VIII THE STOMACH AND ITS APPENDAGES 239 



the action of which digestion appears to begin in some cases 

 before the food reaches the stomach proper. 



4. The Stomach. — At the posterior end of the oesophagus lies 

 the muscular pouch known as the stomach, in which the digestion 

 of the food is principally performed. This organ may be, as in 

 Limax^ no more than a dilatation of the alimentary canal, or it 

 may, as is usually the case, take the form of a well-marked bag 

 or pocket. The two orifices of the stomach are not always 

 situated at opposite ends ; when the stomach itself is a simple 

 enlargement of the wall of one side of the alimentary canal, 

 the cardiac or entering orifice often becomes approximated to 

 the orifice of exit (pyloric orifice). 



The walls of the stomach itself are usually thickened and 

 strengthened by constrictor muscles. In some Nudibranchs 

 (Scyllaea., Bornella) they are lined on the inside with chitinous 

 teeth. In Cyclostoma^ and some Bithynia^ St7'ombus^ and Trochus 

 there is a free chitinous stylet within the stomach.^ In Melon- 

 gena (Fig. 146) the posterior end of the oesophagus is provided 

 witli a number of hard plate-like ridges, while the stomach is 

 lined with a double row of cuticular knobs, which are movable 

 on their bases of attachment, and serve the purpose of triturating 

 food.2 Aplysia has several hard plates, set with knobs and spines, 

 and similar organs occur in the Pteropoda. But the most formid- 

 able organ for the crushing of food is possessed by the BuUidae, 

 and particularly by Scajjhander (Fig. 145). Here there is a 

 strong gizzard, consisting of several plates connected by powerful 

 cartilages, which crush the shells, which are swallowed whole. 



Into the stomach, or into the adjacent portions of the diges- 

 tive tract, open the ducts which connect with the so-called 

 liver. The functions of this important organ have not yet been 

 thoroughly worked out. The liver is a lobe-shaped gland of a 

 brown-gray or light red colour, which in the spirally-shelled 

 families usually occupies the greater part of the spire. In the 

 Cephalopoda, the two ducts of the liver are covered by append- 

 ages which are usually known as the pancreatic coeca ; the bil- 

 iary duct, instead of leading directly into the stomach, passes 



1 According to Moquin-Tandon {Moll, de France., i. p. 44) this process in 

 Bithynia is attached by one end to the wall of the stomach. Vivipara., with two 

 jaw pieces, does not possess this stylet ; Bithynia, which does possess it, has no jaw. 



2 J. H. Vanstone, Journ. Linn. Soc. xxiv. p. 369. 



