238 



THE OESOPHAGUS 



1 



CHAP. ■ I 



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 muscular structure of its 

 walls changes so slowly that it is difficult to say where oesopha- 

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



"'J p.CLC- 



FiG. 145. — Gizzard of Scaphander 

 lignarius L. : A, showing posi- 

 tion with regard to oesophagus 

 (oe) and intestine {i) , the latter 

 being full of comminuted frag- 

 ments of food; p, left plate; 

 p', right plate; p.ac, accessory- 

 plate; B, the plates as seen 

 from the front, with the envel- 

 oping membranes removed, let- 

 tering as in A. Natural size. 



Fig. 144. — Alimentary canal, etc., 

 of Sepia officinalis L. : a, 

 anus; h.d, one of the biliary- 

 ducts; h.m, buccal mass; c, 

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

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

 of the liver; oe, oesophagus; 

 p, pancreatic coeca ; r, rec- 

 tum ; s.g, salivary glands ; st, 

 stomach. (From a specimen 

 in the British Museum.) 



Fig. 146. — Section of the stomach of 

 Melongena, showing the gastric plates 

 {g.p, g.p,) for the trituration of food; 

 b.d, l)iliary duct; g.g, genital gland; 

 i, intestine; I, liver; oe, oesophagus; 

 6-^, stomach. (After Vanstone.) 



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

 In the Pulmonata and Opisthobranchiata it often Avidens 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 



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