t 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 mor

bove that of the water in which they live, but in the mackerel and oth

e fishes (Dipnoans, Crossopterygians, Ganoids) the air-bladder retains its original function of a lung. In other cases some peculiar structure exists in connection with t

and allowed to pass out through the gill-openings, thus bathing the gills. In

s, 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

asket of Lampre

aborate branchial basket formed of cartilage. (b) The plate-gills, found among the sharks, rays, and chim?ras, thence called Elasmobranchs, or plate-gills. In these the gill-structures are flat lamin?, attached by one side to the gill-arches. (c) The fringe-gills found in ordinary fishes, in which the gill-filaments containing the capillaries ar

higher fishes its functions in this regard are rudimentary, and in some cases it has taken collateral functions (as

and air-bladder of Carp. (

catfish, which form the majority of fresh-water fishes, its anterior end is connected through a chain of modified vertebr? to the ear. Sometimes its posterior end fits into an enlarged and hollow interh?mal 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 constri

eans 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 sa

in the closely related colias or chub mackerel (Scomber japonicus) the organ is very evident. In other families, as the rockfishes (Scorp?nid?), genera with and those without the air-bladder are scarcely distinguishable externally. In general, fishes which lie on the bottom, thos

nic-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

still far from understood. The following summary of the various views in this rega

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

means of the air-bladder. Each of these fishes contains air, by rubbing and moving of which the noise is produced.' The blad

nowledge, 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 bl

cent of O2, and the remainder nitrogen. The most remarkable fact discovered about this mixture was that it frequently consisted almost entirely of oxygen,

h 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,

ng 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 S?rensen to be untenable. Müller also stated that in some fish, at least, the

r, acquainting the animal with the degree of pressure that is exerted by the gases in the air-bladder against its walls. This pressure necessarily varie

tion sound was produced, the sound being intensified by the air-bladder, which acted as a resonator. He also b

ternal 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 e

essure. S?rensen 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. S?rensen (1895) held that there is but little evid

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, S?rensen states, is borne out by the anatomical structure found in M

lancelets, lampreys, sharks, rays, and chim?ras 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

ction of the air-bladder and of its development and decli

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 n

Dipnoan, Crossopterygian, and Ganoid the air-bladder has an effective pneumatic duct. This in the Ganoids opens into the dorsal side of the ?sophagus, 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 ?sophagus, 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

essity 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

y 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

e 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

reathe the air. In Polypterus the air-bladder resembles the Dipnoan lung in having lateral divisions and a ventral connection with the ?sophagus, 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

Teleosteans, the air-containing bladders of the others and the Ganoids, and the lung of the Dipnoans, and the i

ent 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 w

ering 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

athed 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 lamin?, or plates, which undoubtedly perform an oxygenating function. The most interesting member of this family is Anaba

ong reason to believe that air-breathing in fishes was originally performed, as it probably often is now, by the unchanged walls of the ?sophagus. Then these walls expanded inwardly, forming a simple cavity, partly closed by a fold of membrane, like that of the Ophiocephalid?. 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 compressi

ch. 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

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 in

ch the lung development had not far progressed. It may indeed have been a leading influence in the development of the Teleostea

till 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,

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,

e 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

ced far forward, just behind the branchial cavity, and separated from the abdominal cavity by a sor

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 contr

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 beco

. 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 blo

the different groups, and a comparative study of the direc

ively slow, and its temperature is raised bu

TNO

hapter in Evolution. America