IV. As to the structure of the respiratory organs, the Ganoid Fishes agree essentially with the Bony Fishes. They all possess free pectinated gills attached to branchial arches, and enclosed in a branchial chamber, which is protected by an operculum, and is closed by a branchiostegal membrane, usually supported by branchiostegal rays. Besides the ordinary branchiae there is frequently an additional gill, called the "opercular branchia," attached to the interior of each operculum, and below this a false gill or "pseudo-branchia," which receives arterialised blood only. Acipenser and Polypterus have "spiracles" placed on the top of the head and communicating with the mouth.
V. There is always a swim-bladder, which is often divided by partitions into several cells, and is always connected with the gullet by an air-duct, as in the Malacopterous division of the Teleostean fishes. In Polypterus the air-bladder is double and sacculated.
VI. As to the structure of the heart, the Ganoids differ from the Bony Fishes, and agree with the Sharks and Rays in having a rhythmically contractile bulbus arteriosus, which is furnished with a special coat of striated muscular fibres, and is separated from the ventricle by several rows of valves. This is a decided advance in structure, as in this way the arterial bulb is enabled to act as a continuation of the ventricle.
VII. The intestine is often furnished with a spiral reduplication of its mucous membrane, forming a spiral valve, such as we shall afterwards see in the Sharks and Rays.
The classification of the Ganoid fishes has hitherto proved a matter of extreme difficulty; and probably no arrangement that has been as yet proposed can be regarded as being, in all its details, more than provisional. A convenient primary division is that into Lepidoganoids, in which the body is furnished with scales of moderate size, and the endoskeleton is generally more or less perfectly ossified, and Placoganoids, in which the skeleton is imperfectly ossified, and the head and more or less of the body are protected by large ganoid plates, which in many cases are united together by sutures. Accepting this division, the order Ganoidei may be divided into the following sub-orders:
Section i. Lepidoganoidei.
Sub-order A. Amiadae.
" B. Lepidosteidae.
" C. Crossopterygidae.
" D. Acanthodidae. (Extinct.)
Section 2. Placoganoidei.
Sub-order E. Ostracostei. (Extinct.) " F. Chondrosteidae.
The best known living fishes belonging to the Lepidoganoids are the Bony Pike and the Polypterus. The Bony Pike (Lepidosteus, fig. 266, A) inhabits the rivers and lakes of North America, and atttains a length of several feet. The body is entirely clothed with an armour of ganoid scales, arranged in obliquely transverse rows. The vertebral column is exceedingly well ossified, and is reptilian in its characters, the bodies of the vertebrae being "opisthocoelous." The jaws form a long narrow snout, armed with a double series of teeth ; and the tail is heterocercal.
The Polypteri, of which several species are known, inhabit the Nile, Senegal, and other African rivers, and are remarkable for the peculiar structure of the dorsal fin (fig. 265, A), which is broken up into a number of separate portions, each composed of a single spine in front, with a soft fin attached to it behind. They belong to the Crossopterygious Ganoids, in which the pectoral fins always, and the ventral fins often, consist of a central lobe or stem, which is covered with scales, and to the sides of which the fin-rays are attached. Two species of Polypterus have recently been shown to possess external branchiae when young, losing them when fully grown. Calamoichthys, also from the rivers of Africa, resembles Polyplerus in most respects, but the body is serpentiform, and there are no ventral fins. Another group of Lepidoganoids is formed by the Trout like Amiae of the fresh waters of the United States, in which the scales are rounded and overlap one another, the tail is slightly heterocercal, and the vertebral column is ossified. The air-bladder in Amia is subdivided, and can be used as a functional respiratory organ.
Fig. 266. - A, Lepidosteus osseus, the "Gar-Pike" of the American Lakes. B, Aspi-dorhynchus, restored (after Agassiz) ; a Jurassic Ganoid allied to Lepidosteus, but having a homocercal tail.
The section Placoganoidei includes the largest and best known of all the living Ganoid fishes - namely, the Sturgeons - and it also contains some highly singular fossil forms. The sub-order is defined by the fact that the skeleton is always imperfectly ossified, and often retains the notochord, whilst the head and more or less of the body are usually protected by large ganoid plates, which in many cases are united together at their edges by sutures. The tail is heterocercal.
The family Chondrosteidae, or Sturionidae, comprises the various species of Sturgeon, which are found in the seas of the northern hemisphere, whence they ascend the great rivers for the purpose of spawning. The vertebral column in the Sturgeon remains permanently in an embryonic condition. The notochord is persistent, and the vertebral centra are wanting, but the neural arches of the vertebrae reach the condition of cartilage. The mouth is destitute of teeth, and the head (fig. 253, B) is covered with an armour of large ganoid plates joined together at their edges by suture. Rows of detached ganoid plates also occur on the body. The various species of Sturgeon attain a great size, one - the Beluga - often measuring twelve or fifteen feet in length. They are commercially of considerable importance, the swimming-bladder yielding most of the isinglass of commerce, whilst the roe is largely employed as a delicacy under the name of caviare. In the Paddle-fishes (Spatularia) the skin is not provided with an exoskeleton. Both Spatularia and Scaphirhynchus are found in the rivers of North America; but two species of the latter have recently been discovered in Asia.
Only a few fossil forms belonging to the Sturionidae are at present known ; and by far the greater number of extinct Placoganoids belong to the family Ostracostei, established by Owen, and characterised by the fact that the head, and generally the anterior part of the trunk as well, was encased in a strong armour composed of numerous large ganoid plates, immovably joined to one another. The posterior extremity of the body was more or less completely unprotected, and, whilst the notochord was persistent, the peripheral elements of the vertebrae - namely, the neural and haemal spines - may be ossified.
Fig 267. - Cephalaspis Lyellii.
The Ostracostei, or "Placoderms," are entirely confined to the Devonian and Upper Silurian rocks, and the most important genera comprised in the group are Cephalaspis (fig. 267), Pteraspis, Fterichthys, and Coccosteus.