54 



BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



The muscles of the solitary Ritteria retracta are numerous, the body- 

 muscles being 15 in number, as I would number them. Body muscles 

 I to X are complete hoops, while body muscles XI to XV are interrupted 

 ventrally, as is also the intermediate muscle. The oral muscles are 

 distinctly of the Cyclosalpa type. There is a single oral retractor which 

 passes outside the intermediate muscle. Apparently three sphincters 

 are present in the lower Up, one of which (ZJ, very wide, seems to 

 lie on the incurved portion of the lip. There is in Hitter's figure a 

 delicate, imperfect, first sphincter at the edge of the upper lip, at the 



i.m. 1 



a.o. 



a,o. 



Fig. 25.— Ritteria retracta, solitary form: A , dorsal view; B, view from the left side. 



DIAMETERS. FROM RlTTER (1906, b). 



Xl{ 



angles of the mouth, and a second, very broad sphincter in the upper 

 Up, of whose connections ventrally I am not quite sure from Ritter's 

 figure, though it seems to connect with the second and third sphincters 

 of the lower lip. Dorsal, horizontal muscle bands lie, one on each 

 side, between this broad sphincter and the upper ends of the inter- 

 mediate muscle, but are not connected with the latter. Ritter's 

 dorsal and side views disagree as to the union of this horizontal band 

 with the sphincter The atrial muscles, likewise, are distinctly of tho 

 Cyclosalpa type, most resembling those of the solitary C. virgula in 

 the character, position, and connections of both retractor and sphinc- 

 ters, but there is resemblance also to the aggregated C. virgula in 

 the fact that all the distal atrial sphincters are connected with the 

 retractor. 



