238 



THE OESOniAGUS 



lies between the pharynx and the stomach (in Pelecypoda 

 between the mouth and stomach) is known as the oesophagus. 

 Its exact limits are not easy to define, since in many cases the 

 tube widens so gradually, while the nuiscular structure of its walls 

 changes so slowly that it is difficult to say where oesophagus 

 ends and stomach begins. As a ride, the oesophagus is fairly 



Fig. 1-15. — Gizzard of Scaphander 

 lignarius L. : A, showing posi- 

 tion with regard to oesojjhagus 

 (oe) and intestine (/), the latter 

 being full of comminuted frag- 

 ments of food ; ^j, left plate ; jj', 

 right plate ; ij.ac, accessory 

 plate ; B, the plates as seen from 

 the frout, with the enveloping 

 membranes removed, lettering 

 as in A. Natural size. 



Fig. 144. — Alimentary canal, etc., 

 of Sepia officinalis L. : a, 

 anus; h.cl, one of the biliary 

 ducts ; h.m, buccal mass ; c, 

 coecum ; i, ink-sac ; i.d, duct 

 of same ; j, jaws ; l.I, lobes 

 of the liver ; oe, oesophagus ; 

 p, pancreatic coeca ; r, rec- 

 tum ; s.g, salivary glands ; st, 

 stomach. ( From a specimen iu 

 the British Museum. ) 



Fig. 146. — Section of the stomach of 

 Alelongena, showing the gastric plates 

 (r/.p, ii.p,) for the trituration of food ; 

 h.d, biliary duct ; fj.g, genital gland ; 

 /, intestine ; /, liver ; oe, oesophagus ; 

 st, stomach. (After Vanstone.) 



simple in structure, and consists of a straight and narrow tube. 

 In the Pulmonata and Opisthobranchiata it often widens out 

 into a ' crop,' which appears to serve the purpose of retaining a 

 quantity of masticated food before it passes on to the stomach. 

 In Octopus and Patella the crop takes the form of a lobular 

 coecum. In the carnivorous Mollusca the oesophagus becomes 

 complicated by the existence of a varying number of glands, by 



