Fig. 309.—Larval Protopterus on the seventeenth day. c, Cement organ; c.g, cutaneous gills; op, operculum; p.l, pectoral limb; pv.l, pelvic limb; y.s, yolk-sac. (From Budgett.)
A month-old larva has much the aspect of a larval Newt. It has four pairs of vascular plumose cutaneous gills (Fig. 309), which are retained as vestiges for a long time or even throughout life, and two pairs of synchronously-developed limbs. As an interesting instance of a nocturnal and protective change of colour, it may be mentioned that the dark chromatophores of the skin of the larva expand in the day-time and the young Fish becomes darker in colour, and therefore less conspicuous when seen against a background of black mud or soil. At night the contraction of the colour-sacs renders the larva more transparent and probably less easily visible than if opaque. The commencement of pulmonary respiration is coincident with the degeneration of the cutaneous gills, which takes place about seven weeks after the deposition of the eggs, and about a month after the larvae leave the nest. Protopterus is said to attain a length of six feet.
Lepidosiren paradoxa,[[607]] probably the only species of the genus, is confined to South America. It occurs along the course of the main Amazon river, entering some of its larger affluents, such as the Ucayale, the Madeira, the Rio Negro, and the Tapajóz, and also in the Chaco Boreal to the west of the Upper Paraguay river. The home of the Lepidosiren (or "Lolach," as the natives call the Fish) of the Chaco country is to be found in the wide-spreading marshes and swamps, which for a great part of the year are almost choked by a luxuriant growth of their own peculiar vegetation and covered by a floating carpet of surface weeds, with here and there deeper and clearer water and slow-flowing streams. In the dry season the water gradually shrinks and the swamps eventually become dried up. Of sluggish habits, the Fish wriggles slowly about at the bottom of the swamp like an Eel, using its hind limbs in an irregular bipedal fashion as it wends its way through the dense network of subaqueous plants. Lepidosiren is not exclusively carnivorous. The large fresh-water snail, Ampullaria, which lives in the swamps in enormous numbers, seems to be its favourite food; but masses of confervoid Algae are also eaten, and in its earlier stages it is probable that the Fish is more herbivorous than carnivorous. The Jacare (Caiman sclerops) feeds on Lepidosiren, and this fact, and probably also the cannibal habits of the Fish itself, may explain the capture of specimens with mutilated tails and regenerated, branched, pectoral limbs. Like other living Dipneusti, Lepidosiren rises to the surface to breathe. The intervals are, however, very variable, and no doubt depend on the relative purity or impurity of the water. Both expiration and inspiration are said to take place through the mouth. The snout is protruded on the surface, and the creature expires. After being withdrawn for a moment the head is again projected, and inspiration takes place through the partially open lips.
Fig. 310.—Pelvic limb of the male Lepidosiren during the breeding season. (From Graham Kerr.)
When the Fish finally sinks a few bubbles of surplus air escape through the gill-clefts. A nocturnal and protective change of colour, similar to that described in Protopterus, has been observed, and although most strikingly manifest in the larvae, it also occurs in individuals of older growth. The flesh is much esteemed as food by the Indians, who wade into the swamps and transfix the Fishes with spears. During the rainy season the Lepidosiren eats voraciously, and a reserve of fat is stored up in the tissues. Like its African relative, the Fish ceases to feed on the approach of the dry season, and eventually hibernates at the dilated extremity of a deep tubular burrow, the entrance to which is plugged by a small lump of clay perforated by several round holes. On the rising of the water at the next rainy season the Lepidosiren pushes out the plug and soon emerges from its burrow.[[608]] The breeding season begins soon after the escape of the Fish. The eggs are deposited in nests in the form of underground burrows excavated in the black peaty soil at the bottom of the swamp, with an entrance about 4-5 inches wide. At a depth of about a foot the burrow takes a horizontal course, its total length varying from 2-5 feet. After the eggs are laid the male remains to guard them. During the breeding season the pelvic limbs of the male enlarge and become covered by a rich growth of highly vascular, blood-red filaments 2-3 inches in length[[609]] (Fig. 310). The use of these curious structures is uncertain, but it is not improbable that they act as accessory gills to enable the male to guard the eggs in the nest without being forced to resort to the surface to breathe air. The development is essentially similar to that of Protopterus. The larva (Fig. 311) has four pairs of cutaneous gills in relation with the first, second, third, and fourth branchial arches, inclusive, the first three pairs being the homologues of the cutaneous gills of the tailed Amphibia; and also a cement-organ which disappears shortly before the larval metamorphosis. At that period the circulation in the cutaneous gills becomes sluggish, and very soon these organs completely atrophy. About the same time the hyo-branchial cleft closes up, as in Protopterus. The young Lepidosiren soon begins to breathe air and to become more active and lively in its habits.[[610]] The adult may attain a length of four feet.
