130 



ZOOLOGY 



SECT. 



e-nd.a 



passage of the Lamprey, including its blind pouch, is a persistent 

 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. 810) 

 is remarkable for having only 

 two semicircular canals, corre- 

 sponding to the anterior (a.s.c.) 

 and posterior (p.s.c.) of the typical 

 organ. 



Organs of taste are present on 



O..S.C 



.s.e 



sac 



CLU.cl.7l 



n.ca 



FIG. sio. Auditory sac of Petromyzon. the wall of the pharynx between 



a. s. c. anterior semicircular canal ; aiul.n. ,1 -n j 



auditory nerve : end. s. endolymphatic the glll-SaCS, and neuromast- Or 



sac ; p.s.c. posterior canal ; sac. sacculus ; lpt pra l linp nro-an arp nrppnf nn 



vtr. utriculus. (After Retains.) 



the head and trunk. 



Urinogenitai Organs. The kidneys (Figs. 811 and 812, k) are 

 long strap-shaped bodies developed from the mesonephros of the 

 embryo. The tubules have no nephrostomes. Each kidney 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 (ur.), 

 which is the undivided pronephric 

 duct. The ureters open posteriorly 

 into a small urinogenital sinus (Fig. 

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

 rectum, and opening, by a urinogenital 

 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 communica- 

 tion with the ccelome. 



The gonad (Fig. 806, ov, Fig. 811 

 ts) is a large unpaired organ occupy- 

 ing the greater part of the abdominal 

 cavity and suspended by a sheet of 

 peritoneum. The sexes are separate, 

 but ova have been found in the testis 

 of the male* The reproductive pro- 

 ducts are shed into the ccelome and 

 make their way by the genital pores 

 into the urinogenital sinus, and so to the surrounding water, where 

 impregnation takes place. 



Development. The oosperm is telolecithal, having a con- 

 siderable accumulation of yolk in one hemisphere ; in correspon- 



int 



FIG. 811. Petromyzon marinus. 



Transverse section of trunk, cd. 

 cardinal veins; d. ao. dorsal aorta; 

 /. r. fin-rays ; /. t. fibrous tissue 

 of spinal canal ; int. intes- 

 tine, the line pointing to the 

 spiral valve ; k, kidneys ; lit. sub- 

 vertebral lymph-sinus ; m. body- 

 muscles ; my. spinal cord ; we. noto- 

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

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



