RESPIRATION.] ICHTHYOLOGY 657 (magnified) of the Perch. a, branch of arteria brunchi- alis ; b, ascending branch of the same ; c, branch of vena branchialis ; d, de scending branch of the same ; e, trans verse section through the branchial arch. bearing a complete gill have two series of those rods, one along each edge ; those with uniserial gills bear one row of rods only. The rods are not part of the arch, but fixed in its integument, the several rods of the one row corresponding to those of the other, forming pairs (feuillet, Cuvier) (fig. 47). Each rod is covered by a loose mucous membrane passing from one rod to its fellow opposite, which again is finely plaited transversely, the extent of surface being greatly increased by these plaits. In most Tclcostei the branchial lamellaj are compressed, and taper towards their free end, but in the Lopho- branchs their base is attenuated and the end enlarged. The mucous membrane contains the finest terminations of the vessels, which, being very superficial, impart a blood-red colour to living gills. The arteria branchialis, the course of which lies in the open canal in the convexity of the branchial arch, emits a branch (a) for every pair of lamellse, which ascends (b) along the inner edge of the lamella, and supplies every one of the transverse plaits with a branchlet. The latter break up into a fine net work of capillaries, from which the oxygenized blood is collected into venous branchlets, return ing by the venous branch (d), which occupies the outer edge of the lamella. The so-called pseudobranchiae (fig. 48) are the remains of an anterior gill which had respira tory functions during the embryonic life of the individuals. By a change in the circulatory system these organs have lost those functions, and appear in the adult fish as retia mirabilia, p IO 47 ^ p a j r as they receive oxygenized blood, which, after branchial lamella; having passed through their capillary system, is carried to other parts of the head. In Pahcichthycs the pseudobranchia is a rete mira- bile caroticum for the brain and eye ; in Tele- osteans a rete mirabile ophthalmicum only. Pseudobranchiae are as frequently absent as present in Chondropterygians as well as Tele- osteans. Among the Ganoids, the organs occur in Ceratodus, Acipcnscr, Polyodon, andLcpidosteus, and are absent in Lcpidosircn, Protoptcrus, Scaphirhynchus, Poly- ptcrus, and Amia. In Chondropterygians and sturgeons the pseudobranchioe are situated within the spiracles ; in those in which spiracles have be come obliterated, the pseudo- branchise lie on the suspen- sorium, hidden below cellular tissue ; but pseudobranchire are not necessarily coexistent with spiracles. In the other Ganoids and Teleosteans the pseudo- branchiae (fig. 48, h) are within the gill-cavity, near the base of the gill-cover ; in Ceratodus even rudiments of the gill- rakers (x 1 , x") belonging to this embryonic gill are preserved, part of them (x") being at tached to the hyoid arch. Pseudobranchire are frequently hidden below the integuments of the gill-cavity, and have the appearance of a glandular body rather than of a gill. Accessory respiratory organs for retaining water or breathing air are found in the Labyrinthici, Ophioce- phalidce, certain Silurida, Fl and Lutodira. Air-bladder. The air- bladder, one of the most characteristic organs of fishes, is a hollow sac, formed of fieveral tunics, containing gas, situated in the abdominal cavity, but outside the peritoneal sac, entirely closed or communicating by a duct with the intestinal tract. Being compressible, its special functions consist in altering the specific gravity of the fish or in changing the centre of gravity. In a few fishes it assumes the function of the organ of higher vertebrates of which it is the homologue Ceratodus. r, arena aorta; ; gl, glossohyal ; ch, ceratohyal ; , attachment of the first gill to the walls of the gill-cavity ; h, pseudo- branrhia ; x , x", two series of gill-rakers belonging to the pseudobranchia viz., of a lung. The gas contained in the air-bladder is secreted from its inner surface. In most freshwater fishes it consists of nitrogen, with a very small quantity of oxygen and a trace of carbonic acid ; in sea-fishes, especially those- living at some depth, oxygen predominates, as much as 87 per cent, having been found. Davy found in the air- bladder of a fresh-run salmon a trace of carbonic acid and 10 per cent, of oxygen, the remainder of the gas being- nitrogen. An air-bladder is absent in Leptocardii, Cydostomi, Chondropterygii, and Uolocrphali, but occurs in all Ganoids, in which, besides, its respiratory functions mora or less clearly manifest themselves. Its occurrence in Tele osteans is very irregular, closely allied species sometimes differing from each other in this respect ; it shows in this subclass the most extraordinary modifications, but has no respiratory function whatever. Constantly situated within the abdominal cavity, below the vertebral column, but outside the sac of the peritoneum which covers only its ventral portion, the air-bladder is fre quently prolonged into the tail, the prolongation being either single and lodged between the non-united parapo- physes, or double and penetrating between the muscles and hsemapophyses of each side. In the opposite direction pro cesses of the air-bladder may penetrate into the skull, as has been mentioned above (p. 653). In some fishes the- air-bladder is almost loose in the abdominal cavity, whilst in others it adheres most intimately by firm and short tissue to the vertebral column, the walls of the abdomen, and the intestines. In the Cobitina and many Siluroids it is mora or less completely enclosed in osseous capsules formed by the vertebra. There are two tunics in the greater number of air- bladders, an extremely thin internal one, frequently shin ing with a silvery lustre, containing crystalline corpuscles, sometimes covered with a pavement-epithelium, and a thicker outer one of a fibrous texture, which sometimes attains to considerable thickness and yields isinglass. The outer wall is strengthened in many fishes by muscular layers for the compression of the whole organ or of some portion of it. A distinction has been made between air-bladders which communicate by a duct with the intestinal tract and those which are entirely closed. It is to be remembered, however, that at an early stage of development all air-bladders are provided with such a duct, which in some fishes is more or less completely obliterated, being then represented by a fine ligament only. Air-bladders without duct are found in Acanthopterygians, Pharyngognaths, Anacanths, and Lophobranchs. They may consist of a single cavity, or may be divided by constrictions into two or three chambers situated behind one another; they may consist of two lateral divisions, assuming a horseshoe-like form, or of a single sac with a pair of simple or bifid processes in front or behind. The families of Sciwnidce and Polynemidce possess air-bladders with a most extraordinary develop ment of appendages rising from each side. Air-bladders with a pneumatic duct are found in Ganoids and Physo- stomes, the duct entering the dorsal side of the intestinal tract, with the exception of Polypterus and the Dipnoi, in which it enters on the ventral side of the oesophagus. In most cases the orifice is in the oesophagus, but in some it is in the cardiac portion of the stomach, as in Adpenser, or in its blind sac, as in many Clupeoids. The air-bladder may be single, or may consist of two divisions situated one behind the other (fig. 45) ; its inner surface may be per fectly smooth, or it may form manifoH pouches and cells. If two divisions are present the anterior possesses a middle elastic membrane which is absent in the posterior; each division has a muscular layer, by which it can be separately
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