xxiv BRITISH FRESHWATER FISHES 



in the Loaches it is reduced and wholly or partly 

 enclosed in a bony capsule. 



The usual function of the air-bladder appears to 

 be a hydrostatic one, enabling the fish to accommodate 

 itself to changes of pressure at different depths by 

 the absorption or secretion of gas ; what purpose is 

 served by the connection with the auditory organ is 

 not definitely known, but probably an increased 

 sensitiveness to changes of pressure results. 



It would be out of place in this work to give an 

 account of the nervous, vascular, digestive, excretory, 

 and reproductive systems, or of the internal skeleton, 

 as it will seldom be necessary to mention them. 



In the distinction of species the greatest depth of 

 the body is usually compared with the length of the 

 fish, measured from the extremity of the snout to the 

 base of the caudal fin ; the length of the head is the 

 distance from the tip of the snout to the end of the 

 operculum. The diameter of the eye and the width 

 of the interorbital region are usually compared with 

 the length of the head, and the position of the 

 posterior end of the maxillary with reference to the 

 eye is noted. The least depth of the caudal peduncle, 

 i.e. the tail just in front of the caudal fin, is compared 

 with its length, measured from the vertical through 

 the end of the base of the anal fin to the actual base 

 of the middle caudal rays. The number of scales in 

 the lateral line, or if these be enlarged the number in 

 a longitudinal series a little above the lateral line, 

 from above the gill-opening to the caudai fin is 

 enumerated, and the number in a transverse series is 

 usually reckoned from the dorsal fin to the lateral 

 line, and thence to the base of the pelvic fins. The 



