PHYLUM MOLLUSCA 



251 



entering the mouth, food is passed through a short esophagus into 

 the saclike stomach. Here it comes in contact with a digestive fer- 

 ment produced by the digestive gland which is discharged into 

 each side of the stomach through ducts. The crystalline style, a 

 diverticulum of the intestine, and found only in mollusks, produces 

 an enzyme mixed with the stomach content which undoubtedly 

 facilitates the digestion of carbohydrates. The food, having been 

 mostly digested and partly absorbed in the stomach, is passed on 



ryjfcalline .style 

 1-* t^ucous qiands 



%-—Intest\r)e 



Fig. 143. — Cross section through the style sac and intestine of Lampsilis anodon- 



toides. (Modified after Nelson.) 



into the intestine which makes one or more loops in the foot, passes 

 through the pericardium and terminates in the anus near the dorsal 

 siphon. 



Respiration 



Respiration is carried on through two pairs of vascularized gills 

 which hang down into the mantle cavity on each side of the foot. 

 Oxygenated water drawn in through the ventral siphon is passed 

 through a rather complicated series of water tubes in the gills. 

 Oxygen is absorbed by the capillaries and carbon dioxide passed 

 into the water where it is discharged to the outside through the 

 dorsal siphon. 



