444 CYCLOSTOMATA [CH. 



stomodaeum, as above explained. This single sac is drawn out 

 into a long tube passing beneath the brain, and in one order, 

 the Myxinidae or Hag-fishes, this opens into the roof of the 

 stomodaeum. The tube-like prolongation is really the pituitary 

 body, which in the embryo develops close to the nasal sac. The 

 groove connecting the two organs becomes closed so as to form a 

 canal, and then by the great development of the roof of the suc- 

 torial mouth the external openings of the two organs are widely 

 removed from one another, although internally their cavities com- 

 municate with one another. 



FIG. 218. The Musk Lamprey, Petromyzon wilderi, in the act of spawning. 

 From Bashford Dean and Sumner. 



The eye develops no proper cornea or aqueous humour, the 

 lens remaining in connection with the skin. The ear is represented 

 either by two semicircular canals and a vestibule (or sacculus) 

 in the Lamprey, or by a single circular membranous tube in the 

 Hag-fish which corresponds to the vestibule and one semicircular 

 canal. 



The gill-slits, usually seven in number, have the form of regular 

 gill-sacs, recalling those of the Hemichorda, only without the 



