DIGESTIVE ORGANS OF MOLLUSCA. 



159 



the Liver of the higher classes of animals. This organ ac- 

 quires still greater size and imporUnce in the Mollusca, 

 where it frequently envelops the stomach, pouring the bile 

 into its cavity by numerous ducts.* As the structure and 

 course of the intestinal canal varies greatly in diflcrcnt tribes 

 of Mollusca, they do not admit of being comprised in any 

 general description. The only examples I think it necessa- 

 ry to give, in this class, are those of the 

 Patella, or Limpet, and of the Pleiiro- 

 branchiis. The intestinal tube of the 

 Patella is delineated in Fig. 329; where 

 M is the mouth; t, the tongue folded 

 back; o, the oesophagus; and s, the sto- 

 mach, from which the tortuous intestinal 

 tube is seen to be continued. All the 

 convolutions of this tube, as well as the 

 stomach itself, are enclosed, or rather 

 imbedded, in the substance of the liver, 

 which is the largest organ of the body. 



The Pleurobranchiis Peronii (Cuv.) is remarkable for 

 the number and complication of its organs of 

 digestion. They are seen laid open in Fig. 

 330; where c is the crop; g, the gizzard; p, a 

 plicated stomach,resemblingthc third stomach 

 of ruminant quadrupeds; and d, a fourth ca- 

 vity, being that in which digestion is com- 

 pleted. A canal of communication is seen at 

 T, leading from the crop to this last cavity: b 

 is the point where the biliary duct enters. 



In the Cephalopoda, the structure of these 

 organs is very complicated; for they are pro- 

 vided with a crop, a muscular gizzard, and a 

 caecum, which has a spiral form. In these 

 animals we also discover the rudiment of another auxiliary 



* Transparent crystalline needles, the nature ami uses of which are quite 

 unknown, are frequently found in the biliary ducts of this class of animals. 



