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 full in their denser element by the applica- 
tion of the same physical law which is made use of by our 
aéronauts, to scale the heavens or to descend again upon the 
The Chondropterygii are subdivided into three orders : 
(a) Sturionide (sturgeons), with free gills. 
(0) Selacii (rays, sharks), with gills fixed and a mouth formed for mastication. 
(ce) Cyclostomata (lamprey, myxine), with gills fixed and a mouth formed for 
suction. 
The osseous fishes, which are far more numerous, are subdivided into six 
orders: ® 
(a) 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, 
&e. 
The three following orders of the osseous fishes have the rays that support the 
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 
(4) Malacopterygii abdominales are suspended beneath the abdomen, and 
behind the pectorals ; in the 
(c) Malacopterygii subbrachiales beneath the pectorals; and in the 
(d) 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 malacopterygians, the single family of the 
anguilliform 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 
(f) Plectognathi—comprising the file, porcupine, and sun fishes—are distin- 
guished by their maxillaries and premaxillaries being joined immoyably to each 
other, so as to render the upper jaw incapable of protrusion. 
