668 TROCEEDINGS OP THE ACADEMY OF [DeC. , 



figured in fig. 23, tl e inner circular muscle layer suddenly ends 

 and the first pair of cfeca of the middle intestine appear. On 

 examining the sections anterior to fig, 23, it is found that the 

 inner circular muscle extends forward for about 1.8 mm. as a very 

 thin layer of a few fibres (see Plate XLII, fig. 30, i.C.M.), 

 the thickened region, as in fig. 23, only extending over a few sec- 

 tions. 



At its beginning the inner circular muscle, as seen in cross sec- 

 tions, measures about .006 mm. dorso-ventrally; over the greater 

 part of its extent the measurements range from .012— .017 mm. ; 

 then suddenly increase from .023 to .06 in about five sections of 

 6/x each; finally just before the end, fig. 23, the layer is .087 mm. 

 thick. 



The inner circular muscle encloses a band of longitudinal muscle 

 fibres on the dorsal side of the stomach, below the rhynchocoel, Plate 

 XLII, figs. 23, 30, i.L.M. 



The fibres of the inner circular muscle layer are direct continua- 

 tions of fibres from the circular layer of the proboscis sheath. 

 Furthermore, in the anterior part of the layer, the fibres come 

 from the inner surface of the proboscis sheath, bend out, ci'oss the 

 outer fibres and then continue down around the stomach. This 

 bending out and crossing is shown in fig. 30, z. In a more poste- 

 rior position, the inner circular layer becomes thicker and consists 

 of fibres from the outermost part of the proboscis sheath, together 

 with fibres from ihe circular muscle of the body wall. It is thus 

 seen that the partial origin of the inner circular muscle layer of 

 Zyge^ipoUa from the proboscis sheath circular muscle is beyond a 

 doubt. 



An inner circular muscle layer has not been heretofore described, 

 as such, for any Heteronemertean. I have found a very similar 

 layer in Micrnra cteca, with a considerable thickening at the poste- 

 rior end, but this has not Ijcen mentioned in any published work, 

 so far as I am aware. Coe (1901) describes in Micrnra alaskensis 

 a structure that might properly be called an inner circular 

 muscle layer. He says, p. 72: " The delicate layer of circu- 

 lar and longitudinal muscular fibers which surrounds the epi- 

 thelial lining of the esophagus in most of the Heteronemer- 

 teans becomes remarkably developed in this species. At the 

 very posterior end of the esophagus — just anterior to the 



