134 



ZOOLOGY 



end.s 



■utr 



sac 



^IG. 800.— Auditory sac of Fetromyzon. 

 a. s. c, anterior semicircular canal ; aud.yi. 

 auditory nerve ; end. s. endolymphatic 

 sac ; p.s.c. posterior canal ; sac. sacculus ; 

 uir. utriculus. (After Retzius.) 



immense upper lip {up. I.), is gradually shifted to the top of the 



head (C, D), the process being accompanied by elongation of the 



pituitary sac, into which the olfactory sac opens posteriorly. 



Where the pituitary sac comes in 

 contact with the infundibulum it 

 gives off numerous small follicles 

 which become separated off and 

 give rise to the pituitary body 

 (Fig. 798, pty. h.). Thus the entire 

 nasal passage of the Lamprey, 

 including its blind pouch, is a per- 

 sistent pituitary sac into which the 

 single olfactory organ opens. More- 

 over, owing to the extraordinary 

 displacement undergone during 

 development, the pituitary sac 

 perforates the skull-floor from 



above instead of from below, as in all other Craniata. 



The auditory organ (Fig. 800) is remarkable for having only 



two semicircular canals, corresponding to the anterior {a.s.c.) and 



posterior {p.s.c.) of the typical organ. 



Organs of taste are present on the wall of the pharynx between 



the gill-sacs. 



Urinogenital Organs.— The kid- 

 neys (Figs 801 and 802, Ic) are long 



strap-shaped bodies developed from 



the mesonephros of the embryo. The 



tubules have no nephrostomes. Each 



is attached along one edge to the 



dorsal wall of the body-cavity by a 



sheet of peritoneum ; along the other 



or free edge runs the ureter (tor.), 



which is the undivided pronephric 



duct. The ureters open posteriori)? 



into a small urinogenital sinus (Fig. 



802, ii.g.s.), placed just behind the 



rectum, and opening, by a iirino- 



genital papilla {u.g.p.), into a pit in 



which the anus {a) also lies. The 



side-walls of the sinus are pierced by 



a pair of small apertures, the genital 



pores (y), which place its cavity in 



communication with the coelome. 

 The go7iad (Fig. 796, ov, Fig. 801, ts) 



is a large unpaired organ occupying 



the greater part of the abdominal cavity and suspended by a sheet 



of peritoneum. The sexes are separate, but ova have been found 



d.ao 



mt 



Fio.sOl.— Petromyzon marinus. 



Transverse section of abdomen, cd. 

 cardinal veins ; d. ao. dorsal aorta ; 

 /. T. fin-rays (neural spines) ; /. (. 

 fibrous tissue of spinal canal ; int. 

 intestine, the line pointing to the 

 spiral valve ; k, kidneys ; li/. sub- 

 vertebral lymph-sinus ; m. body- 

 muscles ; my, spinal cord ; nc. noto- 

 chord ; n. ca. spinal canal ; ts. testis ; 

 ur. ureter. (From Parker's Zootomy.) 



