Lungfish

“ They are aquatic, free-moving, cold-blooded (their blood maintains the temperature of the surrounding water) , and breathe primarily by means of gills. However, some forms (the lungfishes) breathe by means of a lung developed from the air-bladder.
The most recent fish classification recognizes four major classes, the Agnatha (primitive jawless fishes) , the Placodermi (armored fishes with primitive jaws) , the Chondrichthyes (sharks and related forms with cartilaginous internal skeletons) , and the Osteichthyes (true bony fishes) . ”

“ From this stage onward the life of the lung-fish was a rather uneventful one. It received its daily diet of earthworms with apparent relish, and upon them it thrived and grew. Its yearly increase in size varied between two and three inches; at the time of its death it measured eighteen inches. Its movements in the aquarium were like those of larval salamanders, axolotl, for example. Only on rare occasions did it swim in a fish-like manner by means of caudal fin and undulating body, and only twice a year did it show of what sudden movements and great activity it was capable. ”

“ Nearly one-fourth of the total belonged to a tribe of enamel-scaled fishes, now represented only by a few sturgeon and gar-pike.
Lung fishes have never been a large group but it is noteworthy that they have had existence since Middle Devonian time. Living members of the race, inhabitants of Africa and South America, make a practice of burrowing into the mud of stream channels during dry seasons and are provided with lungs which enable them to breathe air in the manner of higher vertebrates. They survive the complete drying-up of the streams and live for months without water. ”

“ In its origin the lung or air-bladder arises as a diverticulum from the alimentary canal, used by the earliest fishes as a breathing-sac, the respiratory functions lost in the progress of further divergence. Nothing of the nature of lung or air-bladder is found in lancelet, lamprey, or shark. In none of the remaining groups of fishes is it wholly wanting at all stages of development, although often lost in the adult. Among fishes it is most completely functional in the Dipneusti, and it passes through all stages of degeneration and atrophy in the more specialized bony fishes. ”

“ As the water passes over it the oxygen it contains is absorbed through the skin of the gill-fringe into the blood, which thus becomes purified. In the same manner the impurities of the blood pass out into the water, and go out through the gill-openings behind. The fish needs to breathe just as we do, though the apparatus of breathing is not the same. Just as the air becomes loaded with impurities when many people breathe it, so does the water in our jar or aquarium become foul if it is breathed over and over again by fishes. ”

“ Fig. 74.—Teeth of Shark.
In the Ganoids and bony fishes there is a gill cover, and in all but a few Ganoids there is some formation of true bone, whereas [98] in the Elasmobranchs the skeleton is wholly cartilaginous. One of the most striking anatomical features of these groups, and one which distinguishes them from the Elasmobranchs, is the presence of a swim bladder, a large sac-like outgrowth from the upper part of the gut. The function of the swim bladder is that of regulating the specific gravity of the fish, which becomes greater or less according as air is expelled or taken in. ”

“ 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 œsophagus, 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. ”

“ These lung fishes, or double-breathers, have the characteristics both of the fish and the frog. To scales and gills and fins and other features of the fish, they add lungs, nostrils, the beginning of a three-chambered heart, and other features of the frog. Living in regions from which the water periodically disappears, these creatures build around themselves in the dry season a shell of mud and leaves, and there, while awaiting the return of the water, they breathe air, and live on the fat stored up in their tails. ”

“ These are various in structure, sometimes folded like gills, or in other cases cellular, like a rudimental lung in structure.
Among mollusks only the true snails are air-breathing, and some of these inhabit water. They have organs resembling those of the spider, concentrated into a single sac. This is lined with blood-capillaries and constitutes a true external lung. The orifice opens on the right side of the neck, and can be closed by a circular muscle.
In the pond-snails we have a clear illustration of the identity in principle of air and water respiration. ”

“ The Queensland lung-fish lives under somewhat different conditions, for its native rivers do not entirely dry up, but are reduced to a series of deep holes connected by mere trickles of water. These holes become so foul from decaying vegetation and dead fish that the possession of a lung is a vital matter, and if the ceratodus were not able to come to the surface and breathe air it would probably succumb. ”

“ There are three living species of lung fishes, one of which is found in Australia, another in Tropical Africa, and a third in the tributaries of the Amazon. All live under conditions which make ordinary respiration by gills difficult. The Australian species inhabits rivers which become reduced, in the dry season, to stagnant pools of foul water, in which ordinary fish frequently fail to survive. ”

“ Even in streams where a successful deposition of the ova has been accomplished, there never appears, as far as my observations have gone, any disposition in the parent-fish to return to the sea. Their instinct still prompts them to keep swimming up-stream, until you often find them with their noses worn quite off, their heads bruised and battered, fins and tail ragged and torn, bodies emaciated, thin, and flabby; the bright silvery tints dull and leaden in hue, a livid red streak extending along each side from head to tail, in which large ulcerous sores have eaten into the very vitals. ”

“ How difficult a matter it is to decide whether the lung-fish of Brazil and Senegambia belong to the amphibia or to the fishes, which in other instances are known to always breathe by their gills! In the rainless season of the year the swamps, the homes of these animals, dry out; they then coat themselves with a crust of earth, and are dependent on their lungs for every breath they draw. ”

