1901.] 



NATURAL SCIENCES OF PHILADELPHIA. 



675 



side. These fibres may he almost invariably traced out of the 

 circular sheath around the intestine into dorso-ventral fibres coming 

 from the circular muscle of the body wall, or else into the circular 

 muscle of the proboscis sheath. This is illustrated by Diagrams 3 

 ^nd 4. The same is true in regard to the scattered fibres occa- 



PS 



oC M 



Dv f 



B 



/;// 



Diagram 4. — Cross section thronoh the body of Linens sp. ; illustrating 

 the deflection of the dorso-ventral muscle tibres into the inner circular mus- 

 cle layer around the intestine. — P.S., proboscis sheath; o.C.M., outer 

 circular muscle ; Int., intestine; i.C'.M., inner circular muscle; Dv.f., 

 dorso-ventral muscle tibres. 



sionally found above or below the oesophagus in Zygeupolia, which 

 come from the body wall or the proboscis sheath, see Diagram 5. 



It therefore seems that, if we accept the view of Biirger that the 

 dorso-ventral fibres are derived from an original inner circular 

 muscle layer, we may profitably go a stej) farther and say that 

 the " Darmmusculatur " or " esophageal muscle," found around 

 the dorsal as well as the ventral side of the oBSophagus, is derived 

 from the dorso-ventral fibres which have turned aside from their 

 dorso-ventral course and have curved around the oesophagus so as 

 to partially encircle it. If this is accepted we may then say that 

 the " oesophageal muscle" is secondarily derived from the inner 



