THE AIR-BLADDER. 189 



Most fishes possess a remarkable accessory organ of locomo- 

 tion in the air-bladder or swim-bladder which extends to a 

 greater or smaller distance along the ventral surface of the 

 spine, and enables them voluntarily to increase or diminish the 

 specific gravity of their body. When they contract this remark- 

 able gas-reservoir, or press out the included air by means of the 

 abdominal muscles, the bulk of the body is diminished, its 

 weight in proportion to the water is increased, and the fish 

 swims easily at a greater depth. The contrary takes place on 

 relaxing the tension of the abdominal muscles ; and thus we 

 see fishes rise and fall in their denser element by the applica- 

 tion of the same physical law which is made use of by our 

 aeronauts, to scale the heavens or to descend again upon the 



The Chondropteiygii are subdivided into three orders : 

 (a) Sturionidse (sturgeons), ■with free gills. 



(i) Selacii (rays, sharks), with gills fixed and a mouth formed for mastication, 

 (c) Cyolostomata (lamprey, myxine), with gills fixed and a mouth formed for 

 suction. 



The osseous fishes, ■which are far more numerous, are subdivided into six 

 orders : 



(<z) Acanthopterygii ; distinguished by the stiff spines which constitute the 

 first fin-rays of the dorsal fin, or which support the anterior fin of the back in 

 case there are two dorsals. In some cases the anterior dorsal fin is only repre- 

 sented by detached spines. The first rays of the anal fin are likewise spinous, 

 as well as the first ray of the ventral fin. To this extensive order, which com- 

 prises about three-fourths of the osseous fishes, belong, among others, the 

 families of the perches, gurnards, mackerels, mullets, breams, gobies, blennies, 

 &c. 



The three following orders of the osseous fishes have the rays that support tne 

 fins soft and composed of numerous pieces articulated with each other, with the 

 exception in some cases of the first ray of the dorsal, or of the pectoral. Their 

 leading character is afforded by the situation or absence of the ventral fin, which 

 in the 



(6) Malacopterygii abdominales are suspended beneath the abdomen, and 

 behind the pectorals ; in the 

 (c) Malacopterygii subbrachiales beneath the pectorals ; and in the 

 (<2) Malacopterygii apodes are totally wanting. 

 To the abdominal soft-rayed fishes belong the herring, salmon, pike, sly, and 

 carp families ; to the subbrachial, the cod family, the side-swimmers, and the lump 

 fishes ; and, finally, to the apodal malacjpterygians, the single family of the 

 anguillifbrm fishes. The small order of the 



(e) Lophobranchi comprises the pipe-fishes, sea-horses, in whom the gills are 

 not pectinated, as in the preceding subdivisions, but consist of little round tufts ; 

 and, finally, the 



(/) Plectognathi — comprising the file, porcupine, and sun fishes — are distin- 

 guished by their maxillaries and premaxillaries being joined immovably to each 

 other, so as to render the upper jaw incapable of protrusion. 



