BREATHING MECHANISM OF THE LAMPREYS. 



Fi<;. 13,^. A diagram of the respiratory apparatus of P. mctrinns. The possibil- 

 ity of the water entering the gills from the mouth and the external branchiopores at 

 the same time is shown, a, gill sac ; />, ental valve ; c, ectal valve ; (/, muscular 

 gill-pouch ; e, water tube ; ?', velar valve ; k, posterior pharyngeal muscle ; h, velar 

 jaws ; ?', tongue ; /, tongue lobe. 



FIG. 13, B. Diagram of the ectal and ental valves of the external branchio- 

 pores of P. marinus. a, closed valves ; b, position of valves as water enters the gill ; 

 f, position of valves when water leaves the gill. 



