CHAPTER VIII 



THE DIGESTIVE ORGANS, JAW, AND RADULA : EXCRETORY ORGANS 



The digestive tract, or, as it is often termed, the alimentary canal 

 or gut, is a very important feature of the MoUusca. It may be 

 regarded as consisting of the following parts : (1) a mouth or oral 

 aperture; (2) a throat or pharynx; (3) an oesophagus, leading into 

 (-4) a stomach, (5) an intestine and rectum, ending in (6) an 

 anus. 



The primitive positions of mouth and anus were presumably 

 at the anterior and posterior ends of the animal, as in the 

 Amphinem-a and symmetrical Mollusca generally. But the 

 modifications of original molluscan symmetry, which have already 

 been referred to (p. 154, compare pp. 245, 246), have resulted in 

 the anus becoming, in the great majority of Gasteropoda, twisted 

 forward, and occupying a position on some point in the right 

 side in dextral, and in the left in sinistral species. 



The process of digestion, as the food passes from one end of 

 the tract to the other, is performed by the aid of the secretions 

 of various glands, which open into the alimentary canal at 

 different points in its course. The principal of these are the 

 salivary glands, situated on the pharynx and oesophagus, and 

 the liver, biliary or hep)atic gland, connecting with the stomach. 

 With these may be considered the cmal and ink-glands, which, in 

 certain genera, connect with the terminal portion of the rectum. 



1. The mouth is generally, as in the common snail and peri- 

 winkle, placed on the lower part of the head, and may be either 

 a mere aperture, circular or semicircular, in the head-mass, or, as 

 is more usual, may be carried on a blunt snout (compare Fig. 6, 

 p. 10, and Fig. 68, p. 159), which is capable of varying degrees 

 of protrusion. From the retractile snout has doubtless been 



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