ORGANS OF RESPIRATION.
Fishes breathe the air dissolved in water by means of gills or
branchiæ. The oxygen consumed by them is not that which forms the
chemical constituent of the water, but that contained in the air which
is dissolved in water. Hence fishes transferred into water from which
the air has been driven out by a high temperature, or in which the
air absorbed by them is not replaced, are speedily suffocated. The
absorption of oxygen by fishes is comparatively small, and it has been
calculated that a man consumes 50,000 times more than is required by
a Tench. However, some fishes evidently require a much larger supply
of oxygen than others: Eels and Carps, and other fishes of similar
low vitality, can survive the removal out of their elements for days,
the small quantity of moisture retained in their gill-cavity being
sufficient to sustain life, whilst other fishes, especially such as
have very wide gill-openings, are immediately suffocated after being
taken out of the water. In some fishes noted for their muscular
activity, like the _Scombridæ_, the respiratory process is so
energetic as to raise the temperature of their blood far beyond that
of the medium in which they live. A few fishes, especially such as
are periodically compelled to live in water thickened into mud by
desiccation and vitiated by decomposing substances, breathe atmospheric
air, and have generally special contrivances for this purpose. These
are so much habituated to breathing air that many of them, even when
brought into pure water of normal condition, are obliged to rise
to the surface at frequent intervals to take in a quantity of air,
and if they be kept beneath the surface by means of a gauze net,
they perish from suffocation. The special contrivances consist of
additional respiratory organs, lodged in cavities either adjoining the
gill-cavity or communicating with the ventral side of the œsophagus,
or of the air-bladder which enters upon respiratory functions (Dipnoi,
Lepidosteus, Amia).
The water used by fishes for respiration is received by the mouth, and
by an action similar to that of swallowing driven to the gills, and
expelled by the gill-openings, of which there may be one or several on
each side behind the head; rarely one only in the median line of the
ventral surface.
[Illustration: Fig. 58.--Fore-part of the body of an embryon of
Carcharias, showing the branchial filaments (natural size).]
The _gills_ or _branchiæ_ consist essentially of folds of the mucous
membrane of the gill-cavity (_laminæ branchiales_), in which the
capillary vessels are distributed. In all fishes the gills are lodged
in a cavity, but during the embryonic stage the Chondropterygians have
the gill-laminæ prolonged into long filaments projecting beyond the
gill-cavity (Fig. 58), and in a few young Ganoids external gills are
superadded to the internal.
In _Branchiostoma_ the dilated pharynx is perforated by numerous
clefts, supported by cartilaginous rods (Fig. 29, _h_). The water
passes between these clefts into the peritoneal cavity, and makes its
exit by the porus abdominalis situated considerably in advance of the
vent. The water is propelled by cilia.
In the _Cyclostomes_ the gills of each side are lodged in a series of
six or more antero-posteriorly compressed sacs, separated from each
other by intervening septa. Each sac communicates by an inner duct
with the œsophagus, the water being expelled by an outer duct. In
_Bdellostoma_ each outer duct has a separate opening, but in _Myxine_
all the outer ducts pass outwards by one common gill-opening on each
side. In the Lampreys the ducts are short, the outer ones having
separate openings (Fig. 2, p. 39). The inner ducts lead into a single
diverticulum or bronchus, blind behind, situated below the œsophagus,
and communicating in front with the pharynx, where it is provided with
two valves by which the regurgitation of the water into the buccal
cavity is prevented.
The same type of branchial organs persists in _Chondropterygians_,
which possess five, rarely six or seven, flattened pouches with
transversely plaited walls. The septa between them are supported by
cartilaginous filaments rising from the hyoidean and branchial arches.
Each pouch opens by a cleft outwards, and by an aperture into the
pharynx, without intervening ducts. The anterior wall of the first
pouch is supported by the hyoidean arch. Between the posterior wall
of the first and the anterior of the second sac, and between the
adjacent walls of the succeeding, a branchial arch with its two series
of radiating cartilaginous filaments is interposed. Consequently the
first and last pouch have one set of gill-laminæ only, viz. the first
on its posterior and the last on its anterior wall. The so-called
_spiracles_ on the upper surface of the head of Chondropterygians are
to be referred to in connection with the respiratory organs. They are
the external openings of a canal leading on each side into the pharynx,
and situated generally close to and behind the orbit. They frequently
possess valves or an irregularly indented margin, and are found in all
species during the embryonic stage, but remaining persistent in a part
only. The spiracles are the remains of the first visceral cleft of
the embryo, and in the fœtal state long branchial filaments have been
observed to protrude, as from the other branchial clefts.
