From the three families last named, with the Pholidophoridæ, there is an almost perfect transition from the Ganoid fishes to teleosteans of the order of Isospondyli, the primitive order from which all other bony fishes are perhaps descended. The family of Leptolepidæ, differing from Oligopleuridæ in the absence of fulcra, is here placed with the Isospondyli, but it might about as well be regarded as Ganoid.
CHAPTER III
ISOSPONDYLI
The Subclass Teleostei, or Bony Fishes.—The fishes which still remain for discussion constitute the great subclass or series of Teleostei (τελεός, true; οστέον, bone), or bony fishes. They lack wholly or partly the Ganoid traits, or show them only in the embryo. The tail is slightly, if at all, heterocercal; the actinosts of the pectoral fins are few and large, rarely over five in number, except among the eels; the fulcra disappear; the air-bladder is no longer cellular, except in very rare cases, nor does it assist in respiration. The optic nerves are separate, one running to each eye without crossing; the skeleton is almost entirely bony, the notochord usually disappearing entirely with age; the valves in the arterial bulb are reduced in number, and the spiral valve of the intestines disappears. Traces of each of the Ganoid traits may persist somewhere in some group, but as a whole we see a distinct specialization and a distinct movement toward the fish type, with the loss of characters distinctive of sharks, Dipnoans, and Ganoids. In a general way the skeleton of all Teleosts corresponds with that of the striped bass (see Figs. 22, 23, Vol. I), and the visceral anatomy is in all cases sufficiently like that of the sunfish (Fig. 16, Vol. I).
The mesocoracoid or precoracoid arch, found in all Ganoids, persists in the less specialized types of bony fishes, although no trace of it is found in the perch-like forms. With all this, there is developed among the bony fishes an infinite variety in details of structure. For this reason the Teleostei must be broken into many orders, and these orders are very different in value and in degrees of distinctness, the various groups being joined by numerous and puzzling intergradations.
Order Isospondyli.—Of the various subordinate groups of bony fishes, there can be no question as to which is most primitive in structure, or as to which stands nearest the orders of Ganoids. Earliest of the bony fishes in geological time is the order of Isospondyli (ἴσος, equal; σπόνδυλος, vertebra), containing the allies, recent or fossil, of the herring and the trout. This order contains those soft-rayed fishes in which the ventral fins are abdominal, a mesocoracoid or precoracoid arch is developed, and the anterior vertebræ are unmodified and essentially similar to the others. The orbitosphenoid is present in all typical forms. In certain forms of doubtful affinity (Iniomi) the mesocoracoid is wanting or lost in degeneration. Through the Isospondyli all the families of fishes yet to be considered are apparently descended, their ancestors being Ganoid fishes and, still farther back, the Crossopterygians.
Woodward gives this definition of the Isospondyli: "Notochord varying in persistence, the vertebral centra usually complete, but none coalesced; tail homocercal, but hæmal supports not much expanded or fused. Symplectic bone present, mandible simple, each dentary consisting only of two elements (dentary and articulo-angular), with rare rudiments of a splenoid on the inner side. Pectoral arch suspended from the cranium; precoracoid (mesocoracoid) arch present; infraclavicular plates wanting. Pelvic (ventral) fins abdominal. Scales ganoid only in the less specialized families. In the living forms air-bladder connected with the œsophagus in the adult; optic nerves decussating (without chiasma), and intestine either wanting spiral valve or with an incomplete representative of it."
The Classification of the Bony Fishes.—The classification of fishes has been greatly complicated by the variety of names applied to groups which are substantially but not quite identical one with another. The difference in these schemes of classification lies in the point of view. In all cases a single character must be brought to the front; such characters never stand quite alone, and to lay emphasis on another character is to make an alteration large or small in the name or in the boundaries of a class or order. Thus the Ostariophysi with the Isospondyli, Haplomi, and a few minor groups make up the great division of the Abdominales. These are fishes in which the ventral fins are abdominal, that is, inserted backward, so that the pelvis is free from the clavicle, the two sets of limbs being attached to different parts of the skeleton. Most of the abdominal fishes are also soft-rayed fishes, that is, without consecutive spines in the dorsal and anal fins, and they show a number of other archaic peculiarities. The Malacopterygians (μαλακός, soft; πτερύξ, fin) of Cuvier therefore correspond very nearly to the Abdominales. But they are not quite the same, as the spiny-rayed barracudas and mullets have abdominal ventrals, and many unquestioned thoracic or jugular fishes, as the sea-snails and brotulids, have lost, through degeneration, all of their fin-spines.
In nearly but not quite all of the Abdominal fishes the slender tube connecting the air-bladder with the œsophagus persists through life. This character defines Müller's order of Physostomi (φυσός, bladder; στόμα, mouth), as opposed to his Physoclysti (φυσός, bladder; κλεῖστός, closed), in which this tube is present in the embryo or larva only. Thus the Thoracices and Jugulares, or fishes having the ventrals thoracic or jugular, together correspond almost exactly to the Acanthopterygians, (ακανθα, spine; πτερύξ, fin), or spiny-rayed fishes of Cuvier, or to the Physoclysti of Müller. The Malacopterygians, the Abdominales, and the Physostomi are in the same way practically identical groups. As the spiny-rayed fishes have mostly ctenoid scales, and the soft-rayed fishes cycloid scales, the Physostomi correspond roughly to Agassiz's Cycloidei, and the Physoclysti to his Ctenoidei.
But in none of these cases is the correspondence perfectly exact, and in any system of classification we must choose characters for primary divisions so ancient and therefore so permanent as to leave no room for exceptions. The extraordinary difficulty of doing this, with the presence of most puzzling intergradations, has led Dr. Gill to suggest that the great body of bony fishes, soft-rayed and spiny-rayed, abdominal, thoracic, and jugular alike, be placed in a single great order which he calls Teleocephali (τελεός, perfect; κεφαλή, head). The aberrant forms with defective skull and membrane-bones he would separate as minor offshoots from this great mass with the name of separate orders. But while the divisions of Teleocephali are not strongly differentiated, their distinctive characters are real, ancient, and important, while those of the aberrant groups, called orders by Gill (as Plectognathi, Pediculati, Hemibranchii), are relatively modern and superficial, which is one reason why they are more easily defined. There seems to us no special advantage in the retention of a central order Teleocephali, from which the divergent branches are separated as distinct orders.