Fig. 311.—Larval Lepidosiren thirty days after hatching. c, Cement organ; c.g, cutaneous gills; p.l, pectoral limb; pv.l, pelvic limb. (From Graham Kerr.)
The relations of the different genera of Dipneusti to one another has been discussed by Dollo in a remarkably suggestive paper.[[611]] Until the publication of this treatise it was generally believed that the modern Dipneusti, Neoceratodus, Protopterus, and Lepidosiren, especially the first mentioned, were the most primitive and the more nearly related to the ancestral stock, while the older types, such as Dipterus, were regarded in the light of highly specialised offshoots. The continuity of the median fins, the apparently diphycercal character of the tail, and the wholly cartilaginous condition of the chondrocranium in the modern Dipneusti, were contrasted with the divided median fins, the heterocercal tail, and the more extensively ossified chondrocranium of the Palaeozoic forms, and the belief seemed inevitable. Dollo has shown, however, that there is good reason for the view that the evolution of the group has taken place in exactly the opposite direction; that, in fact, the older Dipneusti are the more archaic, and that their modern representatives have been derived from them by a sequence of retrogressive changes; or, in other words, the latter have much the same relation to the former as the degenerate Sturgeons and Paddle-Fishes to their Palaeozoic ancestors, the Palaeoniscidae. Taking Dipterus, the most ancient of all the known Dipneusti, as a starting-point, it is possible to select a series of genera which illustrate the evolution of the group both in structure and in palaeontological sequence.[[612]] The series is as follows:—Dipterus, Scaumenacia, Phaneropleuron, Uronemus, Ceratodus (Neoceratodus), Protopterus and Lepidosiren. Briefly, the more important structural modifications observable in the transition from the older to the recent genera are (a) the gradual union of isolated median fins to form a continuous fin[[613]]; (b) the substitution of a gephyrocercal tail for a heterocercal[[613]]; (c) the degeneration of the squamation, the thick ganoid scales of the earlier types being replaced by thin, non-ganoid scales; (d) a reduction in the number of cranial dermal bones and the loss of their original ganoid investment; (e) the suppression of the jugular plates; and (f) a reduction in the size of the opercular bones. In the last two genera of the series, in which specialisation in some respects and degeneration in others have reached their maximum, the body no longer retains the fusiform and more typically Fish-like shape of the older genera, but, in accordance with Eel-like habits and mode of progression, has become more or less Eel-like in form.[[614]] The paired fins are almost vestigial, while the scales, so deeply insunken in the skin as to be externally invisible, suggest that the modern Dipneusti are approximating to a final scaleless as well as to an ultimately limbless condition. As to the origin of the Dipneusti as a group, it seems reasonable to look for their ancestors in the early Devonian Crossopterygii with acutely lobate fins, or, with greater probability, to some still more primitive Crossopterygian with simple, non-rhizodont teeth, capable by fusion of giving rise to massive tritoral plates, and involving as a consequence the substitution of an autostylic for an originally hyostylic skull, and the suppression of the secondary upper jaw. In fact, when our knowledge of the development of the surviving Dipneusti and Crossopterygii is more complete, it is not improbable that the inclusion of the two series of Fishes in subordinate divisions of the Teleostomi will prove to be amply justified. The relations of the Dipneusti to the Amphibia are somewhat deceptive, and it seems improbable that the former group stands in the direct line of Amphibian descent. In most of their structural features not directly or remotely associated with air-breathing the Dipneusti are true Fishes, and the striking resemblances which they present to the Amphibians in the vascular system and lungs seem to be rather the outcome of physiological convergence, associated with adaptive and parallel modifications in structure, and due to the influence of a similar environment, than indicative of direct ancestral relations. With more reason it may be inferred that both the Dipneusti and the Amphibia have been derived from some primitive Crossopterygian ancestor with Elasmobranch tendencies, and subsequently became modified in certain respects on parallel lines.