PETROMYZON BRANOHIALIS. 429 



specimens measure seven inches. It is very hardy, and its habits are like 

 those of the earth-worm. Its ovaries are well developed. 



The fish remains in this state for from three to four years (Fig. 211). 

 And then the metamorphosis is completed in about ten days, during which 

 the oral disc with its tentacles and teeth are formed, the barbel-like processes 



Mouth. 

 Fig. 211. LARVAL FORM OF PETROMYZON BRANCHIALIS, FORMERLY KNOWN AS AMMOC(ETES. 



in the mouth disappear, the eyes develop, the respiratory tube is completed, 

 the intestine and whole body shorten, and the reproductive organs become 

 opaque. The bright silver colour is put on, and the fins grow and become 

 yellowish- white. With these modifications the habits of the fish change, 

 and now having become adult it seeks clear water, uses its suctorial disc, 

 and no longer limits its diet to microscopic forms of food. 



