The fourth order of fishes is the large and important one of the Ganoid fishes, represented, it is true, by few living forms, but having an enormous development in past geological epochs. For this reason the study of the Ganoid fishes is one which claims considerable attention.
At the present day, the order Ganoidei comprises only seven genera - viz., Lepidosteus, Polypterus, Calamoichthys, Amia, Acipenser, Scaphirhynchus, and Spatularia - all of which are found only in the northern hemisphere, and are wholly or partially confined to fresh water.
The order Ganoidei may be defined by the following characters : The endoskeleton is only partially ossified, the vertebral column mostly remaining cartilaginous throughout life, especially amongst the extinct forms of the Palaeozoic period, in which the notochord is often persistent. The skull is furnished with distinct cranial bones, and the lower jaw is present. The exoskeleton is in the form of ganoid scales, plates, or spines. There are usually two pairs of limbs, in the form of fins, each supported by fin-rays. The first rays of the fins are mostly in the form of strong spines. The pectoral arch has a clavicle, and the posterior limbs (ventral fins) are placed close to the anus. The caudal fin is mostly un-symmetrical or "heterocercal." The swim-bladder is always present, is often cellular, and is provided with an air-duct. The intestine is often furnished with a spiral valve. The gills and opercular apparatus are essentially the same as in the Bony Fishes. The heart has one auricle and a ventricle, and the base of the branchial artery is dilated into a bulbus arteriosus, which is rhythmically contractile, is furnished with a distinct coat of striated muscular fibres, and is provided with several transverse rows of valves.
Of these characters, the ones which it is most important to remember are the following:
I. The endoskeleton is rarely thoroughly ossified, but varies a good deal as to the extent to which ossification is carried. In some forms, including most of the older members of the order, the chorda dorsalis is persistent, no vertebral centra are developed, and the skull is cartilaginous, and is protected by ganoid plates. Even in these forms, however, the peripheral elements of the vertebrae may be ossified. In others, the bodies of the vertebrae are marked out by osseous or semi-cartilaginous rings, enclosing the primitive matter of the noto-chord. In others, the vertebrae are like those of the Bony-Fishes; that is to say, deeply biconcave or "amphicoelous." In one Ganoid, however - the Bony Pike (Lepi-dosteus) - the vertebral column consists of a series of "opisthocoelous" vertebrae; that is to say, vertebrae which are convex in front and concave behind. This is the highest point of development reached in the spinal column of any fish, and its structure is more Reptilian than Piscine. In Poly-ptems and Amia the vertebrae are ossified and amphicoelous. The remaining existing genera have a persistent notochord.
II. The exoskeleton consists in most Ganoid fishes of scales, plates, or spines, which are said to possess ganoid characters. The peculiarities of these scales are that they are composed of two distinct layers - an inferior layer of bone and a superficial covering of a kind of enamel, somewhat similar to the enamel of the teeth, called "gano-ine" In form the ganoid scales most typically exhibit themselves as rhom-boidal plates, placed edge to edge, without overlapping, in oblique rows, the plates of each row being often articulated to those of the next by distinct processes (fig. 247, e). In some cases, however, the scales are circular, and overlap one another, as in the ordinary Bony Fishes. In Acipenser (fig. 253, B) and Scaphirhynchus there are detached dermal plates of true bone; whilst Spatularia has the skin naked.
III. As to the fins, both pectorals and ventrals are usually present, and the ventrals are always placed far back in the neighbourhood of the anus, and are never situated in the immediate vicinity of the pectorals. In some living and many extinct forms the fin-rays of the paired fins are arranged so as to form a fringe round a central lobe (fig. 265). This structure characterises a large and important division of Ganoid fishes, called by Professor Huxley, for this reason, "Fringe-finned" Ganoids or Crossopterygidae. The same form of fin is seen in Ceratodus among the Dipnoi, in which the limb (fig. 264) consists of a median cartilaginous axis, formed by a succession of joints, which, in turn, support on each side a lateral series of jointed branches, these finally bearing the fin-rays. The form of the caudal fin varies, the Ganoids being in this respect intermediate between the Bony Fishes, in which the tail is "homocercal," and the Sharks and Rays, in which there is a "heterocercal" caudal fin. In the majority of Ganoids, then, the tail is unsymmetrical or "heterocercal," but it is sometimes equi-lobed or "homocercal."
Fig. 264. - Skeleton of the pectoral fin of Ceratodus, showing the median axis and divergent branches on each side. c Carpal cartilage. (After Gunther.)
Fig. 265. - Ganoid Fishes. A, Polypterus; B, Osteolepis (extinct). a One of the pectoral fins, showing the fin-rays arranged round a central lobe; b One of the ventral fins ; c Anal fin ; d Dorsal fin ; d' Second dorsal fin.