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  Section: Anatomy of Vertebrate Animals » Organisation of the Vertebrata Skeleton
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The Limbs of Fishes


The limbs of Fishes have an endoskeleton which only imperfectly corresponds with that of the higher Vertebrates. For while homologues of the cartilaginous, and even of the bony, constituents of the pectoral iind pelvic arches of the latter are traceable in Fishes, the cartilaginous, or ossified, basal and radial supports of the fins themselves cannot be identified, unless in the most general way, with the limb-bones, or cartilages, of the other Vertebrata.

In its least modified form, as in Lepidosiren, the endoskeleton of the fish's fin is a simple cartilaginous rod, divided into many joints; and articulated, by its proximal end, with the pectoral arch. The Elasmohranchii possess three basal cartilages which articulate with the pectoral arch, and are called, respectively, from before backward - propterygial, mesopterygial, and metapterygial basalia. With these are articulated linear series of radial cartilages, upon which osseous, or horny, dermal fin-rays are superimposed. (Fig. 15.)

Among the Granoid fishes, the fins of Polypterus are, fundamentally, like those of the Elasmobranchii; but the propterygial, mesopterygial, and metapterygial basalia, are more or less ossified, and are succeeded by a series of elongated rudialia, which are also, for the most part, ossified. Beyond these follow some small additional radialia, which remain cartilaginous, and are embraced by the bases of the fin-rays. In the other Ganoids the propterygial basale disappears, and some of the radialia, pushing themselves between the mesopterygial and metapterygial basalia, articulate directly with the pectoral arch. The mesopterygial basale is embraced by, and becomes more or less incorporated with, the large ante rior fin-ray.

The right pectoral member of the Monkfish (Squatina): h, propterygium - ma, mesopterygilum; mt metapteryglum.
Fig. 15. - The right pectoral member of the Monkfish (Squatina): h, propterygium - ma, mesopterygilum; mt metapteryglum.

From these Ganoids the passage is easy to the Teleostei, in which, also, the mesopterygial basale always becomes fused with the anterior fin-ray, whence the latter seems to articulate directly with the shoulder-girdle. Four bones, of very similar general form, usually articulate with the pectoral arch, beneath and behind the mesopterygial basale and its fin-ray. At their distal ends small cartilaginous nodules may lie, and these are embraced by the fin-rays. Of these four bones, or partially-ossified cartilages, the lowermost and hindermost answers to the metapterygial basale of the Shark; the others seem to be radialia. (See the figure of the Pike's pectoral fin, infra.)

The ventral fins have basal and radial cartilages and finrays, more or less resembling those of the fore-limbs. In most Ganoids and Teleosteans the pectoral and pelvic arches are, in part, or completely, ossified; the former frequently presenting distinct scapular and coracoid bones. To these, in all Ganoids and Teleosteans, membrane bones, representing a clavicle, with supra-clavicular and post-clavicular ossifications, are added.

In all Elasmobranchs and Ganoids, and in a large proportion of the Teleosteans, the pelvic fins are situated far back on the under side of the body, and are said to be "ventral" in position; but, in other Teleosteans, the ventral fins may move forward, so as to be placed immediately behind, or even in front of, the pectoral fins. In the former case they are said to be "thoracic," in the latter "jugular."

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