(1755). We thus arrive at the important conclusion that different portions of the eocoskeleton become approximated in character to those of the endoskeleton, or, in truth, really convertible into true bone; and with this fact before us it becomes easy to understand the nature of various parts of the skeleton of a fish, which, upon any other supposition, would be not a little puzzling to the comparative osteologist.

(1756). The nature of the rays of the dorsal and anal fin of the Perch, for example, together with the interspinous bones upon which they are sustained, is quite unintelligible if they are regarded as belonging to the endoskeleton; and no dismemberments of the osseous system as yet imagined, or supposed subdivisions of the vertebrae into a greater number of elemental pieces than we have enumerated, has been able to solve the difficulty; but if they are regarded as ossified derivations from the exoskeleton, all difficulties at once vanish.

(1757). Again, the opercular bones (fig. 312, 28, 30, 32, 33) forming the gill-covers of an osseous fish have been a fruitful source of discussion; and M. Geoffroy St.-Hilaire* was reduced to the necessity of recognizing in these broad plates the ossicles of the human ear, which, after dwindling to a rudiment in the descending scale of vertebrate animals, suddenly reappeared in a new and exaggerated form. "J'ai peu vu dans la serie des etres de ces resurrections d'organes se remontrant subitement dans une classe apres avoir disparu dans une ou deux de celles qui la precede dans l'echelle," are the impressive words of Cuvier upon a similar occasion; and it is certainly far more simple to imagine the epidermic plates of the Sturgeon ossified and converted into bone, than to be compelled to have recourse to the bold speculations of the French anatomist regarding the real nature of these opercular portions of a fish's skeleton 1.

* Philosophic Anatomique des Pieces osseuses des Organes respiratoires. 8vo. Paris, 1818.

1 The different opinions on the nature or homology of the opercular bones may be reduced to two principles: first, that they are modifications of parts of the ordinary skeleton; secondly, that they are superadded bones peculiar to Fishes: the latter view is that taken by Cuvier. According to the former, which is the more philosophical mode of considering them, three opinions have been offered. The first by Spix and Geoffroy, that they are gigantic representatives of the ossicles of the ear, otherwise absent in the skeleton of Fishes: this view has been adopted by Professor Grant.

(1758). In connexion with the locomotive organs we must here notice one of the most elegant contrivances met with in the whole range of animated nature, by which the generality of fishes are enabled to ascend towards the surface, or to sink to any required depth, without exertion.

(1759). The apparatus given for this purpose is called the swimming-bladder, and consists of a reservoir of air (fig. 317, p) placed beneath the spine, in which position it is firmly bound down by the peritoneum. The outer coat of this bladder is very strong, and composed of a peculiar fibrous substance from which isinglass is obtained, but it is lined internally with a thin and delicate membrane. The shape of the swimming-bladder varies considerably in different tribes. In the Perch it is a simple cylinder closed at both extremities; sometimes it gives off branched appendages; sometimes, as in the Cyprinidge, it is divided into two portions, one anterior and the other posterior, by a deep central constriction; but, whatever its shape, its office is the same, namely, to alter the specific gravity of the fish, and thus to cause it to rise or sink in the medium it inhabits. By simply compressing this bladder by approximating the walls of the abdomen, or occasionally by means of a muscular apparatus provided for the purpose, upon a principle with which every one is familiar, the fish sinks in proportion to the degree of pressure to which the contained air is subjected; and as the compressed ' air is again permitted to expand, the creature becoming more buoyant rises towards the surface.

(1760). In the Perch, and many other fishes, this organ is entirely closed, so that there is no escape for the contained air; and in such it has been found that if they are suddenly brought up by means of a line from any great depth, the gas, being no longer compressed by the weight of the column of water above, and having no exit, bursts the swimming-bladder, and sometimes distends the abdomen to such an extent, that it pushes the stomach and oesophagus into the fish's mouth.

(1761). In other cases, however, a provision is made apparently with the view of obviating such an accident, and a kind of safety-valve provided through which the air may be permitted to escape: thus, in the Carp, a tube communicates between the interior of the air-bladder and the oesophagus, and in the Herring a similar communication is met with between this organ arid the stomach.

(1762). The gas which fills the air-bladder has been found in many cases to be nearly pure nitrogen; but in fishes that live at a great depth, Messrs. Configliacchi* and Biot ascertained that oxygen was substituted, whence it has been presumed that this apparatus was in some way or other an auxiliary in respiration; and some authors have even gone so far as to see in the swimming-bladder the representative of the lungs of aerial Vertebrata. But, however this may be, the gas enclosed is indubitably a product of secretion, being derived either from the lining membrane of the viscus, or from a glandular structure which may frequently be distinctly pointed out in its interior.

Secondly, that they are dismemberments of the lower jaw, which by the detachment of the opercular bones from the ramus is rendered more simple in its composition than in Reptiles, - a view proposed by M. de Blainville and temporarily adopted by Bojanus and Oken, but refuted by the complicated structure of the lower jaw in certain sauroid fishes, as the Lepidosteus, which likewise possesses the opercular bones. Thirdly, that they are parts of the dermal skeleton - in short, scales modified in subserviency to the breathing function, - an opinion first proposed by Professor Owen, in his Lectures on Comparative Anatomy at St. Bartholomew's Hospital in 1835, and which is the view here adopted.