STONE-SUCKERS. 321 



few in number, about fourteen species being the 

 whole of those known to naturalists. They are 

 found both in fresh and salt waters, principally 

 in the northern parts of the world. Six of the 

 number are enumerated as British. 



Genus Petromyzon. (Linn.) 



The Lampreys have a smooth, elongated, 

 cylindrical body like that of an Eel. There are 

 seven gill-apertures on each side ; the mouth is 

 circular, and its inner surface is studded with 

 hard, crusted tubercles, answering the purpose 

 of teeth. The tongue, which moves backwards 

 and forwards like a piston, has two rows of 

 small teeth. The skin, elevated in a fold around 

 the extremity of the body, answers to dorsal, 

 caudal, and anal fins. 



The generic name applied to these fishes, 

 Petromyzon, signifies Stone-sucker ; and refers 

 to a curious habit depending on the structure 

 of the mouth. The animal applying its circular 

 lip to the surface of a stone or other solid body 

 in the water, draws in the piston-like tongue ; 

 a vacuum is thus produced in the mouth, while 

 the pressure of the super-incumbent body of 

 water causes the lip to adhere to the stone with 

 immense tenacity, until by the protrusion of the 

 tongue the vacuum is voluntarily destroyed. 



It is supposed that the Lamprey resorts to 

 this singular expedient to prevent its being con- 

 stantly carried down by the current of the rivers 

 in which it lives ; its powers of locomotion being 

 feeble. But Sir William Jardine has shown that 

 a much more obvious end is effected by the same 



Y 



