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PHYLUM CHORDATA 



125 



opens just outside the stomodseum instead of within it as in other 

 Craniata: its inner or blind end extends to the ventral surface 

 of the fore-brain and terminates just below the infundibulum (inf.). 

 As development goes on, the olfactory and pituitary invaginations 

 become sunk in a common pit (B), which, by the growth of the 

 immense upper lip (up.l.\ is gradually shifted to the top (C, D) of 

 the head, 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 



Ticfl 



Fiii. 755. Petromyzon. Diagrams of four stages in the development of the olfactory and 

 pituitary sacs. ent. mesenteron ; inf. infundibulum ; 1. Ip. lower lip ; nch. nptochord ~ 

 off. s. olfactory sac ; pn. pineal body ; pty. s. pituitary sac ; stdm. stomodaeum ; u. Ip. 

 upper lip. (Altered from Dohrn.) 



give rise to the pituitary body (Fig. 754, pty. &.). Thus the entire 

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

 persistent pituitary sac into which the single olfactory organ opens. 

 Moreover, 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. 756) is remarkable for having only 

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

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



Urinogenital Organs. The kidneys (Figs. 757 and 758, k.) are 

 long strap-shaped bodies developed from the mesonephros of the 



