Phylum Chordata 573 



genital sinus, passing out through the urogenital opening. Fertilization 

 is external. In the adult lamprey, the sexes are always separate. In 

 the immature form, however, the gonad is not differentiated into either 

 testis or ovary. 



The Life History of Petromyzon marinus. — When the sea lam- 

 prey becomes sexually mature in early summer, it migrates up streams 

 to shallow areas over gravel bottoms. Here they move stones about 

 by means of the buccal cavity to produce a small depression in which 

 the eggs are deposited. The female attaches herself to a stone in the 

 nest by means of the buccal cavity while the male twines himself about 

 her. The eggs and sperm are discharged simultaneously with a single 

 female releasing from 24,000 to 107,000 eggs at a time. The larvae 

 hatch in about 20 days and gradually drift downstream with the cur- 

 rent. When they encounter a mud bottom, the larvae burrow to form 

 a U-shaped tube and remain in this area for approximately four years. 

 At this time, the larvae are known as ammocoetes. In many ways they 

 resemble amphioxus in appearance : they are blind and feed on mate- 

 rial carried in by the water currents ; also they possess an endostyle 

 which produces mucus for trapping and concentrating food. 



VELUM 



PHARYNX 



ENDOSTYLE 



Fig. 192. — Ammocoetes larva of the lamprey. 



After remaining as an ammocoetes for about four years, the lam- 

 prey metamorphoses into the adult. It then must feed on other fish. 

 It does this by attaching to the fish by means of the buccal funnel, rasp- 

 ing a hole into the body wall with its teeth, and then sucking the blood 

 and bits of fiesh. The effect of these attacks on fish is often fatal 

 to the host. 



Economic Importance of Petromyzon. — Attacks of this sort on 

 economically important food fish often attain great importance. This 

 is particularly true in areas where the lampreys have but recently mi- 

 grated. In recent years, the damage done by the lampreys has become 

 of great importance in the fresh-water fisheries of the Great Lakes. 