The _Holocephali_ and _Ganoidei_ show numerous deviations from the
Chondropterygian type, all leading in the direction towards the
Teleosteans. As a whole they take an intermediate position between the
preceding types and the Teleosteans, but they show a great variation
among themselves, and have in common only the imperfect separation
of the branchial sacs and the presence of a single outer branchial
aperture.
In _Chimæra_ the septum separating the branchial sacs is confluent
with the wall of the gill-cavity in a part of its extent only, and
still more imperfect is the separation of those branchial divisions
in _Ceratodus_ (Fig. 60). The other Ganoids show no such division
whatever. In _Chimæra_ the first gill is incomplete (uniserial), and
belongs to the hyoid; then follow three complete gills; the last,
belonging to the fourth branchial arch, being again incomplete.
_Acipenser_, _Scaphirhynchus_, _Lepidosiren_, _Protopterus_,
and _Lepidosteus_, possess likewise an anterior incomplete gill
(_opercular gill_), followed by four complete gills in the Sturgeons
and _Lepidosteus_, whilst in _Lepidosiren_ and _Protopterus_ a part
of the branchial arches is gill-less. In _Polyodon_, _Ceratodus_, and
_Polypterus_, an opercular gill is absent, the two former having four
complete gills, the latter three and a half only. _Spiracles_ are still
in some Ganoids present, viz. in the Sturgeons and _Polypterus_. In all
the Ganoids an osseous gill-cover is now developed.
In the _Teleostei_ the gills with their supporting branchial arches
lie in one undivided cavity; more or less wide clefts between the
arches lead from the pharynx to the gills, and a more or less wide
opening gives exit to the water after it has washed the gills. The
interbranchial clefts have sometimes nearly the same extent as the
branchial arches; sometimes they are reduced to small openings, the
integuments stretching from one arch to the other. Sometimes there
is no cleft behind the fourth arch, in which case this arch has only
an uniserial gill developed. The gill-opening likewise varies much
in its extent, and when reduced to a foramen may be situated at any
place of the posterior boundary of the head. In the _Symbranchidæ_
the gill-openings coalesce into a single narrow slit in the median
line of the isthmus. In the majority of Teleosteans the integument of
the concave side of the branchial arches develops a series of horny
protuberances of various form, the so-called _gill-rakers_. They are
destined to catch any solid corpuscles or substances which would be
carried into the gill-cavity with the water. In some fishes they are
setiform, and form a complete sieve, whilst in others they are merely
rough tubercles, the action of which must be very incomplete if they
have any function at all.
Most Teleosteans possess four complete gills, but frequently the
fourth arch is provided with an uniserial gill only, as mentioned
above, or even entirely gill-less. The most imperfect gills are found
in _Malthe_, which has two and a half gills only, and in _Amphipnous
cuchia_, in which one small gill is fixed to the second arch.
The gills of the Teleosteans as well as of the Ganoids are supported by
a series of solid cartilaginous or horny pointed rods, arranged along
the convex edges of the branchial arches. Arches bearing a complete
gill have two series of those rods, one along each edge; those with
uniserial gills bear one row of rods only. The rods are not part
of the arch, but fixed in its integument, the several rods of one
row corresponding to those of the other, forming pairs (_feuillet_,
Cuvier) (Fig. 59). Each rod is covered by a loose mucous membrane
passing from one rod to its fellow opposite, which again is finely
transversely plaited, the general surface being greatly increased by
these plaits. In most Teleostei the branchial lamellæ are compressed,
and taper towards their free end, but in the Lophobranchs their base
is attenuated and the end enlarged. The mucous membrane contains the
finest terminations of the vessels, which, being very superficial,
impart the blood-red colour to living gills. The _Arteria branchialis_,
the course of which lies in the open canal in the convexity of the
branchial arch, emits a branch (_a_) for every pair of lamellæ which
ascends (_b_) along the inner edge of the lamella, and supplies every
one of the transverse plaits with a branchlet. The latter break up into
a fine net of capillaries, from which the oxygenised blood is collected
into venous branchlets, returning by the venous branch (_d_), which
occupies the outer edge of the lamella.
[Illustration:
Fig. 59.--A pair of branchial lamellæ (magnified) of the Perch.
