in the same direction. In fact
we have strong reason to believe that air-breathing in fishes was
originally performed, as it probably often is now, by the unchanged
walls of the oesophagus. Then these walls expanded inwardly, forming a
simple cavity, partly closed by a fold of membrane, like that of the
Ophiocephalidæ. A step further reduced this membranous fold to a narrow
opening, leading to an inner pouch. As the air-breathing function
developed, the opening became a tube, and the pouch a simple lung, with
compressing muscles and capillary vessels. By a continuation of the
process the smooth-walled pouch became sacculated, its surface being
increased by folding into breathing cells. Finally, a longitudinal
constriction divided it into two lateral pouches, such as we find in
the lung of the Dipnoans. This brings us to the verge of the lung of
the amphibians, which is but a step in advance, and from that the line
of progress is unbroken to the more intricate lung of the higher land
animals.
"The dorsal position of the bladder and its duct would be a difficulty
in this inquiry, but for the fact that the duct is occasionally
ventral. This dorsal position may have arisen from the upward pressure
of air in the swimming fish, which would tend to lift the original
pouch. But in the case of fishes which made frequent visits to the
shore new influences must have come into play. The effect of gravity
tended to draw the organ and its duct downward, as we find in the
Crossopterygians and in all the Dipnoans, and its increased use in
breathing required a more extended surface. Through this requirement
came the pouched and cellular lung of the Dipnoans. Of every stage of
the process here outlined examples exist, and there is great reason
to believe that the development of the lung followed the path above
pointed out.
"When the carboniferous era opened there may have been many lung- and
gill-breathing fishes which spent much of their time on land, and some
of which, by a gradual improvement of their organs of locomotion,
changed into batrachians. But with the appearance of the latter, and
of their successors, the reptiles, the relations of the fish to the
land radically changed. The fin, or the simple locomotor organ, of the
Dipnoans could not compete with the leg and foot as organs of land
locomotion, and the fish tribe ceased to be lords of the land, where,
instead of feeble prey, they now found powerful foes, and were driven
back to their native habitat, the water. Nor did the change end here.
In time the waters were invaded by the reptiles, numerous swimming
forms appearing, which it is likely were abundant in the shallower
shore-line of the ocean, while they sent many representatives far out
to sea. These were actively carnivorous, making the fish their prey,
the great mass of whom were doubtless driven into the deeper waters,
beyond the reach of their air-breathing foes.
"In this change of conditions we seem to perceive an adequate cause
for the loss of air-breathing habits in those fishes in which the lung
development had not far progressed. It may indeed have been a leading
influence in the development of the Teleostean or bony fishes, as
it doubtless was in the loss of its primitive function by, and the
subsequent changes of, the air-bladder.
"Such of the Crossopterygians and Dipnoans as survived in their old
condition had to contend with adverse circumstances. Most of them in
time vanished, while their descendants which still exist have lost
in great measure their air-breathing powers, and the Dipnoans, in
which the development of the lung had gone too far for reversal, have
degenerated into eel-like, mud-haunting creatures, in which the organs
of locomotion have become converted into the feeble paddle-like limbs
of Neoceratodus and the filamentary appendages of the other species.
"As regards the presence of a large quantity of oxygen in the bladders
of deep-swimming marine fishes, it not unlikely has a respiratory
purpose, the bladder being, as suggested by Semper, used as a reservoir
for oxygen, to serve the fish when sleeping, or when, from any cause,
not actively breathing. The excess of oxygen is not due to any like
excess in the gaseous contents of sea-water, for the percentage of
oxygen decreases from the surface downward, while that of nitrogen
remains nearly unchanged. In all cases, indeed, the bladder may
preserve a share of its old function, and act as an aid in respiration.
Speaking of this, Cuvier says: 'With regard to the presumed assistance
which the swim-bladder affords in respiration, it is a fact that when a
fish is deprived of that organ, the production of carbonic acid by the
branchiæ is very trifling,' thus strongly indicating that the bladder
still plays a part in the oxygenation of the blood.
