During the dry season, specimens living in shallow waters which periodically dry up, form a cavity in the mud, the inside of which they line with a protecting capsule of mucus, and from which they emerge again when the rains refill the pools inhabited by them. Whilst they remain in this torpid state of existence, the clay-balls containing them are frequently dug out, and if the capsules are not broken, the fishes imbedded in them can be transported to Europe, and released by being immersed in slightly tepid water. Protopterus is exclusively carnivorous, feeding on water-insects, frogs, and fishes, and attains a length of six feet.
[Owen, “Trans. Linn. Soc.” 1841, xviii.]
Ceratodus.—Body elongate, compressed, with one continuous vertical fin. Limbs paddle-shaped, with broad, rayed fringe. Vomerine teeth incisor-like; molars with flat, undulated surface, and lateral prongs. No external branchial appendages. Conus arteriosus with transverse series of valves. Ovaries transversely lamellated.[39]
Fig. 139.—Ceratodus miolepis.
Two species, C. forsteri and C. miolepis, are known from fresh waters of Queensland. The specimens hitherto obtained have come from the Burnett, Dawson, and Mary rivers, some from the fresh waters of the upper parts, others from the lower brackish portions. The fish is said to attain to a weight of twenty pounds and to a length of 6 feet. Locally, the settlers call it “Flat-head,” “Burnett- or Dawson-Salmon,” and the aborigines “Barramunda,” a name which they appear to apply also to other large-scaled freshwater fishes, as the Osteoglossum leichardti. In the stomach there is generally found an enormous quantity of the leaves of plants growing on the banks of rivers, evidently eaten after they had fallen into the water and when in a decomposing condition. The flesh of the fish is salmon-coloured, and much esteemed as food.
The Barramunda is said to be in the habit of going on land, or at least on mud-flats; and this assertion appears to be borne out by the fact that it is provided with a lung. However, it is much more probable that it rises now and then to the surface of the water in order to fill its lung with air, and then descends again until the air is so much deoxygenised as to render a renewal of it necessary. It is also said to make a grunting noise, which may be heard at night for some distance. This noise is probably produced by the passage of the air through the œsophagus when it is expelled for the purpose of renewal. As the Barramunda has perfectly developed gills, beside the lung, we can hardly doubt that, when it is in water of normal composition, and sufficiently pure to yield the necessary supply of oxygen, these organs are sufficient for the purpose of breathing, and that the respiratory function rests with them alone. But when the fish is compelled to sojourn in thick muddy water charged with gases, which are the products of decomposing organic matter (and this must be the case very frequently during the droughts which annually exhaust the creeks of tropical Australia), it commences to breathe air with its lung in the way indicated above. If the medium in which it happens to be is perfectly unfit for breathing the gills cease to have any function; if only in a less degree the gills may still continue to assist in respiration. The Barramunda, in fact, can breathe by either gills or lungs alone, or by both simultaneously. It is not probable that it lives freely out of the water, its limbs being much too flexible for supporting the heavy and unwieldy body, and too feeble generally to be of much use in locomotion on land. However, it is quite possible that it is occasionally compelled to leave the water, although we cannot believe that it can exist without it in a lively condition for any length of time.
Of its propagation or development we know nothing, except that it deposits a great number of eggs of the size of those of a newt, and enveloped in a gelatinous case. We may infer that the young are provided with external gills, as in Protopterus and Polypterus.
Fig. 140.—Tooth of fossil Ceratodus from Aust., near Bristol, natural size.