CHORDA I'A 



271 



they are popularly classed as fishes, they differ so strikingly 

 from the true fishes that zoologists place them in a class by 

 themselves. Lampreys (Fig. 282) are found in both fresh and 

 salt water, and the salt-water species ascend the rivers to spawn ; 

 the hag-fishes are marine. They all have elongated bodies < 

 ered with a smooth, slimy skin. The paired appendages which. 

 are present in most Vertebrata are entirely wanting. There may 

 be one or two dorsal fins and a caudal fin more or less well de- 



FlG. 282. Petromyzon wilderi, the musk lamprey, in the act of spawning. (From Shipley 



and MacBride's Zoology.) 



veloped. The largest lamprey is a marine species, Petromyzon 

 marinus, which attains a length of a meter ; the fresh-water 

 species vary from about fifteen to sixty centimeters. At the 

 anterior end of the body on the ventral side is a large, funnel- 

 shaped depression, whose walls are covered with conical chiti- 

 nous projections or teeth (Fig. 283). At the deep end of the 

 funnel is the circular mouth, without jaws, through which the 

 muscular tongue, provided with rasplike teeth, may be protruded. 

 There is a pair of well-developed eyes, and on the dorsal side of 