“ The anterior dorsal fin composed of four stiff spines. Vertebræ twenty-four.
The “Grey Mullets” inhabit in numerous species and in great numbers the coasts of the temperate and tropical [502] zones. They frequent brackish waters, in which they find an abundance of food which consists chiefly of the organic substances mixed with mud or sand; in order to prevent larger bodies from passing into the stomach, or substances from passing through the gill-openings, these fishes have the organs of the pharynx modified into a filtering apparatus. ”

“ In most cases there is no absolute limit to growth, the species growing as long as it lives. But while some species endure many years, others are certainly very short [Pg 31] lived, and some may be even annual, dying after spawning, perhaps at the end of the first season.
Teeth are wholly absent in several groups of fishes. They are, however, usually present on the premaxillary, dentary, and pharyngeal bones. In the higher forms, the vomer, palatines, and gill-rakers are rarely without teeth, and in many cases the pterygoids, sphenoids, and the bones of the tongue are similarly armed. ”

“ No fish has hairs or feathers; most are covered with scales, but some have a rough or smooth skin. The tongue is hard, often toothed; and sometimes so much adherent that it seems to be wanting. The eyes have no lids; nor are any ears or nostrils visible, for what takes the place of nostrils is a blind cavity. Nevertheless they have the senses of tasting, smelling, and hearing. All have blood. All scaly fishes are oviparous, but the cartilaginous fishes (with the exception of the Sea-devil, which Aristotle places along with them) are viviparous. All have a heart, liver, and gall-bladder ”

“ Parental interest is, perhaps, better evinced in the case of the “paidle-fish” standing guard over his nest with unwearied vigilance, exhibiting all the care of a brooding hen for his future offspring. Again, in the case of the dog-fish—the plague of fishermen—how different. Here the young are brought forth, after the fashion of mammals, fully developed. A common sight in fishing boats where these pests are brought on board, is to see the finger-long juveniles swimming briskly about in the bilge-water with the yolk-sac still attached. ”

“ Those Fishes which have no Lungs, have Gills, which do the Office of Lungs, receiving and expelling alternately the Water, whereby the Blood-Vessels suffer the same alteration of Dimensions, that they do in the Lungs of more perfect Animals.
The Lungs or Air-Vessels of Insects, are yet exceedingly more different in Structure, Distribution, and Situation from those of perfect Animals, than those of Fishes are, and yet in their Use and Action agree perfectly with both; that is, receiving and expelling the Air, and varying the Dimensions and Capacities of the Blood-Vessels. ”

“ They then breathe by gills, like fishes, taking oxygen from the air which is suspended in water, before lungs are acquired which afterwards enable the animals to take oxygen directly from the air. The amphibian sometimes sheds the gills, and leaves the water to live on land. Sometimes gills and lungs are retained through life in the same individual. This amphibian condition of lung and gill being present at the same time is paralleled by a few fishes which still exist, like the Australian Ceratodus, the lung-fish, an ancient type of fish which belongs to early days in geological time. ”

“ The development of fore and hind limbs from fins, of lung from swim-bladder, and of instincts appropriate to the new medium, would strike all conservative fishes as highly immoral deviations from that biological tradition which had given stability to the glorious race of fishes. Nothing could seem more a-biological. ”

“ No doubt the oxygen they thus swallow enters the body-cavity, and slightly assists them in aërating the blood, though much of it, also, may pass in the ordinary way through the gills, which are the regular and normal respiratory organs. It is always interesting to me, however, to watch fish when they come up thus to drink air at the surface, as goldfish often do when thoughtlessly confined in too small a glass basin; for in this instinctive act, as modern biologists now generally allow, we have the first faint beginnings of the evolution of lungs and the habit of air-breathing. ”

“ The mouth and throat of a fish cannot be very sensitive when it is considered that it swallows, whole, such prey as [115] sunfish and catfish fry, bristling with sharp fins and spines, and those of the catfish are always erect, even if swallowed head first. Marine fishes also swallow crabs, lobsters, prawns, besides mollusks, sea-urchins, and other creatures that would be exceedingly irritating and painful to a sensitive throat. ”

“ Whenever we find in a fish long delicate filaments, developed in connection with the fins or the extremity of the tail, we may conclude that it is an inhabitant of still water and of quiet habits. Many deep-sea fishes (Trachypteridæ, Macruridæ, Ophidiidæ, Bathypterois) are provided with such filamentous prolongations, the development of which is perfectly in accordance with their sojourn in the absolutely quiet waters of abyssal depths. ”

“ 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. ”

“ Dorsal fin very long, the anterior portion composed of numerous short isolated spines; anal fin with spines anteriorly. Ventrals none. The humeral arch is not suspended from the skull. Gill-openings reduced to a slit at the lower part of the side of the head.
Freshwater-fishes characteristic of and almost confined to the Indian region. The structure of the mouth and of the branchial apparatus, the separation of the humeral arch from the skull, the absence of ventral fins, the anatomy of the abdominal organs, affords ample proof that these fishes are Acanthopterygian eels. ”

“ 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. ”