CHAPTER IX.. Organs of Respiration.

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.

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.

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.

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.

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.

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 Polypterus 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.

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.

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.

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.]