A Guide to the Study of Fishes, Volume 1 (of 2)

In the Ctenodontidæ the tail is diphycercal, and no jugular plates are present in the known specimens. In Ctenodus and Sagenodus there is no jugular plate and there are no marginal teeth. The numerous species of Ctenodus and Sagenodus belong chiefly to the Carboniferous age. Ctenodus wagneri is found in the Cleveland shale of the Ohio Devonian. Sagenodus occidentalis, one of the many American species, belongs to the coal-measures of Illinois.

As regards the succession of the Dipneusti, Dr. Dollo regards Dipterus as the most primitive, Scaumenacia, Uronemus, Ctenodus, Ceratodus, Protopterus, and Lepidosiren following in order. The last-named genus he thinks marks the terminus of the group, neither Ganoids nor Amphibians being derived from any Dipnoans.

Order Sirenoidei.—The living families of Dipneusti differ from these extinct types in having the cranial roof-bones reduced in number. There are no jugular plates and no marginal teeth in the jaws. The tail is diphycercal in all, ending in a long point, and the body is covered with cycloid scales. To these forms the name Sirenoidei was applied by Johannes Müller.

Family Ceratodontidæ.—The Ceratodontidæ have the teeth above and below developed as triangular plates, set obliquely each with several cusps on the outer margin. Nearly all the species, representing the genera Ceratodus, Gosfordia, and Conchopoma, are now extinct, the single genus Neoceratodus still existing in Australian rivers. Numerous fragments of Ceratodus are found in Mesozoic rocks in Europe, Colorado, and India, Ceratodus latissimus, figured by Agassiz in 1838, being the best-known species.

The abundance of the fossil teeth of Ceratodus renders the discovery of a living representative of the same type a matter of great interest.

In 1870 the Barramunda of the rivers of Queensland was described by Krefft, who recognized its relationship to Ceratodus and gave it the name of Ceratodus forsteri. Later, generic differences were noticed, and it was separated as a distinct group by Castelnau in 1876, under the name of Neoceratodus (later called Epiceratodus by Teller). Neoceratodus forsteri and a second species, Neoceratodus miolepis, have been since very fully discussed by Dr. Günther and Dr. Krefft. They are known in Queensland as Barramunda. They inhabit the rivers known as Burnett, Dawson, and Mary, reaching a length of six feet, and being locally much valued as food. From the salmon-colored flesh, they are known to the settlers in Queensland as "salmon." According to Dr. Günther, "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 deoxygenized 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 besides 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 lung alone or by both simultaneously. It is not probable that it lives freely out of 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.

"The discovery of Ceratodus does not date farther back than the year 1870, and proved to be of the greatest interest, not only on account of the relation of this creature to the other living Dipneusti and Ganoidei, but also because it threw fresh light on those singular fossil teeth which are found in strata of Triassic and Jurassic formations in various parts of Europe, India, and America. These teeth, of which there is a great variety with regard to general shape and size, are sometimes two inches long, much longer than broad, depressed, with a flat or slightly undulated, always punctated, crown, with one margin convex, and with from three to seven prongs projecting on the opposite margin."

Development of Neoceratodus.—From Dean's "Fishes, Recent and Fossil," pp. 218-221, we condense the following account (after the observations of Dr. F. Semon) of the larval history of the Barramunda, Neoceratodus forsteri:

It offers characters of exceptional interest, uniting features of Ganoids with those of Cyclostomes and Amphibians.

The newly hatched Neoceratodus does not strikingly resemble the early larva of shark. No yolk-sac occurs, and the distribution of the yolk material in the ventral and especially the hinder ventral region is suggestive rather of lamprey or amphibian; it is, in fact, as though the quantum of yolk material had been so reduced that the body form had not been constricted off from it. The caudal tip in this stage appears, however, to resemble that of the shark, and, as far as can be inferred from surface views, a neurenteric canal persists. Like the shark there then exists no unpaired fin; the gill-slits (five?) are well separated and there is an abrupt cephalic flexure. In this stage pronephros (primitive kidney) and primitive segments are well marked, and are outwardly similar to those structures in Ganoid; the mouth is on the point of forming its connection with the digestive cavity; the anus is the persistent blastophore; the heart, well established, takes a position, as in Cyclostomes, immediately in front of the yolk material.

