BREATHING MECHANISM OF THE LAMPREYS. \^ 



forms a short tube connecting the gill sac with the exterior. 

 The ental valves extend lat- 

 erad and at the same time 

 dorsad and ventrad to the 

 outer end of the tubular 

 branchiopore to which the 

 ectal valve is attached. Each 

 has its concave face outward 

 and the two cover about two 

 thirds of the lumen of the 

 branchiopore. Each overlaps 

 its fellow by nearly half its 

 dorso-ventral diameter. 

 The ectal valve is a thin 



.... FlG. 10. The external branchiopore show- 



membrane which is attached ing the normal position of the ectal and ental 



to the cephalic half of the valves when water is being discharged from the 



outer margin of the external S il! sac - a > ectal valve ; <*. ental valve ; "; car ' 



, . , tilaeinous ring ; </, buccal fringe ; e, point on 



branchiopore and stretches 



the cartilaginous ring. 



loosely over the cephalic half 



of its lumen (Fig. 9). The ental valve is attached at the internal 



b c 



FIG. ii. Position of the ental valves when the ectal valve is stretched taut. </, 

 cartilaginous ring ; <?, ental valve ; c, laminre of the gill sac ; b, central divided 

 lamellae. 



end of the external branchiopore and on its caudal margin while 



