68 



BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



lip, attached to the ventral branch of the retractor. The first of 

 these, the adrnarginal sphincter, is of moderate width. The second 

 is still narrower. Distally it fuses with the third sphincter. The 

 dorsal horizontal bands, which in most species lie between the basal 

 oral sphincter and the first body muscle, are wanting in this species. 

 There is an atrial retractor muscle on each side (fig. 43), internal 

 to the sphincters, of which there are fourteen in each of several indi- 

 viduals studied. The basal two sphincters are broad and strong; 

 sphincters 6 to 12 (numbering from the aperture) are attached to the 

 ventral edge of the retractor. In sphincters 7 or 8 to 12 only part 



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IM.z 



flg. 42.— rltteria nexagona, aggregated form, oral muscles of the left side seen from the 

 inside. (Drawn by Hoyt S.Hopkins.) 



of the fibers bend and run into the retractor; other fibers pass across 

 the retractor, making complete hoops. The distal five sphincters are 

 not connected with the retractor muscle. The terminal three are 

 very delicate. The fibers of the sphincter muscles, like those of the 

 body muscles, form an even single layer. The fibers are flat, and lie 

 edge to edge, presenting a perfectly regular appearance. The fibers 

 of the retractor, on the other hand, are elliptical, not flat, in cross 

 section, and arc more irregularly arranged. 



In all the body muscles and in the wider atrial sphincters one fiber, 

 or sometimes two or three fibers, at each edge of each muscle band 



