CYCLOSTOMATA 421 



genital pores into the iirinogenital sinus, then out through the papilla 

 to the environmental water where fertilization occurs. 



The life history may be summarized as follows: 



a. The eggs which contain considerable yolk in the vegetal por- 

 tion and are about one millimeter (%5 inch) in diameter are laid in 

 fresh-water streams, usually between March and June for all kinds 

 of lampreys. The eggs first stick to objects, then fall in the sand. 

 They are fertilized in the water almost immediately after laying. 

 Cleavage follows, in about six hours when the optimum temperature 

 of 22.5° C. prevails. At 20° C, this division requires nine days. 



b. The adults spawn but once and then die. 



c. A tadpolelike larval form, ammocoetes or mud lamprey, hatches 

 from the egg and lives from four to five years in the mud along the 

 streams where the eggs are laid. 



d. At the end of four or five years the ammocoetes undergo meta- 

 morphosis to become adult. They remain under the mud from July 

 or August to February or March while undergoing this transforma- 

 tion to adult condition, 



e. The sea lamprey then migrates to the ocean and the lake lamprey 

 moves down stream to a large fresh-water lake. They both become 

 parasitic on other fish and continue this existence for from one and 

 one-half to three and one-half years, when they return to fresh-water 

 streams to breed again. 



