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TEXTBOOK OF ZOOLOGY 



the single nasal chamber which is located immediately anterior to 

 the brain. Extending ventrally from the nasal chamber and project- 

 ing beneath the brain to end blindly just above the esophagus is the 

 pituitary pouch. As it passes beneath the diencephalon it makes 

 contact with the infundibulum. The eyes of this animal are not 

 highly developed, and sight is not used extensively by it. The audi- 

 tory organ, which does not include an organ of hearing, is only for 

 equilibrium ; it consists simply of a vestibule and two vertical semi- 

 circular canals. The sense of taste centers in taste buds located in 

 the respiratory tubes between the gill slits and possibly near the 

 inner margin of the buccal funnel. 



The urinogenital system shows only fair development. The rib- 

 bonlike kidneys lie, one at each side of the notochord and just dorsal 



P AU HB OA SC N PfJ VN OE MM M 





LP-. 



OH OP 



cc vf a;jc t c 



VA V A G L F 



AN CF 



Fig. 227. — Ammocoetes larva of the lamprey, Entosphentos tridentatus. A, 

 auricle of heart; AN, anus; ANC, anterior end of notochorcl ; AU, ear; BA, bran- 

 chial arteries (afferent) ; G, duct connecting pharynx and thyroid ; CC, cranial 

 cartilage, extending from tip of upper lip to a point slightly anterior to end of 

 notochord, where it divides to form two lateral rods ; CF, caudal fin ; DA, dorsal 

 aorta ; F, folds in intestinal wall, suggesting a possible origin of the spiral valve ; 

 FB, forebrain ; G, gall bladder ; HB, hind brain ; I, intestine ; L, liver ; LP, upper 

 lip, supported by cranial cartilage; M, body muscles (myotomes) ; MB, midbrain; 

 MA'', position in which mesonepliros will develop; N, notochord; O, eye; OE, esorh- 

 agus ; OH, oral hood; OL, olfactory organ; OP, oral papillae; P, pineal body; 

 PC, pericardial cavity ; PN, pronephros, showing pronephric tubules with their 

 ciliated funnels (nephrostomes) ; SC, spinal cord; T, thyroid; UL, under lip; V, 

 ventricle of heart; VA, ventral aorta; VE, velum; VN, hepatic vein. (Courtesy 

 of Albert E. Galigher, Inc.) 



to the peritoneal lining of the body cavity. A mesonephric duct ex- 

 tends posteriorly along the free edge of each to join the small urino- 

 genital sinus. This is located just posterior to the rectum and opens 

 externally by the urinogenital papilla just behind the anus. The single 

 gonad is rather large and is suspended by a peritoneal fold into the 

 coelom. The sexes are presumably separate, but hermaphroditic con- 

 ditions are occasionally found. Germ cells when mature are dis- 

 charged from the gonad into the body cavity and go by way of two 



