Popular Science Monthly/Volume 9/September 1876/Amphibious Fishes
By E. SAUVAGE.
IN the swamps of the Gambia, after they have been dried by the tropical sun, there are to be found here and there beneath the surface clods of earth uniform in shape, and usually about the size of a man's two fists. These clods inclose living animals, which have been led by instinct to hide themselves away toward the close of the rainy season, and before the coming of the season of drought, by burying themselves in the mud while it was yet soft, and before it had been hardened by the scorching rays of the sun.
On breaking one of these lumps of mud, it is found to be a sort of pouch or cocoon, with thin walls, and with projections here and there corresponding to the form of the animal concealed within. Its larger end is rounded, but its narrower end is closed by a slightly convex lid with a narrow opening in the centre. If the surface of the cocoon be even gently touched, a pretty loud cry is heard which Natterer has compared to the mewing of a cat.
For a long time it was supposed that the animal buried itself amid the leaves which surround its protecting sheath. In a special memoir published in the Bulletins of the St. Petersburg Academy of Sciences, Leuckart expressed the opinion that the epidermis, by becoming detached, supplied the materials for this envelope. But since his time it has been demonstrated that the cocoon is formed from a dense secretion of mucus; such is the result of observations made by Paulson and Richard Owen, and repeated by Auguste Duméril, Professor of Ichthyology at the Museum. He has himself witnessed the formation of the cocoon, and his description of the process we repeat here in his own words. He says:
"Two protopteri, that had been restored to freedom by the gradual softening of the clods in which they had been inclosed, evinced signs, after living for a month in an aquarium, that the time had come for them to seek, in the soft earth covered by the water, the shelter which they require during the dry season. Their restlessness, their abundant secretion of mucus, their attempts at burrowing, all showed an irresistible desire to find a medium different from that in which they then lived.
"I therefore took pains to surround them with conditions analogous to those they meet with when, after the water has retreated, the soil first becomes dry, and then hardens. The water in the aquarium was drawn off little by little as soon as the animals had burrowed into the mud. Three weeks had scarcely passed, and already the hardened earth showed a number of cracks; through these a small quantity of air is admitted, which supports respiration.
"On the seventieth day I examined the earth, and found that the two animals had met with such conditions as enabled them successfully to live through the artificially-produced dry season: they were enveloped in cocoons, and were full of life, as was shown by their motion on being touched ever so lightly."Thus the cocoon is a protecting sheath formed of the mucous secretion. The abundant secretion of mucus, in the first place, coats and strengthens the walls of the burrow made by the protopterus, and hence the subterraneous canal which it had excavated had its sides smooth, and as it were polished. Then, after the animal has reached the required depth, the secretion becomes still more abundant, and the mucus dries, forming a membranaceous envelope of remarkable structure."
The animal doubles itself up in its envelope, the tail being brought up in front of the head; the mouth is free, and through it passes the air needed for respiration, which, of course, is exclusively pulmonary, owing to the conditions in which the animal lives. In fact, the protopterus is able to respire in two ways, viz., either directly in the atmosphere, or indirectly by separating dissolved oxygen from water by means of its gills.
The external openings to the latter are two small apertures, one on each side of the neck. Each of these gives access to a chamber of moderate capacity, in which are floating certain filamentous appendages. On these are distributed the blood-vessels, which constitute but ill-developed tufts. In water, the animal respires by means of these; when it lives in its burrow, it respires by means of its lung.
Most fishes have, beneath the vertebral column, a sort of capsule, which seems to act the part of a floating apparatus. By means of this, the fish can rise in the water or descend at pleasure; it is known as the air-bladder. The sounds emitted by certain fishes, Triglœ, for example, are caused by vibrations communicated to the gases in this organ.
In the protopterus the air-bladder discharges the physiological functions of a lung when the animal can no longer respire through the gills. To attain this end, it divides up into a number of little cellular lobes, over the walls of which are spread a multitude of blood-vessels, containing blood to be oxygenated, though it is only partially venous. To prevent mixture of the two kinds of blood, that which has respired the oxygen and that which has discharged its physiological function, the auricle becomes divided in two by a partition. The left chamber receives the red blood, just as in the higher animals. A muscular frœnum, or fold, forming a sort of rudimentary septum, rises from the floor of the ventricle; this frœnum acts as a piston, preventing the return of the blood into the vessels by contracting when the heart contracts.
