502 Arthur E. Shipley 



The mouth is a transverse slit in the median posterior part of the 

 ciliated groove: itleads luto a somewhat narrow Oesophagus which lies 

 against the posterior curved border of the triangulär septum (fìg. lì. At 

 the bottoni of this border, the Oesophagus turns toward the posterior end 

 of the animai and immediately enlarges into the stomach . This is giobular 

 in shape and receives upon each side the openiug of the liver (fìg. 5) . 

 The stomach narrows posteriorly and passes into a conical intestine which 

 bends towards the ventral shell between the two brood pouches and ends 

 between the occlusor muscles. There is no amis. The w^alls of the 

 alimentary canal are of nearly the same thickness throughout. being 

 only a little thinner in the stomach and a little thicker in the posterior 

 part of the intestine vrhere the lumen begins to disappear. They are 

 lined throughout by cilia, continuous v^ith those of the ciliated groove. 

 On the outside the alimentary canal is enclosed by a membraneous sheath 

 which is prolonged into numerous Strands and mesenteries ; these pass 

 into the body wall lining the shell and serve to support the structure. 

 Within this sheath is a basement membrane and within this a thick layer 

 which stains deeply and appears to be composed of muscular fibres. The 

 inside is lined by very high epithelium cells which bear the cilia. 



The inner coat of the Oesophagus and stomach is sometimes broken 

 up into broad ridges, by narrow depressions, but this is not always the case. 



The alimentary canal is connected with the posterior ridge of the 

 septum and with the dorsal shell by a uumber of connective tissue 

 Strands. Öimilar Strands are also found on the ventral side of the canal; 

 these unite to form a double mesentery which passes from the canal to 

 the body wall. More posteriorly the double mesentery fuses to form a 

 single band which lies in the median line and is at first connected with 

 the body wall behind the sub-oesophageal nerve ganglion, but after- 

 wards it unites with that part which covers the median ridge of the ven- 

 tral shell (fìg. 6and7;^^j. This corresponds with the mesentery of 

 Huxley. In the anterior part where the mesentery is double, each sheet 

 is continuous with the inner wall of the lophophore, so that the cavity 

 of the lophophore opens into that part of the body cavity which lies 

 between these two membranes. The other mesentery serving to support 

 the intestine corresponds with Huxley's gastro-parie tal band. It 

 Stretches from the line where the lophophore fuses with the mantle, to 

 the side of the intestine, dividing the body cavity into two parts. Io 

 the half next the shell, the ova and part of the occlusor muscles He, 

 in the other the branches of the liver (fìg. 6 gp] . 



Anteriorly this mesentery splits into two layers, oue passiug to fuse 