_a_, Branch of Arteria branchialis; _b_, Ascending branch of
the same; _c_, Branch of Vena branchialis; _d_, Descending
branch of the same; _e_, Transverse section through the
branchial arch.]
The so-called _Pseudobranchiæ_ (Fig. 60) are the remains of an anterior
gill which had respiratory functions during the embryonic life of the
individuals. By a change in the circulatory system these organs have
lost those functions, and appear in the adult fish as retia mirabilia,
as they receive oxygenised blood, which, after having passed through
their capillary system, is carried to other parts of the head. In
Palæichthyes the pseudobranchia is a rete mirabile caroticum for the
brain and eye; in Teleosteans a rete mirabile ophthalmicum only.
Pseudobranchiæ are as frequently absent as present in Chondropterygians
as well as Teleosteans. As to the Ganoids, they occur in _Ceratodus_,
_Acipenser_, _Polyodon_, and _Lepidosteus_, and are absent in
_Lepidosiren_, _Protopterus_, _Scaphirhynchus_, _Polypterus_, and
_Amia_.
In Chondropterygians and Sturgeons the pseudobranchiæ are situated
within the spiracles; in those, in which spiracles have become
obliterated, the pseudobranchiæ lie on the suspensorium, hidden below
cellular tissue; but pseudobranchiæ are not necessarily co-existent
with spiracles. In the other Ganoids and Teleosteans the pseudobranchiæ
(Fig. 60, _h_) are within the gill-cavity, near the base of the
gill-cover; in _Ceratodus_ even rudiments of the gill-rakers (_x’_,
_x”_) belonging to this embryonic gill are preserved, part of them
(_x”_) being attached to the hyoid arch. Pseudobranchiæ are frequently
hidden below the integuments of the gill-cavity, and have the
appearance of a glandular body rather than of a gill.
[See Müller, “Vergleichende Anatomie des Gefäss-systems der
Myxinoiden;” and “Ueber den Bau und die Grenzen der Ganoiden.”]
[Illustration: Fig. 60.--Gills of Ceratodus.
_x_, Arcus aortæ; _gl_, Glossohyal; _ch_, Ceratohyal; _u_,
Attachment of the first gill to the walls of the gill-cavity;
_h_, Pseudobranchia; _x’_, _x”_, two series of gill-rakers
belonging to the Pseudobranchia.]
_Accessory respiratory organs_ for retaining water or breathing air,
such as are found in the _Labyrinthici_, _Ophiocephalidæ_, certain
_Siluridæ_, and _Lutodira_, are structures so specialised that they are
better described in the accounts of the Fishes in which they have been
observed.
* * * * *
_Air-Bladder._--The air-bladder, one of the most characteristic organs
of fishes, is a hollow sac, formed of several tunics, containing gas,
situated in the abdominal cavity, but without the peritoneal sac,
entirely closed or communicating by a duct with the intestinal tract.
Being compressible, its special functions consist in altering the
specific gravity of the fish or in changing the centre of gravity. In a
few fishes it assumes the function of the organ of higher Vertebrates,
of which it is the homologue--viz. of a _lung_.
The gas contained in the air-bladder is secreted from its inner
surface. In most freshwater fishes it consists of nitrogen, with a very
small quantity of oxygen and a trace of carbonic acid; in sea-fishes,
especially those living at some depth, oxygen predominates, as much
as 87 per cent having been found. Davy found in the air-bladder of a
fresh-run Salmon a trace of carbonic acid and 10 per cent of oxygen,
the remainder of the gas being nitrogen.
An air-bladder is absent in _Leptocardii_, _Cyclostomi_,
_Chondropterygii_, and _Holocephali_; but occurs in all Ganoids, in
which, besides, its respiratory functions more or less clearly manifest
themselves. Its occurrence in Teleosteans is most irregular, closely
allied species sometimes differing from each other in this respect; it
shows in this sub-class the most extraordinary modifications, but has
no respiratory function whatever.
Constantly situated within the abdominal cavity, below the vertebral
column, but without the sac of the peritoneum which covers only its
ventral portion, the air-bladder is frequently prolonged into the
tail, the prolongation being either simple and lodged between the
non-united parapophyses, or double and penetrating between the muscles
and hæmapophyses of each side. In the opposite direction processes of
the air-bladder may penetrate into the skull, as has been mentioned
above (p. 117). In some fishes the air-bladder is almost loose in the
abdominal cavity, whilst in others it adheres most intimately by firm
and short tissue to the vertebral column, the walls of the abdomen, and
the intestines. In the Cobitina and many Siluroids it is more or less
completely enclosed in osseous capsules formed by the vertebræ.
