35 



oesophageal ami iiuirr hiucal sinuses and entering tlie lower sid(> of the phai-ynx 

 opens into ilie mouth at the tip of the tongue. 



The Oesophagus. Tlie oesopliagus extends from the jtharynx through the skull, 

 the oesophageal nerve ring and the liver to the stomach. It passes directly 

 backwai'd in the oesoi)hagcal sinus imtil it has passed under the cerebral ganglion, 

 then it turns slightl\- ujiward and passes thru the fascia that closes the large 

 foramen ui' the skull. This fascia separates the liver from the oesophageal nerve 

 ring, covers the visceral ganglion, and closes the dorsal end of the inner buccal 

 sinus. Having left the head , the oesophagus passes obliquely upward and back- 

 ward thru a veiy deep groove (apparently a canal) in the liver to its upp(!r 

 surface. It continues backward in a shallow groove on this surface until some 

 distance beyond the middle of the liver. At this point it passes obli(|uely downward 

 thru the liver and emerges on its lower surface between the hepatic ducts. The 

 oesophagus after leaving the livei-, passes between it and the nephridial sac, and 

 being deflected slightly to the right enters the ventral end of the stomach near 

 the middle of the visceral mass. The wall of the oesophagus is formed by a tlnCk 

 outer layer of circular muscle fibres, a thin inner layer of longitudinal muscle 

 fibres and a columnar epithelium which secretes a thin chitinous cuticle. This 

 cuticle is continuous with that which covers the surface of the odontophore and 

 the inner sides of the palatine lobes and with the lining of the stomach. 



The Stomach. The stomach and caecum are oval muscular sacs which lie 

 on the right and lett side respectively' and in-ojcu-t into the coelom fiom ir.s 

 ventral end. The nephridia and the branchial and systemic hearts are below 

 these organs ; the dorsal end of the liver is above them ; and the gonad lies 

 above and behind them. The stomach and the caecum are very distensible and 

 may occupy only the ventral third of the coelom or may extend nearly to its 

 dorsal end. 



The stomach is a thick-walled, muscular, oval sac, whose inner or left surface 

 is adherent to the inner surface of the caecum. Its wall is formed by five lay(us 

 viz. 1) the peritoneum, a sheet of ciliated pavement epithelium: 2) a thin outer 

 layer of oblique muscle fibres; 3) a thick middle layer of circulai- muscle fibres; 

 4) a thin inner sheet of oblique muscular fibres; 5) a palisadal ejiithelium that 

 secretes a chitinous cuticle which lines the stomach. There are no mucous or 

 other secretory glands in the walls of the stomach and oesophagus 



The Caecum. The caecum has a very thin wall. It is larger than the stomach 

 and is closely connected with it. The openings (which will be more fully 

 described below) connecting the caecum with the stomach , intestine , and liver 

 are close together at the ventral end of the caecum. They lie in the crest of a 



