CHAPTER YIII 



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 Mollusca. It 

 may be regarded as consisting of the following parts : (1) a 

 mouth or oral aperture : (2) a throat ov pharynx ; (3) an oesoph- 

 agus, leading into (4) a stomachy (5) an mtestine 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 

 Amphineura 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 Gastero- 

 poda, 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 hepatic gland., connecting with the stomach. 

 With these ma}^ be considered the anal and ink-gla7ids^ 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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