POUCH-GILLED VERTEBRATES. 153 



erally found in deep water, where they are parasitic on 

 various fishes. The teeth are represented by two comb- 

 like rows on the tongue. There is also a single median 

 or middle tooth (Fig. 191, a). They secrete an enormous 

 quantity of slime, and a single hag has so filled four cubic 

 feet of water that the mass could be lifted out with a 

 stick, forming, according to Couch, a continuous sheet 

 The eggs are large, and covered with horny cases, having 

 short filaments that wind about sea-weed. 



FIG. 191. Organs of respiration in the Myxine. a, single hooked tooth ; 

 b b b b, double rows of lingual teeth ; c t branchial cells ; d d d d, tentar.- 

 ula ; e, mucous glands, 



Lamprey (Petromyzon). The lamprey eel (Fig. 192) 

 lives in both salt and fresh water, and attains a length ot 

 three feet. When young they are blind and toothless, and 

 were long considered separate animals (Ammocartcs), (Fig 

 192). The adults have sunken eyes, and teeth on the car- 

 tilage supporting the lips. The mouth is a sucker, the 

 tongue acting as a piston, and during the breeding-season 

 they use it to advantage in building their nests. They 

 follow the shad up rivers in the spring, deposit their eggs, 

 and return to the ocean in the autumn. ^ They are eaten 

 in England. 



NOTE. The nests are formed of piles of rocks weighing several 

 pounds, which are brought from up-stream by successively lifting them 

 from the bottom, allowing the tide to carry them along. Some of the 

 nests are three feet high and four in circumference, and in some cases 

 the eggs are deposited in mere hollows in the sand. The young remain 

 in their castles until able to protect themselves, and do not assume the 



