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. 
Fic. 191.—Organs of respiration in the Myxine. a, single hooked tooth; 
6664, double rows of lingual teeth; c, branchial cells; @ dd d, tentac 
ula ; ¢, mucous glands. 
Lamprey (Petromyzon).—The lamprey eel (Fig. 192) 
lives in both salt and fresh water, and attains a length of 
three feet. When young they are blind and toothless, and 
were long considered separate animals (Ammocetes), (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. 
Nortre.—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 thesand. The young remain 
in their castles until able to protect themselves, and do not assume the 