The tunics of the majority of air-bladders are an extremely fine
internal one, frequently shining silvery, containing crystalline
corpuscles, sometimes covered with a pavement-epithelium; and a thicker
outer one of a fibrous texture, which sometimes attains to considerable
thickness and yields isinglass. This wall is strengthened in many
fishes by muscular layers for the compression of the whole organ or of
some portion of it.
A distinction has been made between air-bladders which communicate by
a duct with the intestinal tract and those which are entirely closed.
However, it is to be remembered that at an early stage of development
all air-bladders are provided with such a duct, which in a part of the
fishes more or less completely obliterates, being then represented by
a fine ligament only. In young _Lucioperca_ of six to eight inches in
length the duct may be found still open for a considerable distance;
and, on the other hand, in adult _Physostomi_, that is Teleosteous
fishes with a ductus pneumaticus, not rarely the whole duct is found
very narrow, or, for some part of its length, even entirely closed.
[Illustration: Fig. 61.--Air-bladder of Otolithus sp.]
[Illustration:
Fig. 62.--Vertical section through abdominal cavity of
Collichthys lucida. _b_, air-bladder; _l_, liver; _s_,
stomach; _epp_ and _ipp_, external and internal laminæ of
peritoneum parietale; _epv_ and _ipv_, external and internal
laminæ of peritoneum viscerale; _dv_, dorsal air-vessels;
_vv_, ventral air-vessels.]
Air-bladders without duct are found in Acanthopterygians,
Pharyngognaths, Anacanths, and Lophobranchs. They may consist of
a single cavity or divided by constrictions into two or three
partitions situated behind one another; they may consist of two lateral
partitions, assuming a horseshoe-like form, or they may be a single
sac with a pair of simple or bifid processes in front or behind (Fig.
61). The families of _Sciænidæ_ and _Polynemidæ_ possess air-bladders
with a most extraordinary development of appendages rising from each
side of the air-bladder. In the Sciænoid (Fig. 63) fifty-two branches
issue from each side, each branch being bifurcate and bearing smaller
appendages. In _Pogonias chromis_ (Fig. 64) the sides of the anterior
half is provided with irregular broad-fringed appendages, the hindmost
of which communicates by a narrow duct with the posterior extremity
of the air-bladder. In _Collichthys lucida_ (Fig. 62) twenty-five
appendages issue from each side; the anterior ones are directed towards
the front, but the lateral assume a more posterior direction, the
nearer they are to the posterior extremity of the air-bladder, where
they form an assemblage giving the appearance of a cauda equina. All
these appendages soon bifurcate in a dorsal and ventral stem; these
stems bifurcate again and again, and either terminate after the first
or second bifurcation or are so far prolonged as to reach the median
line of the ventral and dorsal sides, anastomosing with the branches
of the other side. The branches being enveloped in laminæ of the
peritonæum, form a dorsal and ventral sac of beautiful appearance,
caused by the regular arrangement of the air-vessels. The dorsal sac is
situated between the air-bladder and the roof of the abdominal cavity
without being attached to the latter. The ventral sac receives within
its cavity the intestine, liver, and ovaries.--A peculiar mechanism
has been observed in the air-bladder of the _Ophidiidæ_, the anterior
portion of which can be prolonged by the contraction of two muscles
attached to its anterior extremity, with or without the addition of a
small bone.
[Illustration: Fig. 63.--Air-bladder of a Sciænoid.
I. Visceral surface opened at _b_, to show openings of the
lateral branches.
II. Isolated lateral branch; _a_, its opening into the
cavity of the air-bladder.]
Air-bladders with a pneumatic duct are found in Ganoids and
Physostomes, the duct entering the dorsal side of the intestinal
tract, with the exception of _Polyp__terus_ and the _Dipnoi_, in
which it enters on the ventral side of the œsophagus. In the majority
the orifice is in the œsophagus, but in some, as in _Acipenser_, in
the cardiac portion of the stomach, or in its blind sac, as in many
Clupeoids. The air-bladder may be single, or consist of two divisions
situated one behind the other (Fig. 52); its inner surface may be
perfectly smooth, or form manifold pouches and cells. If two divisions
are present the anterior possesses a middle elastic membrane which
is absent in the posterior; each division has a muscular layer, by
which it can be separately compressed, so that part of the contents of
the posterior may be driven into the elastic anterior division, and
_vice versa_. The posterior division being provided with the ductus
pneumaticus does not require the elasticity of the anterior.
