CHAPTER X 

 ORGANS OF DIGESTION AND FOOD OF THE OYSTER 



Page 



Mouth — - - — - 219 



Esophagus and stomach - 219 



Gastric shield - - - 222 



Crystalline style. 223 



Formation --- -- - --- 225 



Chemical composition... 225 



Midgut and rectum 226 



Digestive diverticula 226 



Alimentary tract and formation of feces 226 



Digestion 228 



The pH content of the gut andstomach 230 



Absorption of food by the gills and mantle .- 231 



Food and feeding. 231 



Artificial feeding. 234 



Bibliography 235 



The system of organs concerned with the in- 

 gestion and digestion of food and elimination of 

 feces consists of the mouth, short esophagus, 

 stomach, crystalline style sac, digestive diver- 

 ticula, midgut, rectum, and anus (fig. 197). With 

 the exception of a short section of the rectum, the 

 entire alimentary canal lies within the visceral 

 mass and is completely immobilized by the sur- 

 rounding connective tissue. In the absence of 

 peristaltic movements the food is moved from the 

 mouth toward the anus exclusively by the strong 

 ciliary motion of the epithelial lining of the system. 

 It is difficult to reveal all anatomical details of the 

 alimentary tract by dissecting the tissues. Better 

 preparations can be obtained by making casts of 

 latex or other suitable material injected through 

 the esophagus or via the rectum and left until it 

 sets. Yonge (1926a) used for this purpose a warm, 

 concentrated solution of gelatin. Awati and Rai 

 (1931) employed a mixture of paraffin and resin 

 colored with carmine. Satisfactory results were 

 obtained in the Bureau's shellfish laboratory by 

 using red or yellow latex mjected into the esop- 

 hagus through a wide mouth pipette with a rubber 

 balloon. The preparation was immediately placed 

 in 5 percent formalin in which the latex sets and 

 remains tough and flexible for a long time. Casts 

 of the entire alimentary canal or of its parts can 

 be obtained in this way. 



FISHERY bulletin: VOLUME 6 4, CHAPTER X 



MOUTH 



The mouth is a compressed U-shaped slit be- 

 tween the two lips (fig. 104) and is lined with 

 columnar ciliated epithelium set on a narrow basal 

 membrane. The epithelial cells of the mouth are 

 taller than those of the labial palps and contain 

 only a few mucous glands. In the surrounding 

 connective tissue are large vesicular cells, num- 

 erous muscle fibers, and blood spaces which are 

 occasionally filled with leucocytes. Leucocytes 

 are also found in narrow spaces between the tissue 

 cells and on the surface of the mantle lining from 

 which they are discarded. 



ESOPHAGUS AND STOMACH 



The esophagus, a short, funnel-shaped, and 

 dorso-ventrally compressed tube, is lined with 

 epithelium similar to that of the mouth. It leads 

 to the stomach, which occupies a central position 

 within the visceral mass (fig. 197). The stomach 

 is an irregularly shaped, large sac (figs. 198 and 

 199) with several outgrowths or pouches. At the 

 entrance of the esophagus the wall of the stomach 

 forms an anterior chamber, a, which leads into a 

 broader posterior chamber, b (figs. 198 and 199). 

 An oblique outgrowth or pouch called the caecum, 

 c, is the most conspicuous structure which arises 

 from the ventral side of the anterior chamber. 

 Both the anterior and posterior ends of the caecum 

 are curved and form the anterior and posterior 

 appendices (a.ap., p.ap.). The larger posterior 

 appendix is a strip curved ventrally and toward 

 the right of the stomach. The configuration and 

 relative sizes of the appendices vary but the 

 structures are recognizable in all the casts. A 

 groove along the wall of the caecum leads to the 

 opening of the midgut (m.g.) and serves for sorting 

 of food (Yonge, 1926a). On the left side below 

 the caecum the wall of the stomach bulges out to 

 form a broad pyloric caecum (p.c), which leads 

 to a long outgrowth alongside the midgut, the 

 crystalline sac (cr.s.). 



219 



