LOCOMOTION OF FISHES. 9 



not exist, and are only found in a rudimentary state, hidden 

 under the skin. The lower jaw has in like manner only one 

 piece on each side, and the opercular apparatus is generally 

 altogether, wanting. The vertebral column is sometimes chiefly 

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

 the nerves, but not divided into distinct vertebra. The bodies 

 of the vertebrae are also frequently pierced through, so that the 

 gelatinous substance which fills up the intervals of these bones 

 forms a continued line through the whole column. The arrange- 

 ment of the bones of the shoulder, of the pelvis, and of the fins, 

 varies. Lastly, the hyoidean apparatus which supports the gills 

 is usually formed very nearly in the same manner as amongst 

 ordinary fish ; but in the lowest groups of this class (amongst the 

 Lampreys for example) the branchial arches are absent. 



526. The greater number of Fishes swim with great agility ; 

 we are told that the Salmon, for example, advances sometimes 

 with a rapidity of twenty-six feet in a second, and travels in 

 one hour the space of from twenty to twenty-five miles. In 

 general, it is through lateral strokes on the water, by the alter- 

 nate bendings of the tail and the body, that they can move in 

 this manner ; and the muscles which are destined to bend the 

 vertebral column laterally, are so developed, that they usually 

 constitute the greatest part of the mass of the body. The fins 

 on the central line, that is to say, the caudal, the dorsal, and 

 the anal, serve to increase the extent of this kind of oar : but 

 the lateral fins the pectoral and the ventral act but little in 

 progression, or forward movement ; and their principal use is 

 generally to influence the direction of the course, and especially 

 to support the animal in equilibrium. 



527. A peculiarity in the organisation of Fishes, which is 

 a great assistance to them in swimming, is the existence of a 

 kind of bag filled with air, and constructed in such a manner as 

 to be capable of being compressed at will. This swimming 

 bladder is situated in the abdomen, and under the dorsal spine ; 

 and it usually communicates with the oesophagus, or with the 

 stomach, by a canal, through which the air contained in its interior 

 can escape ; but this fluid does not appear to penetrate into it by 