[Illustration: Fig. 64.--Air-bladder of Pogonias chromis.]
Some Siluroids possess a peculiar apparatus for voluntarily exercising
a pressure upon the air-bladder. From the first vertebra a process
takes its origin on each side, expanding at its end into a large round
plate; this is applied to the side of the air-bladder, and by pressing
upon it expels the air through the duct; the small muscle moving the
plate rises from the skull.
The connection of the air-bladder with the organ of hearing in some
Physostomes has been described above, p. 117.
In the modifications of the air-bladder, hitherto mentioned, the
chief and most general function is a mechanical one; this organ
serves to regulate the specific gravity of the fish, to aid it in
maintaining a particular level in the water, in rising or sinking,
in raising or depressing the front part of its body as occasion may
serve. Yet a secretion of gas from the blood into its cavity must take
place; and if this be so, it is not at all impossible that also an
exchange of gases between the two kinds of blood is effected by means
of the extraordinary development of _retia mirabilia_ in many
air-bladders.
In all fishes the arteries of the air-bladder take their origin from
the aorta or the system of the aorta, and its veins return either to
the portal, or vertebral, or hepatic veins; like the other organs of
the abdominal cavity it receives arterial blood and returns venous
blood. However, in many fishes the arteries as well as veins break
up below the inner membrane into _retia mirabilia_ in various ways.
The terminal ramifications of the arteries may dissolve into fan-like
tufts of capillaries over almost every part of the inner surface,
as in Cyprinoids. Or these tufts of radiating capillaries are more
localised at various places, as in _Esocidæ_; or the tufts are so
aggregated as to form gland-like, _red bodies_, the capillaries
reuniting into larger vessels, which again ramify freely round the
border of the red body; the red bodies are formed not only by minute
arteries but also by minute veins, both freely anastomosing with
its kind, and being inextricably interwoven. The rest of the inner
surface of the air-bladder receives its blood, not from the red bodies,
but from normally ramifying vessels. This kind of rete mirabile or
“vaso-ganglion” is found in the Perch and Gadoids; it is generally
distributed in closed air-bladders, but also sometimes observed in
air-bladders’ with pneumatic duct. In _Anguilla_ and _Conger_ two
similar vaso-ganglia are situated at the sides of the opening of the
pneumatic duct.
[Illustration: Fig. 65.--Lung of Ceratodus opened in its lower
half to show its cellular pouches. _a_, Right half; _b_, Left
half; _c_, Cellular pouches; _e_, Vena pulmonalis; _f_, Arterial
blood-vessel; _oe_, Œsophagus opened, to show glottis (_gl._)]
Whilst the air-bladders of some _Ganoids_, anatomically as well as
functionally, closely adhere to the Teleosteous type, that of _Amia_
is more cellular and lung-like in its interior than the Teleosteous
air-bladder, and _Polypterus_ approaches the Dipnoi not only in
having a laterally divided air-bladder but also in its pneumatic duct
entering the _ventral_ side of the œsophagus. The air-bladder of the
_Dipnoi_ possesses still more the anatomical characteristics of a lung
and assumes its functions, though, as it co-exists with gills, only
periodically or in an auxiliary manner. The ductus pneumaticus is a
membranous bronchus, entering the ventral side of the œsophagus, and
provided at its entrance with a _glottis_. In _Ceratodus_ (Fig. 65) the
lung is still a single cavity, but with a symmetrical arrangement of
its internal pouches; it has no pulmonal artery, but receives branches
from the _arteria cœliaca_. Finally, in _Lepidosiren_ and _Protopterus_
the lung is completely divided into lateral halves, and by its cellular
structure approaches most nearly that of a reptile; it is supplied with
venous blood by a true pulmonary artery.
[Illustration: Fig. 66.--Heart of Lepidosteus osseus.
I. External aspect. II. Conus arteriosus opened.
_a_, Atrium; _b_, Conus arteriosus; _v_, Ventricle; _h_,
Branchial artery for 3d and 4th gill; _k_, for the second;
_l_, for the first; _m_, branch for the opercular gill; _d_,
Single valve at the base of the conus; _e-g_, Transverse rows
of Ganoid valves.]