64 



BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



The upper lip bears a broad third sphincter with a very delicate second 

 sphincter branching from its base and a rather narrow admarginal 

 sphincter attached at its base to both the oral retractor and the third 

 dorsal sphincter. The lower lip has a rather narrow admarginal 

 sphincter, continuous at each angle of the mouth with a rudimentary 

 oral retractor muscle whose unconnected posterior end lies horizon- 

 tally between the intermediate muscle and body muscle I. A broad 

 second sphincter lies near the base of the incurved flap of the lower lip. 



O.r. 



FlO. 36.— RlTTERlA IIEXAGONA, SOLITARY FORM, ORAL MUSCLES OF THE LEFT SIDE, SEEN FROM THE INSIDE 

 X 5 DIAMETERS. (DRAWN BY HOYT S. HOPKINS.) 



A third sphincter arises from the oral retractor in front of the second. 

 It is at first broad, but soon narrows to a thread, which lies just 

 beneath the second sphincter, that is, on the endodermal side of the 

 incurved flap. Both lips are seen to be incurved, the ventral more 

 strongly than the dorsal. There are no horizontal muscles lying 

 between the basal oral sphincter and body muscle I, as there are inmost 

 species. 



On each side of the body, posteriorly, is a well-developed horizontal 

 atrial retractor muscle (fig. 37) continuous with the very numerous 



