BREATHING MECHANISM OF THE LAMPREYS. 9 



other and quite rapidly tapers to its point. The space between 

 the jaws when they are open is a broad but incomplete ellipse ; 

 when closed, the points of the jaws come together forming an 

 entire but narrower ellipse. 



The jaws extend forward from the free end of the united walls 



FIG. 5. Dorsal view of the head of P. niarinns dissected so as to expose the 

 cavities of the pharynx and oesophagus, a, lateral tongue lobe ; l>, tongue ; c, pharyn- 

 geus muscle ; /, velar jaws ; e, posterior pharyngeus ; _/", gill sac ; g, gill pouch ; h, 

 external branchipore ; /, oesophagus ; x, anterior lobe of the tongue. 



of the water tube and oesophagus and in the same plane so that 

 the greater part of their bulk lies ventral to the oesophagus. The 

 position is not clearly shown in Fig. i . The plane of the jaws 

 is moreover oblique to the long axis of the pharynx which dips 