"Under the hypothesis here presented the process of evolution involved
may be thus summed up. Air-breathing in fishes was originally performed
by the unchanged walls of the oesophagus perhaps at specially vascular
localities. Then the wall folded inward, and a pouch was finally
formed, opening to the air. The pouch next became constricted off,
with a duct of connection. Then the pouch became an air-bladder with
respiratory function, and finally developed into a simple lung. These
air-breathing fishes haunted the shores, their fins becoming converted
into limbs suitable for land locomotion, and in time developed into
the lung- and gill-breathing batrachia, and these in their turn into
the lung-breathing reptilia, the locomotor organs gradually increasing
in efficiency. Of these pre-batrachia we have existing representatives
in the mud-haunting Dipnoi, with their feeble limbs. In the great
majority of the Ganoid fishes the bladder served but a minor purpose
as a breathing organ, the gills doing the bulk of the work. In the
Teleostean descendants of the Ganoids the respiratory function of the
bladder in great measure or wholly ceased, in the majority of cases
the duct closing up or disappearing, leaving the pouch as a closed
internal sac, far removed from its place of origin. In this condition
it served as an aid in swimming, perhaps as a survival of one of its
ancient uses. It gained also in certain cases some connection with the
organ of hearing. But these were makeshift and unimportant functions,
as we may gather from the fact that many fishes found no need for them,
the bladder, in these cases, decreasing in size until too small to be
of use in swimming, and in other cases completely disappearing after
having travelled far from its point of origin. In some other cases,
above cited, the process seems to have begun again, in modern times,
in an eversion of the wall of the oesophagus for respiratory purposes.
The whole process, if I have correctly conceived it, certainly forms a
remarkable organic cycle of development and degeneration, which perhaps
has no counterpart of similarly striking character in the whole range
of organic life."
=The Heart of the Fish.=--The heart of the fish is simple in structure,
small in size, and usually placed far forward, just behind the
branchial cavity, and separated from the abdominal cavity by a sort of
"diaphragm" formed of thickened peritoneum. In certain eels the heart
is remote from the head.
The heart consists of four parts, the sinus venosus, into which the
veins enter, the auricle or atrium, the ventricle, and the arterial
bulb at the base of the great artery which carries the blood to the
gills. Of these parts the ventricle is deepest in color and with
thickest walls. The arterial bulb varies greatly in structure, being in
the sharks, rays, Ganoids, and Dipnoans muscular and provided with a
large number of internal valves, and contracting rhythmically like the
ventricle. In the higher fishes these structures are lost, the walls
of the arterial bulb are not contractile, and the interior is without
valves, except the pair that separate it from the ventricle.
In the lancelet there is no proper heart, the function of the heart
being taken by a contractile blood-vessel situated on the ventral
side of the alimentary canal. In the Dipnoans, which are allied to
the ancestors of the higher vertebrates, there is the beginning of a
division of the ventricle, and sometimes of the auricle, into parts by
a median septum. In the higher vertebrates this septum becomes more
and more specialized, separating auricle and ventricle into right and
left cavities. The blood in the fish is not returned to the heart after
purification, but is sent directly over the body.
=The Flow of Blood.=--The blood in fishes is thin and pale red
(colorless in the lancelet) and with elliptical blood-corpuscles. It
enters the _sinus venosus_ from the head through the jugular vein,
from the kidney and body walls through the cardinal vein, and from
the liver through the hepatic veins. Hence it passes to the auricle
and ventricle, and from the ventricle through the arterial bulb, or
conus arteriosus to the ventral aorta. Thence it flows to the gills,
where it is purified. After passing through the capillaries of the
gill-filaments it is collected in paired arteries from each pair of
gills. These vessels unite to form the dorsal aorta, which extends the
length of the body just below the back-bone. From the dorsal aorta
the subclavian arteries branch off toward the pectoral fins. From a
point farther back arise the mesenteric arteries carrying blood to the
stomach, intestine, liver, and spleen. In the tail the caudal vein
carries blood to the kidneys. These secrete impurities arising from
waste of tissues, after which the blood again passes to the heart
through the _cardinal vein_. From the intestine the blood, charged with
nutritive materials in solution, is carried by the _portal vein_ to
the liver. Here it again passes by the _hepatic sinus_ to the _sinus
venosus_ and the heart.
The details of the circulatory system vary a good deal in the different
groups, and a comparative study of the direction of veins and arteries
is instructive and interesting.
The movement of the blood in fishes is relatively slow, and its
temperature is raised but little above that of the surrounding water.
FOOTNOTES:
[5] The Origin of Lungs: A Chapter in Evolution. American Naturalist,
December, 1892.