The air enters either through the mouth or through the nostrils, which debouch near the posterior margin of the upper lip; thence it passes into a trachea, which traverses the wall of the œsophagus; finally, having entered a sort of membranous sac, through two large openings, it reaches the lungs, whereof there are two, and which are like the lungs of serpents.
These singular animals, being, as we have said, truly amphibious, have received from naturalists the expressive name of Dipnoi, a term formed from two Greek words, meaning animals with twofold respiration.
Two genera, each comprising only one species, make up this subclass Dipnoi. Gambia, Zanzibar, Senegal, the region of the White Nile, and the Niger, are the native haunts of the African species, the Protopterus annectens, or anguilliformis; the other species, Lepidosiren paradoxa, is found in the valley of the Amazonas.
The latter species is but ill represented in collections; there are in Europe only a very few specimens. According to Mr. Bates, the natives call it zambaki mboya; this naturalist says that the Lepidosiren has even penetrated to the great lakes in the vicinity of the Tapajos and the Madeira. M. de Castelnau has caught this animal in a marsh on the left bank of the Amazonas, above Villanova, at a place called Caracauca.
In Lepidosiren the tail is pointed; the pectoral and ventral fins, which stand far apart, are not long, and consist of a single ray, not divided into segments. The general form is that of an eel, with two threads hanging on each side. In color the animal is dark brown-gray, or olive, with round spots of lighter color, about the size of the scales, and indistinct on the head and the middle of the back. The species appears to grow to the length of about one metre.
The protopterus, or African representative of the group, is olive-green in color, this tint being varied with-a number of irregular brown or blackish spots. The lower portions are violet. The young are marked with fine lines of light color, which cross each other, forming a regular network. The extremity of the tail is tapering. The pectoral and ventral fins are long, and consist of one ray made up of jointed segments. The bones of the skeleton are of a green hue.
For our first acquaintance with these animals we are indebted to the naturalist Natterer, who, during his visit to Brazil, obtained two specimens, which he placed in the Vienna Museum. For a long time the Protopterus and Lepidosiren were classed with those batrachians in which the tail persists, as in the axolotl. Later they were considered as forming a sort of intermediate class between reptiles and fishes, and as forming the connecting link between the two. At present naturalists class Lepidosiren and Protopterus among fishes.
The Dipnoi are not the only class of animals that bury themselves in the dried-up mud after the water has been evaporated by the heat of summer. There is another fish that does the same the—mud-fish (Amia), which is found in the fresh waters of the United States. The Amia, too, is indisputably a fish. There appears to exist some relation between this animal's mode of life and the cellular structure of its air-bladder. Still, the Amia is not an amphibian, in the strict sense of the term. For, though its air-bladder resembles the lung of the serpent, it certainly receives only blood that has previously been aërated; hence we find in this animal no true aërial respiration alternating with strictly aquatic respiration.
But, though the Amia is not amphibious, and hence not to be considered in this place, nevertheless we must not omit to mention the fact that, while at present the genus is restricted within rather narrow geographical limits, it appears to have existed in Europe during the epoch known to geologists as the Middle Tertiary.
Thus there have been found at Oeningen (Switzerland), Kutschlin (Bohemia), Ménat and Armissan (France), fossil remains of Cyclurus, which has a close affinity with Amia. It is highly probable, not to say certain, that these fishes buried themselves in the mud during the dry season. The little tertiary lakes of Limagne appear to have undergone in past times alternations of drought and humidity, like the marshes of tropical and intertropical countries.
The presence in Europe of a genus closely allied to the Amia of America would seem to show that, at a relatively late period, these two divisions of the world were connected. The study of tertiary insects, to which E. Oustalet has devoted himself, and a thorough investigation of fossil fishes, would, we think, tend greatly to confirm Oswald Heer's hypothesis, according to which an Atlantis—not an historical Atlantis, as understood by Plato, but a geological Atlantis—connected the north of Europe with America toward the close of the great Tertiary epoch.