In a later stage the unpaired fin has become perfectly established, the tail increasing in length; the gill-slits have now been almost entirely concealed by a surrounding dermal outgrowth, the embryonic operculum; a trace of the pectoral fin appears; the lateral line is seen proceeding down the side of the body; near the anal region the intestine[165] becomes narrower, and the beginnings of the spiral valve appear. In a larva of two weeks a number of developmental advances are noticed; the fish has become opaque; the primitive segments are no longer seen; the size of the yolk mass is reduced; the anal fin-fold appears; sensory canals are prominent in the head region; lateral line is completely established; the rectum becomes narrowed; and the cycloidal body-scales are already outlined. Gill-filaments may still be seen beyond the rim of the outgrowing operculum. In the ventral view of a somewhat later larva the following structures are to be noted: the pectoral fins, which have now suddenly budded out,[166] reminding one in their late appearance of the mode of origin of the anterior extremity of urodele; the greatly enlarged size of the opercular flap; external gills, still prominent; the internal nares, becoming constricted off into the mouth-cavity by the dermal fold of the anterior lip (as in some sharks); and finally (as in Protopterus and some batrachian larvæ) the one-sided position of the anus.

The larva of six weeks suggests the outline of the mature fish; head and sides show the various openings of the tubules of the insunken sensory canals; and the archipterygium of the pectoral fin is well defined. The oldest larva figured is ten weeks old; its operculum and pectoral fin show an increased size; the tubular mucous openings, becoming finely subdivided, are no longer noticeable; and although the basal supports of the remaining fins are coming to be established, there is as yet little more than a trace of the ventrals.

The early development of a lung-fish has thus far been described (Semon) only from the outward appearance of the embryo. The egg of Neoceratodus has its upper pole distinguished by its fine covering of pigment. From the first fine planes of cleavage it will be seen that the yolk material of the lower pole is not sufficient to prevent the egg's total segmentation. The first plane of cleavage is a vertical one, passing down the side of the egg as a shallow surface furrow, not appearing to entirely separate the substance of the blastomeres, although traversing completely the lower hemisphere. A second vertical furrow at right angles to the first is seen from the upper pole. The third cleavage is again a vertical one (as in all other fishes, but unlike Petromyzon), approximately meridional; its furrows appear less clearly marked than those of earlier cleavages, and seem somewhat irregular in occurrence. The fourth cleavage is horizontal above the plane of the equator. Judging from Semon's figure, at this stage the furrows of the lower pole seem to have become fainter, if not entirely lost. In a blastula showing complete segmentation the blastomeres of the upper hemisphere are the more finely subdivided. In the earlier stage the dorsal lip of the blastopore is crescent-like; in the later the blastopore acquires its oblong outline, through which the yolk material is apparent; its conditions may later be compared to those of a Ganoid.

The next change of the embryo is strikingly amphibian-like; the medullary folds rise above the egg's surface, and, arching over, fuse their edges in the median dorsal line. The medullary folds are seen closely apposed in the median line; hindward, however, they are still separate, and through this opening the blastopore may yet be seen. At this stage primitive segments are shown; in the brain region the medullary folds are still slightly separated.

In an older embryo the fish-like form may be recognized. The medullary folds have completely fused in the median line, and the embryo is coming to acquire a ridge-like prominence; optic vesicles and primitive segments are apparent, and the blastopore appears to persist as the anus. The continued growth of the embryo above the yolk mass is apparent; the head end has, however, grown the more rapidly, showing gill-slits, auditory, optic, and nasal vesicles, at a time when the tail mass has hardly emerged from the surface. Pronephros has here appeared. It is not until the stage of the late embryo that the hinder trunk region and tail come to be prominent. The embryo's axis elongates and becomes straighter; the yolk mass is now much reduced, acquiring a more and more oblong form, lying in front of the tail in the region of the posterior gut. The head and even the region of the pronephros are clearly separate from the yolk-sac; the mouth is coming to be formed.

According to Eastman (Ed. Zittel), the skeleton of Neoceratodus is less developed and less ossified than that of its supposed Triassic ancestors. A similar rule holds with regard to the sturgeons and some Amphibians.

Lepidosirenidæ.—The family Lepidosirenidæ, representing the suborder Diplopneumona, is represented by two genera of mudfishes found in streams of Africa and South America. Lepidosiren paradoxa was discovered by Natterer in 1837 in tributaries of the Amazon. It was long of great rarity in collections, but quite recently large numbers have been obtained, and Dr. J. Graham Kerr of the University of Cambridge has given a very useful account of its structure and development. From his memoir we condense the following record of its habits as seen in the swamps in a region known as Gran Chaco, which lies under the Tropic of Capricorn. These swamps in the rainy season have a depth of from two to four feet, becoming entirely dry in the southern winter (June, July).

