CLASS OF FISHES. 



377 



The cranium is without sutures, and is composed of a single 

 piece, but modelled and pierced like the cranium of a common 

 fish. The upper jaw is formed of pieces analogous to the 

 palatine bones and to the vomer ; the maxillary and inter- 

 maxillary bones either do not exist or are merely rudimentary. 

 The lower jaw is also formed of a single piece on each side, 

 and the opercular apparatus is wanting. The vertebral column 

 is often formed of a single tube, pierced on each side for the 

 passage of the nerves, but not divided into distinct vertebrae ; 

 and the gelatinous-looking substance acting as a ligament to 

 connect the vertebrae to each other, often forms a continuous 

 cord. The arrangement of the bones of the shoulder, pelvis, 

 and fins, varies. Finally, the hyoid apparatus supporting 

 the gills is arranged pretty much as in other, fishes, but in 

 the last of the series, the lampreys, the branchial arches are 

 wanting. 



Fig. 386. Dorsal Fin. 



482. Most fish swim with great agility, and it is said 

 that the salmon goes at the rate of twenty-four feet in a 

 second. They swim by alternate flexions of the tail and 

 trunk, and the muscles intended to move these form the 

 greater part of the mass of the body. The lateral fins are 

 not much used in progression, and seem merely intended to 

 maintain the equilibrium of the body, or slightly to modify 

 the direction of its course. 



483. A remarkable feature in the organization of some 

 fish is the swimming bladder, placed in the abdomen under 

 the dorsal spine, communicating often with the gullet or 

 stomach by a canal, permitting of the escape of air from its 

 interior ; but sometimes no such passage exists, and then it 

 is evident that the air contained in the swimming bladder is 

 a secretion from a gland situated on its walls. By the move- 



