In Chondropterygians and Sturgeons the pseudobranchiæ are situated within the spiracles; in those, in which spiracles have become obliterated, the pseudobranchiæ lie on the suspensorium, hidden below cellular tissue; but pseudobranchiæ are not necessarily co-existent with spiracles. In the other Ganoids and Teleosteans the pseudobranchiæ (Fig. 60, _h_) are within the gill-cavity, near the base of the gill-cover; in _Ceratodus_ even rudiments of the gill-rakers (_x’_, _x”_) belonging to this embryonic gill are preserved, part of them (_x”_) being attached to the hyoid arch. Pseudobranchiæ are frequently hidden below the integuments of the gill-cavity, and have the appearance of a glandular body rather than of a gill. [See Müller, “Vergleichende Anatomie des Gefäss-systems der Myxinoiden;” and “Ueber den Bau und die Grenzen der Ganoiden.”] [Illustration: Fig. 60.--Gills of Ceratodus. _x_, Arcus aortæ; _gl_, Glossohyal; _ch_, Ceratohyal; _u_, Attachment of the first gill to the walls of the gill-cavity; _h_, Pseudobranchia; _x’_, _x”_, two series of gill-rakers belonging to the Pseudobranchia.] _Accessory respiratory organs_ for retaining water or breathing air, such as are found in the _Labyrinthici_, _Ophiocephalidæ_, certain _Siluridæ_, and _Lutodira_, are structures so specialised that they are better described in the accounts of the Fishes in which they have been observed. * * * * * _Air-Bladder._--The air-bladder, one of the most characteristic organs of fishes, is a hollow sac, formed of several tunics, containing gas, situated in the abdominal cavity, but without 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--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_, _Cyclostomi_, _Chondropterygii_, and _Holocephali_; but occurs in all Ganoids, in which, besides, its respiratory functions more or less clearly manifest themselves. Its occurrence in Teleosteans is most irregular, closely allied species sometimes differing from each other in this respect; it shows in this sub-class the most extraordinary modifications, but has no respiratory function whatever.