Kerr on the Habits of Lepidosiren.—The loalach, as the Lepidosiren is locally called, is normally sluggish, wriggling slowly about at the bottom of the swamp, using its hind limbs in irregular alternation as it clambers through the dense vegetation. More rapid movement is brought about by lateral strokes of the large and powerful posterior end of the body. It burrows with great facility, gliding through the mud, for which form of movement the shape of the head, with the upper lip overlapping the lower and the external nostril placed within the lower lip, is admirably adapted. It feeds on plants, algæ, and leaves of flower-plants. The gills are small and quite unable to supply its respiratory needs, and the animal must rise to the surface at intervals, like a frog. It breathes with its lungs as continuously and rhythmically as a mammal, the air being inhaled through the mouth. The animal makes no vocal sound, the older observation that it utters a cry like that of a cat being doubtless erroneous. Its strongest sense is that of smell. In darkness it grows paler in color, the black chromatophores shrinking in absence of light and enlarging in the sunshine. In injured animals this reaction becomes much less, as they remain pale even in daylight.

In the rainy season when food is abundant the Lepidosiren eats voraciously and stores great quantities of orange-colored fat in the tissues between the muscles. In the dry season it ceases to feed, or, as the Indians put it, it feeds on water. When the water disappears the Lepidosiren burrows down into the mud, closing its gill-openings, but breathing through the mouth. As the mud stiffens it retreats to the lower part of its burrow, where it lies with its tail folded over its face, the body surrounded by a mucous secretion. In its burrow there remains an opening which is closed by a lid of mud. At the end of the dry season this lid is pushed aside, and the animal comes out when the water is deep enough. When the waters rise the presence of Lepidosirens can be found only by a faint quivering movement of the grass in the bottom of the swamp. When taken the body is found to be as slippery as an eel and as muscular. The eggs are laid in underground burrows in the black peat. Their galleries run horizontally and are usually two feet long by eight inches wide. After the eggs are laid the male remains curled up in the nest with them. In the spawning season an elaborate brush is developed in connection with the ventral fins.

Protopterus, a second genus, is found in the rivers of Africa, where three species, P. annectens, P. dolloi, and P. æthiopicus, are now known.

The genus has five gill-clefts, instead of four as in Lepidosiren. It retains its external gills rather longer than the latter, and its limbs are better developed. The habits of Protopterus are essentially like those of Lepidosiren, and the two types have developed along parallel lines doubtless from a common ancestry. No fossil Lepidosirenidæ are known.

Just as the last page of this volume passes through the press, there has appeared a bold and striking memoir on the "Phylogeny of the Teleostomi," by Mr. C. Tate Regan of the British Museum of Natural History. In this paper Mr. Regan takes the view that the Chondrostean Ganoids (Palæoniscum, Chondrosteus, Polyodon, Psephurus, etc.) are the most primitive of the Teleostomous fishes; that the Crossopterygii, the Dipneusti, the Placodermi, and the Teleostei (as well as the higher vertebrates) are descended from these; that the Coccosteidæ (Arthrodires) are the most generalized of the Placoderms, the Osteostraci and most of the other forms called Ostracophores (Antiarcha, Anaspida) being allied to the Arthrodires, and to be included with them among the Placodermi; that the cephalic appendage of Pterichthyodes, etc., is really a pectoral fin; that the Heterostraci (Lanarkia, Pteraspis, etc.) are not Ostracophores or Placoderms at all, but mailed primitive sharks, derived from the early sharks as the Chimæras are, and that the Holostean Ganoids (Lepisosteus, Amia, etc.) should be separated from the Chondrostei and referred to the Teleostei, of which they are the primitive representatives.

Mr. Regan especially calls attention to the very close similarity in structure of pectoral and ventral fins in the Chondrostean Ganoids, Psephurus and Polyodon, with that of the anal fin in the same fishes. From this he derives additional evidence in favor of the origin of paired fins from a lateral fold. In his view, the Chondrostei have sprung directly, through ancestors of the Lysopteri and Selachostomi, from pleuropterygian sharks (Cladoselache) of the Lower Silurian, and the true fishes on the one hand and the Crossopterygian-Dipneustan-Placoderm series on the other are descended from these. The absence of the lower jaw in fossil remains of Ostracophores may be due to its cartilaginous structure. "There is no justification for regarding the Crossopterygii as less specialized than the Chondrostei because they were the earlier dominant group."

These views are very suggestive and contain at least some elements of taxonomic advance, although few naturalists of to-day will regard the Chondrostean Ganoids as more primitive than the fishes called Crossopterygii and Placoderms.

These conclusions are summarized by Mr. Regan as follows:

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