364 



COLLEGE ZOOLOGY 



organs of taste are situated between the gill 

 pouches on the pharyngeal wall. The organ 

 of smell is an olfactory sac (Fig. 242) 

 which lies in the nasal capsule; this com- 

 municates with the outside by a nasal open- 

 ing on the dorsal surface between the eyes. 

 The olfactory sac gives off ventrally a tube 

 of unknown function, called the pituitary 



sac. 



The "ears" (balancing organs) of Petro- 

 myzon, which lie in the auditory capsule, 

 have only two semicircular canals instead of 

 the usual three. The hagfish has only one. 

 The eyes of the adult lamprey, though primi- 

 tive, are excellent visual organs. Besides the 

 paired eyes there is a well-developed median 

 pineal eye just behind the nasal opening. 



Endocrine glands 



Where the pituitary sac comes in contact 

 with the infundibulum of the brain, it gives 

 off numerous small follicles which become 

 separated, forming the pituitary gland. The 

 endostyle of the larva, which has been stud- 

 ied by means of radio-iodine, is the fore- 

 runner of the thyroid gland of the adult 

 lamprey. 



Urogenital system 



The excretory and reproductive systems 

 are so closely united in the lamprey that it 

 is customary to treat them together as the 

 urogenital system. The kidneys lie along 

 the dorsal wall of the body cavity, and each 

 pours its secretions by means of the urinary 

 duct into the urogenital sinus, and thence 

 to the outside through the urogenital open- 

 ing. The sexes are separate in the adult; 

 however, the immature gonad is hermaphro- 

 ditic, but later becomes male or female for 

 an individual. The single gonad fills most 

 of the abdominal cavity at the time of sex- 

 ual maturity. There is no genital duct; eggs 

 or sperms break out into the coelom, make 

 their way through two genital pores into 

 the urogenital sinus, and then pass out 



through the urogenital opening into the 

 water, where fertilization occurs. 



Reproduction 



The sea lampreys become sexually mature 

 in May or early June; then both sexes mi- 

 grate into streams, sometimes "hitchhiking" 

 on a passing boat. They seek a gravelly bot- 

 tom under moderately fast-flowing water; 

 and, by means of the oral hood, move 

 stones on the bottom of the stream to form 

 a shallow rounded depression called a nest. 

 The female then fastens to a stone in the 

 nest and the male attaches to the female by 

 the use of their oral funnels. Partly en- 

 twined, they move back and forth as sperms 

 and eggs are discharged, fertilization taking 

 place in the water. Each female sea lamprey 

 produces 24,000 to 107,000 eggs, depending 

 on her size. The average female lays 62,500 

 eggs and dies after spawning. The eggs hatch 

 out into larvae, known as "ammocoetes," 

 in 20 to 21 days. The blind harmless larvae 

 make their way out of the nest and drift 

 downstream until quiet water is reached. 

 Here they dive and burrow into the bottom 

 if it is mud or silt. The larval period spent 

 in burrows is, recent studies indicate, at 

 least 7 to 8 years, and possibly longer in 

 duration. Inspiration of water for respirator)' 

 purposes appears to be largely responsible 

 for drawing of food organisms in the mouth 

 of the larvae. Thereafter, food particles are 

 carried forward to the esophagus by action 

 of the cilia in certain areas of the pharynx. 

 An endostyle on the floor of the pharynx 

 secretes mucus which entangles the food, as 

 in the amphioxus. 



The ammocoetes lies buried in mud and 

 sand and probably keeps its skin free from 

 bacteria, fungi, and other parasitic growths 

 by means of an integumentary secretion. In 

 the winter of the seventh or eighth year, the 

 larval lamprey undergoes a metamorphosis, 

 after which it migrates to the sea, or, if it is 

 in the Great Lakes region, to one of the 

 lakes, where growth to sexual maturity takes 



