184 



ZOOLOGY 



SEf'T. 



e/z d.s 



a.s.c 



"ulr 



s.c 



sac 



cctccl.71 



Fig. SOO.— Auditory sac of Petromyzon. 

 «. s. c. aiitei'iov semicircular canal ; iiud.n. 

 auditorj' nerve ; end. s. endolymphatic 

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

 ntr. ntriculus. (After Retzius.) 



immense upper lip (up. l), 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 postel•iorl3^ 



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, ptij. I.). Thus the entire 

 nasal passage of the Lamprey, 

 including its blind pouch, is a per- 

 sistent pituitary sac into which the 

 single olfjictory organ opens. More- 

 over, owing to the extraordinaiy 

 displacement undergone during 

 development, the pituitary sac 

 perforates the skull-fioor from 



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



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



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



posterior (p.s.f.) of the typical organ. 



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



the gill-sacs. 



Urinogenital Organs. — The hid- 



nci/s (Figs 801 and 802, Jc) 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 (nr.), 



which is the undivided pronephric 



duct. The ureters open posteriori}^ 



into a small urinogcnital sinus (Fig. 



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



rectum, and opening, by a tirino- 



genital p)0L23iUa (u.g.j).), 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 ccfilome. 

 The gonad (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 



Fig. SOI. —Petromyzon marinus. 



Ti-ansverse section of abdomen, al. 

 cardinal veins ; d. an. dorsal aorta ; 

 f. r. fin -rays (neural spines) ; /. t. 

 fibrous tissue of spinal canal ; int. 

 intestine, the line pointing to the 

 spiral valve ; A-, kidneys ; Iv. sub- 

 vertebral lymph-sinus ; w. body- 

 muscles; )/(y, spinal cord ; ?ic. noto- 

 chord ; n. ca. spinal canal ; <.«. testis ; 

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



