362 



COLLEGE ZOOLOGY 



Digestive system 



The adult Petromyzon lives chiefly on the 

 blood of fishes. The expansion of the oral 

 funnel (Fig. 241) causes the mouth to act 

 like a sucker and enables the animal to cling 

 to stones or to fasten itself to fishes such as 

 shad, sturgeon, cod, and mackerel in the 

 ocean, and lake trout, whitefish, yellow pike- 

 perch, and carp in the Great Lakes. With its 

 rasplike tongue, it files a hole through the 

 scales and flesh of its victim and sucks out 

 the blood. 



Figure 241. The head of a sea lamprey showing 

 the oral funnel which serves as a suction cup by 

 which it attaches itself to its prey. It is by means of 

 the sharply pointed horny teeth inside the oral fun- 

 nel and the rasplike tongue that it can penetrate 

 through the scales and flesh of its victim. Arrow 

 points to pistonlike tongue. (Courtesy of Institute 

 for Fisheries Research, Michigan Department of 

 Conservation.) 



The mouth cavity opens at its posterior 

 end into two tubes (Fig. 242), an upper 

 one, the esophagus, and the ventral one, the 

 pharynx. A fold, the velum, at the anterior 



end of the pharynx prevents the passage of 

 food into the respiratory system. 



There is no distinct stomach. The 

 posterior end of the esophagus is separated 

 from the straight intestine only by a valve. 

 A fold in the intestine called the typhlosole 

 forms a sort of spiral valve. The digestive 

 tract ends at the small anus. A liver is 

 present, but there is usually no bile duct in 

 the adult; it is not definitely known whether 

 or not there is a pancreas. 



Circulatory system 



Petromyzon possesses a heart, a number 

 of veins and arteries, and many lymphatic 

 sinuses. The heart (Fig. 242) lies in the 

 pericardial cavity, and consists of a ventricle 

 which forces the blood into the arteries and 

 an atrium which receives the blood from 

 the veins. A renal portal system is absent. 



Respiratory system 



Respiration is carried on by means of 7 

 pairs of gill pouches, which open to the out- 

 side by the gill slits and internally to the 

 pharynx. Each gill pouch contains numer- 

 ous gill filaments that contain many capil- 

 laries in which the blood is oxygenated by 

 the water in the pouch. In the adult lam- 

 prey, water is taken into the gill pouches 

 through the external gill slits and is dis- 

 charged through the same openings (Fig. 

 242). This method, which is unlike that in 

 the true fishes, is necessary because the 

 lamprey, when attached to its food by its 

 oral funnel, cannot take water through the 

 mouth. However, in a larval lamprey, the 

 water used in respiration passes in through 

 the mouth and out the gill slits as in fishes. 



Nervous system 



The brain (Fig. 242) of the adult lamprey 

 is very primitive. The forebrain consists of 

 a large pair of olfactory lobes; behind these 

 are the small cerebral hemispheres attached 



